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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

CHAPTER

1

Tank History, Typical Configurations, Locating, Sizing, and Selecting Ira M. Gabin, P.E. Dixon Engineering

Richard A. Horn, P.E. CB&I

The rapid development and expansion of public water supply systems at the beginning of the 20th century led to the establishment of public health standards for drinking water systems. An area of major concern for these systems was the storage facilities. Early steel reservoirs and standpipes were of riveted construction. Modern welded-steel reservoirs can be built to very large capacities with either domed or column-supported roofs. In the 1970s, it became common for smaller-capacity reservoirs and standpipes to use bolted construction technology, originally developed for industrial and agricultural uses. Prefabricated panels and bolted connections reduced erection costs and made these structures popular in rural areas. The advent of factory-applied ceramic coatings reduced future maintenance costs, adding to the tanks’ attractiveness to water supply systems with limited financial resources. Bolted tanks with diameters greater than 30 ft (9 m) are often built with low-maintenance aluminum geodesic domed roofs, a technology

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

2

Chapter One

FIGURE 1-1 Geodesic dome on bolted-steel reservoir.

commonly found on wastewater plant storage tanks as well. Figure 1-1 shows a geodesic dome on a bolted reservoir. The earliest elevated storage tanks were constructed of wood in the manner of water refilling stations for steam-powered trains. Some were built on stone or brick columns. Limitations as to size and durability, as well as public health concerns, led to steel becoming the material of choice for elevated tanks. Most steel elevated tanks constructed before 1950 were riveted, their legs consisting of opposed channels connected by latticework bracing. Roofs on most small tanks and many larger ones were the familiar cone or “witch’s hat” design (Fig. 1-2). Some larger elevated tanks had hemispherical or ellipsoidal roof designs. Welded construction became the industry norm by the early 1950s and remains the standard for most elevated tanks. Legged tanks continued to be built in great numbers; however, the lattice legs replaced tubular sections. Many larger-capacity legged tanks were of the radial arm design shown in Fig. 1-3. These have been phased out in favor of the toroelliptical legged tank style. Early prototypes of single-pedestal tanks were developed in the 1940s and became a common alternative to legged tanks by the 1950s. The more efficient shape of these structures provided the advantage of lower maintenance costs. In the 1960s, the fluted-column singlepedestal design was introduced, which provided a usable area in the column for pumping equipment, storage, offices, and other municipal uses. Legged tanks continue to be built primarily in sizes up to 1 million gallons (mil gal) (3.8 million liters [ML]) as a lower-cost alternative to single-pedestal or fluted-column tanks. Single-pedestal tanks are widely specified from very small to large capacities. Larger capacities

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g FIGURE 1-2 Witch’s hat roof design.

FIGURE 1-3 Legged tank with radial arm design.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

4

Chapter One (0.75 to 2 mil gal [2.8 to 7.6 ML] or more) are generally single-pedestal or fluted-column tanks. Some fluted-column tanks have even larger capacities. In the late 1980s, composite-tank technology combined a concrete pedestal with the steel-bowl geometry of the fluted-column tank. This addressed one of the concerns of the fluted-column design—the large steel surface area and resulting higher repainting costs. Built generally to hold 0.75 to 2 mil gal (2.8 to 7.6 ML) of water, composite tanks are now in use throughout the United States and Canada. Other materials and technologies are available for specialized applications. However, the steel, glass-lined steel, concrete, and composite tanks discussed in this chapter comprise the large majority of tanks currently in use and being specified for new construction.

Reservoirs A reservoir is a ground-supported, flat-bottom cylindrical tank with a shell height less than or equal to its diameter. Reservoirs are one of the most common types of water storage structure. They are used as a part of the distribution system as well as to hold treated water for pumping into the distribution system. Of the three types of steel water tanks, a reservoir, because of its low height, is generally the most economical to fabricate, erect, and maintain. See Figs. 1-4 and 1-5 for a photo and a cross-sectional view of a welded-steel reservoir; see Figs. 1-6 and 1-7 for a photo and a cross-sectional view of a bolted-steel reservoir. Table 1-1 gives typical sizes of welded-steel reservoirs, and Table 1-2 gives capacities of glass-coated, bolted-steel reservoirs and standpipes. Storage reservoirs for potable water are covered by roof structures, which may be either column supported or self-supporting. Standard tank accessories may include shell and roof manholes, screened roof vents, inside or outside ladders, and connections for pipes as required.

Standpipes Standpipes are ground-supported, flat-bottom cylindrical storage tanks that are taller than their diameter. They are usually built where there is little elevated terrain and where extra height is needed to create pressure for water distribution. See Figs. 1-8 and 1-9 for a photo and a cross-sectional view of a welded-steel standpipe and Figs. 1-10 and 1-11 for a photo and a cross-sectional view of a bolted-steel standpipe. Table 1-3 gives capacities and sizes of typical welded-steel standpipes. Standpipe systems are often designed so that the water in the tank, until it reaches a certain low level, maintains the system pressure. When that low level is reached, pumps come on, valving is changed,

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

FIGURE 1-4 Welded-steel reservoir. (Photo: Gay Porter DeNileon, AWWA)

Roof manholes Overflow pipe

Splash pad

Approved ladder, cage, platform, or safety devices complying with Occupational Safety and Health Act

Roof vent 12 in. (0.3 m) 3⁄4 in. (19 mm) Capacity level

Weir box (optional)

Column support

Inlet–outlet (optional) Base elbow or valve pit

Tank bottom crowned at center

Roof rafters

Column bases

Shell manholes (two required)

Sand pad Compacted backfill Crushed rock or gravel

Concrete foundation

FIGURE 1-5 Cross-sectional view of welded-steel reservoir. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

6

Chapter One

FIGURE 1-6 Bolted-steel reservoir, glass fused to steel. Approved ladder, cage, and platform complying with Occupational Safety and Health Act Roof manway

Floor sloped toward outlet pipe Inlet–outlet

24-in. (0.6-m) round access door

Gravity ventilator

Internal overflow funnel

Overflow pipe

Splash pad

Grade level

FIGURE 1-7 Cross-sectional view of bolted-steel reservoir. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Capacity (US gal) 50,000 60,000 75,000 100,000

(m3 ) 189 227 284 379

125,000

473

150,000

568

200,000

757

250,000

946

300,000

1,136

400,000

1,515

500,000

1,893

600,000

2,271

750,000

2,839

1,000,000

3,785

1,500,000

5,678

2,000,000

7,571

3,000,000 11,356 4,000,000 15,142 5,000,000 18,927 7,500,000 28,391 10,000,000 37,854

Range of Sizes Available Diameter (ft [in.]) 19 [3] 21 [0] 23 [6] 23 [6] 27 [0] 26 [0] 30 [3] 28 [6] 33 [0] 33 [0] 38 [3] 37 [0] 42 [9] 40 [6] 46 [9] 46 [6] 54 [0] 46 [6] 52 [0] 60 [6] 51 [0] 57 [0] 57 [0] 64 [0] 66 [0] 74 [0] 80 [6] 90 [6] 93 [0] 104 [6] 114 [0] 127 [6] 131 [6] 147 [6] 147 [0] 165 [0] 180 [0] 201 [6] 233 [0] 208 [0]

Height to Diameter Height to TCL (ft [in.]) (m) TCL (m) 24 [0] 5.9 7.3 24 [0] 6.4 7.3 24 [0] 7.2 7.3 32 [0] 7.2 9.8 24 [0] 8.2 7.3 32 [0] 7.9 9.8 24 [0] 9.2 7.3 32 [0] 8.7 9.8 24 [0] 10.0 7.3 32 [0] 10.0 9.8 24 [0] 11.7 7.3 32 [0] 11.3 9.8 24 [0] 13.0 7.3 32 [0] 12.3 9.8 24 [0] 14.3 7.3 32 [0] 14.2 9.8 24 [0] 16.5 7.3 40 [0] 14.2 12.2 32 [0] 15.9 9.8 24 [0] 18.4 7.3 40 [0] 15.6 12.2 32 [0] 17.4 9.8 40 [0] 17.4 12.2 32 [0] 19.5 9.8 40 [0] 20.1 12.2 32 [0] 22.6 9.8 40 [0] 24.5 12.2 32 [0] 27.6 9.8 40 [0] 28.4 12.2 32 [0] 31.9 9.8 40 [0] 34.7 12.2 32 [0] 38.9 9.8 40 [0] 40.1 12.2 32 [0] 44.9 9.8 40 [0] 44.8 12.2 32 [0] 50.3 9.8 40 [0] 54.9 12.2 32 [0] 61.4 9.8 32 [0] 71.0 9.8 40 [0] 63.5 12.2

Source: AWWA Manual M42, Steel Water-Storage Tanks. Note: TCL = top capacity level.

TABLE 1-1 Capacities and Sizes of Typical Welded-Steel Water-Storage Reservoirs

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24

28

16 22 27 32 24 31 39 47 33 43 53 64 54 71 88 105 81 107 132 158 114 149 185 220 151 199 246 294 218 286 355 423 326 326 428 530 421 553 685 816 567 744 921 1,099 691 906 1,122 1,337 874 1,147 1,420 1,247 1,637

19 37 54 74 122 183 256 341 491 632 948

44 63 86 142 212 292 388 559 734

49 70 96 159 238 327 436 628 836

54 78 106 176 263 363 483 696

59 86 117 193 289 398 531

65 93 122 210 320 434 578

70 101 137 227 340 469

75 108 148 244 365 505

TABLE 1-2

†

Capacities of Glass-Coated, Bolted-Steel Reservoirs and Standpipes

80 116 158 261 391

86 123 168 278 416

91 96 101 107 131 139 146 154 179 189 199 210 296 313 330 347 442

Capacity in Thousands of Gallons† 112 161 220 364

117 169 230

122 177 241

128 184 251

133 192 261

139 199 272

33 38 43 47 52 57 61 66 70 75 79 84 89 93 98 102 107 112 116 121

To convert feet to meters, multiply by 0.3048. Capacity in thousands of gallons. To convert gallons to cubic meters, multiply by 0.0037854. Source: AWWA Manual M42, Steel Water-Storage Tanks.

∗

14 17 20 25 31 36 42 50 62 70 81 90 101 120

Nominal 15 Diameter (ft)∗

Nominal Height (ft)∗

Tank History, Typical Configurations, Locating, Sizing, and Selecting

8

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g FIGURE 1-8 Welded-steel standpipe with decorative pilasters.

and water is pumped from the lower portion of the standpipe into the system. As with reservoirs, steel standpipes are covered with a roof structure and may be provided with ornamental trim. Standard accessories may include shell and roof manholes, roof vent(s), a fixed outside ladder, and connections or pipes as required. Inside ladders are not recommended in locations where freezing weather can be expected.

Roof Designs for Reservoirs and Standpipes The emphasis on making steel water reservoirs and standpipes attractive as well as functional has led to the development of a wide variety of roof designs. Alternative roof styles for welded tanks include conical, toriconical, umbrella, dome, and ellipsoidal designs. Some are column supported; others are self-supporting. Bolted-steel tanks are usually provided with conical roofs or may be furnished with an aluminum geodesic dome. Column-supported roof structures are not usually used on steel standpipes taller than 50 ft (15 m). Whichever design is selected, it is particularly important to design any rafters, trusses, columns, stiffeners, and connections to minimize potential corrosion sites. All interfaces and connections of such members should be analyzed for their corrosion potential, and protective

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

10

Chapter One Roof vent

Roof plate Roof manholes Overflow pipe Weir box (optional)

Capacity level

Painter’s trolley rail Approved ladder, cage, platform, or safety devices complying with Occupational Safety and Health Act

Tank bottom crowned at center Inlet– outlet (optional) Shell manholes (two required)

Concrete foundation

Base elbow or valve pit

Splash pad

Sand pad Crushed rock or gravel Compacted backfill or undisturbed soil

FIGURE 1-9 Cross-sectional view of typical welded-steel standpipe. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

coatings should be applied to all surfaces deemed necessary from a cost/benefit standpoint.

Column- and Rafter-Supported Cone Roofs The column- and rafter-supported roof (Fig. 1-12) is generally the most economical for a reservoir. The roof has a minimum slope for adequate drainage and provides easy access to the manhole for interior inspection. Column loads are spread to a safe limit by column bases, and concrete footings under the columns are not usually required. A modification of this design incorporates a transition from the shell plate to the roof plate that is a smooth curve rather than a sharp break. This transition, or knuckle plate, is a dished or rolled section that usually requires a stiffener at the rafter attachment point (Fig. 1-13).

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

FIGURE 1-10 Bolted-steel standpipe. Approved ladder, cage, and platform complying with Occupational Safety and Health Act Roof walkway and guard rail

Internal overflow funnel

Roof access

Gravity ventilator

Top elbow

Overflow pipe

Floor sloped toward outlet pipe

Inlet–outlet (optional)

24-in. (0.6-m) round access door

Splash pad

Grade level

FIGURE 1-11 Cross-sectional view of bolted-steel standpipe. (Source: AWWA Manual M42, Steel Water-Storage Tanks) Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

Tank History, Typical Configurations, Locating, Sizing, and Selecting

12

Chapter One Capacity (US gal)

(m3 )

Range of Sizes Available Diameter (ft [in.])

Height to Diameter Height to TCL (ft [in.]) (m) TCL (m)

50,000

189

14 [9]

40 [0]

4.5

12.2

60,000

227

16 [2]

40 [0]

4.9

12.2

75,000

284

18 [0]

40 [0]

5.5

12.2

100,000

379

19 [0]

48 [0]

5.8

14.6

125,000

473

21 [3]

48 [0]

6.5

14.6

150,000

568

23 [3]

48 [0]

7.1

14.6

200,000

757

24 [10]

56 [0]

7.6

17.1

250,000

946

27 [9]

56 [0]

8.5

17.1

300,000

1,136

28 [5]

64 [0]

8.7

19.5

400,000

1,514

32 [10]

64 [0]

10.0

19.5

500,000

1,893

34 [7]

72 [0]

10.5

21.9

600,000

2,271

37 [10]

72 [0]

11.5

21.9

750,000

2,839

42 [6]

72 [0]

12.9

21.9

1,000,000

3,785

46 [4]

80 [0]

14.1

24.4

1,500,000

5,678

56 [9]

80 [0]

17.3

24.4

2,000,000

7,571

65 [6]

80 [0]

20.0

24.4

2,500,000

9,464

69 [10]

88 [0]

21.3

26.8

3,000,000 11,356

76 [6]

88 [0]

23.3

26.8

4,000,000 15,142

84 [6]

96 [0]

25.8

29.3

5,000,000 18,927

94 [6]

96 [0]

28.8

29.3

Source: AWWA Manual M42, Steel Water-Storage Tanks. Note: TCL = top capacity level.

TABLE 1-3 Capacities and Sizes of Typical Welded-Steel Standpipes

Self-Supporting Dome Roof and Umbrella Roof Steel self-supporting roofs are constructed of plates that are butt welded, lap welded, or lap bolted. They are supported directly on the top angle and shell plate. This type of roof is used where an uncluttered interior and smooth exterior appearance are desired. Domeroof sections are pressed to form a spherical shape. Umbrella roofs are formed to a radius in one direction only, forming chords like the cloth between the spines of an umbrella (Fig. 1-14). Structural stiffeners may be used internally on large-diameter roofs to avoid excessive plate thickness on welded or bolted tanks. Sometimes steel trusses may be used to support the roof, but these

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

12 in. (0.3 m)

Top angle

Vent

3⁄4 in. (19 mm)

Channel rafters

Girders are required when more than one column is used

Butt-welded tank shell Column base

3 ⁄16-in. (4.7-mm) lap-welded roof plate Capacity level One or more supporting columns

1⁄4-in. (6.4-mm) lap-welded bottom plate

FIGURE 1-12 Tank with column- and rafter-supported cone roof. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

should be avoided if possible, because they may create corrosion problems. In addition, the trusses should be kept above the water line to prevent damage by ice and accelerated rates of corrosion. A modification of the self-supporting dome is the toriconical roof. This consists of a rolled or pressed knuckle and a higher-pitched selfsupporting center. Aluminum dome roofs are sometimes erected on bolted-steel or welded-steel tanks. These aluminum domes are usually constructed

Knuckle plate

12 in. (0.3 m)

¾ in. (19 mm)

Radius

Channel rafter

Butt-welded tank shell Column base

3/16 in. (4.7-mm) lap-welded roof plate Capacity level One or more supporting columns

¼-in. (6.4-mm) lap-welded bottom plate

FIGURE 1-13 Column- and rafter-supported roof with knuckle. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

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Chapter One 3 ⁄16-in. (4.7-mm) minimum thickness lap- or butt-welded Cap plate roof plate

Vent

Top angle

Capacity level us di ra . al ax ric . m in. he D . m Sp 1.2 0 D = .8 0

Butt-welded tank shell

1⁄4-in. (6.4-mm) lap-welded bottom plate

FIGURE 1-14 Self-supporting dome roof or umbrella roof. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

of triangulated space truss (geodesic) panels. The dead weight of these domes is usually 3 lb/ft2 (143 N/m2 ) or less, compared with 3.8 lb/ft2 (181 N/m2 ) for a bolted-steel roof and 7.6 lb/ft2 (364 N/m2 ) for a welded-steel roof.

Self-Supporting Ellipsoidal Roof The self-supporting ellipsoidal roof is not a true ellipse, but it is formed with two radii yielding major- and minor-axis proportions of approximately 2:1. The transition from shell to roof is a smooth unbroken curve (Fig. 1-15). This roof design is suitable for large- and small-diameter reservoirs and standpipes. On tanks 50 ft (15 m) in diameter or less, the roof is usually free of internal structural members. Larger-diameter tanks usually have radial and circumferential stiffening members or rafters, which may be subject to corrosion problems if they are not properly designed or maintained.

Self-Supporting Cone Roof An inexpensive and very functional type of roof for small-diameter reservoirs and standpipes is the self-supporting cone roof without internal structural members. This roof is usually too steep to walk on. Access to manholes and vents by a roof ladder or steps and handrail should be provided. All means of access should be designed individually and installed to comply with current standards.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

Kn u

1⁄4-in. (6.4-mm) minimum thickness and butt welded in area filled with water

Area above capacity level may be lap welded

le ck

Butt-welded tank shell

Vent

Capacity level

1⁄4-in. (6.4-mm) lap-welded bottom plate

FIGURE 1-15 Self-supporting ellipsoidal roof. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Elevated Tanks An elevated steel water tank has two primary components: the tank itself and its supporting structure. Such tanks are ordinarily used where there is insufficient elevated terrain to ensure distribution of water at suitable pressure by gravity. These tanks are of welded construction. Elevated tanks can be categorized into several different types. The various diameters and head ranges for the tanks described in the remaining figures and tables in this chapter are only representative and may vary with individual fabricators. Specific diameter/head range combinations should be determined by the tank fabricator within the limits indicated in the tables. Height should be specified by the purchaser as the dimension between the top of the foundation and the top capacity level of the tank. Further dimensions, which are a function of the fabricator’s standard, should not be specified. To minimize cost, desired operating ranges should be specified to fall within standard available tank dimensions. However, individual operating needs may dictate nonstandard operating ranges.

Multiple-Column Elevated Tanks Small-Capacity Elevated (Double-Ellipsoidal) Tanks The small-capacity multiple-column elevated (or double-ellipsoidal) tank has a cylindrical sidewall, an ellipsoidal bottom and roof, and a

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

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Chapter One FIGURE 1-16 Double-ellipsoidal tank. (Photo: Gay Porter DeNileon, AWWA)

top capacity level (TCL) in the roof several feet or meters above the top of the cylindrical shell. Although in the past they were constructed in capacities up to 1 mil gal (3.8 ML), today, double-ellipsoidal tanks are typically constructed only in capacities of 200,000 gal (760,000 L) or less. See Figs. 1-16 and 1-17 for a photo and a cross-sectional view of a small-capacity elevated (double-ellipsoidal) tank. Table 1-4 gives capacities and sizes of typical double-ellipsoidal elevated tanks.

Medium-Capacity Elevated Tanks For medium-capacity multiple-column elevated tanks, the toroellipsoidal design provides a lower initial cost by using the strength of steel most efficiently. The features used (torus bottom and ellipsoidal roof) cause the central riser to support, as well as contain, a considerable portion of the stored water, while the major portion of the steel bottom acts as a membrane in tension. These tanks usually have a capacity between 200,000 gal (760,000 L) and 500,000 gal (1.9 ML). See Figs. 1-18 and 1-19 for a photo and a cross-sectional view of a mediumcapacity elevated tank. Table 1-5 gives capacities and sizes of typical medium-capacity elevated tanks.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g Diameter

Balcony or stiffening girder

As required

Purchaser to specify

Head range

6 in. (0.15 m) min.

FIGURE 1-17 Cross-sectional view of double-ellipsoidal tank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Large-Capacity Multiple-Column Elevated Tanks Large-capacity elevated tanks (>500,000 gal [>1,893 m3 ]) provide economical service for communities that need to store a substantial volume of water. Lower operating and pumping costs are ensured because of the low head range, which achieves minimum variation of water pressure throughout the system. See Figs. 1-20 and 1-21 for a photo and a cross-sectional view of a large-capacity elevated tank. Table 1-6 gives capacities and sizes of typical large-capacity elevated tanks.

Pedestal Elevated Tanks Small-Capacity Single-Pedestal Tanks The single-pedestal spherical tank is widely favored for smallercapacity tanks when appearance is a concern. The gracefully flared base contains sufficient space for pumping units and other operating equipment, a feature common to all pedestal-type vessels.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

18

Chapter One Capacity (US gal)

(m3 )

Range of Sizes Available Diameter (ft) 18–20

Head Range (ft) 12.5–15.5

Diameter (m) 5.5–6.1

Head Range (m) 3.3–4.7

25,000

95

30,000

114

18–20

15.0–16.5

5.5–6.1

4.6–5.0

40,000

151

22–23

15.0–17.0

5.7–7.0

4.6–5.2

50,000

189

22–24

18.0–20.0

6.7–7.3

5.5–6.1

60,000

227

22–25

19.0–23.0

6.7–7.6

5.3–7.0

75,000

284

26–30

16.0–24.0

7.9–9.1

4.9–7.3

100,000

379

23–30

20.0–25.0

3.5–9.1

6.1–7.6

125,000

473

30–32

23.0–28.0

9.1–9.7

7.0–8.5

150,000

568

32–34

24.5–29.5

9.7–10.4

7.5–9.0

200,000

757

36–38

28.0–29.5

11.0–11.6

8.5–9.0

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-4 Capacities and Sizes of Typical Double-Ellipsoidal Elevated Tanks

FIGURE 1-18 Medium-capacity welded-steel elevated tank. (Photo: Gay Porter DeNileon, AWWA)

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

Balcony or stiffening girder

As required

Purchaser to specify

Head Range

6 in. min.

FIGURE 1-19 Cross-sectional view of medium-capacity, torus-bottom weldedsteel elevated tank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Ladders to the container and roof are inside to protect against unauthorized access. These tanks are usually constructed in capacities of 200,000 gal (760,000 L) or less. See Figs. 1-22 and 1-23 for a photo and a cross-sectional view of a small-capacity single-pedestal tank. Table 1-7 gives capacities and sizes of typical small-capacity single-pedestal tanks. Small-capacity elevated tanks are also constructed as various combinations of cones and cylinders. An alternative design is shown in Fig. 1-24.

Large-Capacity Single-Pedestal Tanks The tubular supporting pedestal gives the large-capacity singlepedestal tank a distinctively contemporary look. Large capacities (0.2 to 2 mil gal [0.76 to 7.6 ML]) are provided by this low-head-range spheroidal tank design. See Figs. 1-25 and 1-26 for a photo and a cross-sectional view of a large-capacity single-pedestal tank. Table 1-8

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

20

Chapter One Capacity (US gal)

Range of Sizes Available

Diameter (m3 ) (ft)

Height to TCL (ft [in.])

Diameter (m)

Height to TCL (m)

200,000

757

36–38

28 [30]

11.0–11.6

8.5–9.1

250,000

946

38–40

28 [33]

11.6–12.2

8.5–10.1

300,000 1,136

43–45

28 [31]

13.1–13.7

8.5–9.4

400,000 1,514

46–50

30 [36]

14.0–15.2

9.1–11.0

500,000 1,893

50–56

29 [38]

15.2–17.1

600,000 2,271

51–0 57–0

40 [0] 32 [0]

15.6 17.4

750,000 2,839

56–65

34 [45]

17.1–19.8 10.4–13.7

1,000,000 3,785

64–65

45 [46]

19.5–19.8 13.7–14.0

8.8–11.5 12.2 9.8

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-5 Capacities and Sizes of Typical Medium-Capacity Elevated Tanks

FIGURE 1-20 Large-capacity elevated tank. (Photo courtesy of Landmark Structures, Inc.)

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g Diameter

As required

Purchaser to specify

Head range

6 in. (0.15 m) min.

FIGURE 1-21 Cross-sectional view of large-capacity, multicolumn elevated tank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Capacity

Range of Sizes Available

500,000

Diameter Head Diameter Head (ft) Range (ft) (m) Range (m) 1,893 60–65 24–25 18.3–19.8 7.3–7.9

600,000

2,271

(US gal)

(m3 )

65–70

24–25

19.8–21.3

7.3–7.9

750,000

2,839

70–76

25–30

21.3–23.2

7.6–9.1

1,000,000

3,785

75–87

25–35

22.9–25.5

7.6–10.7

1,500,000

5,678

91–98

30–35

27.7–29.9

9.1–10.7

2,000,000

7,571 105–106

34–36

32.0–32.3 10.4–11.0

2,500,000

9,464 108–117

39–41

32.9–35.7 11.9–12.5

3,000,000 11,356 119–127

35–40

36.3–38.7 10.7–12.2

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-6 Capacities and Sizes of Typical Large-Capacity Welded-Steel Elevated Tanks

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

22

Chapter One FIGURE 1-22 Spherical single-pedestal tanks give pleasant silhouette. (Photo: Walter Baas, AWWA)

gives capacities and sizes of typical large-capacity single-pedestal tanks.

Modified Single-Pedestal Tanks The attractive modified single-pedestal tank has a central support column (usually fluted to give structural rigidity) that encloses the riser pipe, overflow pipe, and access ladder to the tank roof. The support column may be constructed of steel or concrete. The space within the column can provide multistory usable floor space for pumping, storage, and office facilities. Although available in all capacities, these tanks are not usually constructed in capacities less than 500,000 gal (1.9 ML). See Figs. 1-27 and 1-28 for a photo and a cross-sectional view of a modified single-pedestal tank. Table 1-9 gives capacities and sizes of typical modified single-pedestal tanks.

Composite Elevated Tanks Composite elevated tanks are of an attractive design that uses the best design features of steel and concrete. Concrete, which is excellent for compression loads, is used as the support column for the steel bowl. The concrete has the advantage of requiring either no painting

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Tank History, Typical Configurations, Locating, Sizing, and Selecting Diameter

As required

Purchaser to specify

Head range

6 in. (0.15 m) min.

FIGURE 1-23 Cross-sectional view of small-capacity spherical single-pedestal tank. (Source: AWWA Manual M42, Steel Water-Storage Tanks)

Capacity (US gal)

(m3 )

Range of Sizes Available Diameter (ft)

Head Range (ft)

Diameter (m)

Head Range (m)

25,000

95

19–20

15–17

5.8–6.1

4.6–5.2

30,000

114

20–21

15–18

6.1–6.4

4.6–5.5

40,000

151

21–23

19–22

6.4–7.0

5.8–6.7

50,000

189

23–24

19–23

7.0–7.3

5.8–7.0

60,000

227

24–26

22–24

7.3–7.9

6.7–7.3

75,000

284

25–28

23–27

7.9–8.5

7.0–8.2

100,000

379

29–30

25–30

8.8–9.1

7.6–9.1

125,000

473

31–33

27–32

9.4–10.0

8.2–9.7

150,000

568

33–34

30–34

10.1–10.4

9.1–10.4

200,000

757

36–38

36–38

11.0–11.6

11.0–11.6

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-7 Capacities and Sizes of Typical Small-Capacity Single-Pedestal Tanks

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

24

Chapter One

FIGURE 1-24 Alternative single-pedestal tank design.

or a low-cost exterior coating for aesthetic purposes. The steel bowl construction is similar to that found on the fluted-column tanks; the bowl can be built with either a cone or a domed roof. The most common designs use a domed concrete floor with a steel liner. Commonly built to store between 750,000 gal and 2 mil gal (2.8 and 7.6 ML), these tanks provide many of the benefits of a fluted-column tank with significantly less area that requires painting, thereby reducing maintenance costs. The diameter of the concrete column is generally somewhat smaller (30 to 60 ft [9 to 18 m]) than for a fluted-column tank, so the area in the column for other uses is reduced. See Figs. 1-29 and 1-30 for a photo and cross-sectional view of a composite elevated tank.

Locating, Sizing, and Selecting a Water Tank Locating, sizing, and selecting a water-storage tank involve the evaluation of several design considerations and require an awareness of zoning and other regulations. The purpose of this section is to discuss these considerations and to provide the reader with a checklist to work through in the effort to arrive at a reasonable solution. Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

FIGURE 1-25 Large-capacity single-pedestal elevated tank. (Photo courtesy of Tnemec/STI/SPFA)

Locating a Water Tank Generally, locating tanks depends on where people are living now and where future neighborhoods will be built within the area served by the water system. In addition, numerous other conditions can significantly influence the choice of a suitable site and therefore the overall cost of the tank project. Answers to the following basic questions must be determined and considered when selecting a location for a new water-storage tank. Hydraulics r What are the maximum and minimum pressures that you want to provide the end users?

r Is it better to pump or use gravity flow to provide the needed pressure?

r What are the local utility costs of pumping during daily and peak demand periods? Proximity to Users r Where is the growth in the community taking place now and projected to be in the future?

r Is land available in the area of future growth?

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

26

Chapter One Diameter

Head range

As required

Purchaser to specify

Access tube

6 in. (0.15 m) min.

FIGURE 1-26 Cross-sectional view of large-capacity single-pedestal elevated tank. (Source: AWWA Manual 42, Steel Water-Storage Tanks)

Capacity (US gal)

(m3 )

200,000 250,000 300,000 400,000 500,000 750,000 1,000,000 1,250,000 1,500,000 2,000,000

757 946 1,136 1,514 1,893 2,839 3,785 4,732 5,678 7,571

Range of Sizes Available Diameter (ft) 40–42 43–47 46–48 50–53 55–60 64–66 74–78 76–80 85–90 90–95

Head Range (ft) 27–30 25–32 30–33 30–40 30–40 38–42 35–40 40–45 45–50 50–55

Diameter (m) 12.2–12.8 13.1–14.3 14.0–14.6 15.2–16.1 16.3–18.3 19.5–20.1 22.5–23.8 22.9–24.4 25.9–27.4 27.4–29.0

Head Range (m) 8.2–9.1 7.6–9.7 9.1–10.1 9.1–12.2 9.1–12.2 11.6–12.3 10.7–12.2 12.2–13.7 13.7–15.2 15.2–16.3

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-8 Capacities and Sizes of Typical Large-Capacity Single-Pedestal Tanks

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Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

FIGURE 1-27 Folded-plate design of a modified single-pedestal tank support. (Photo courtesy of Tnemec/STI/SPFA)

Acquiring Land r What is the cost of the tank site being considered? Is the land even available?

r What is the cost of connecting water mains and permanent

electrical power at each site being considered? Zoning r Is a zoning map available, and are the potential sites zoned to allow a tank project? Federal Aviation Administration (FAA) r Would the FAA allow a tank at the required height to be built on the potential site?

r Are obstruction lights or FAA painting required on the tank at the potential site?

Size of Site Is the site large enough for r Erection equipment, steel storage, staging operations, ground assembly, and crane operations with a safe and adequate distance for items that may be dropped from the tank during erection?

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

28

Chapter One Diameter

Fluted column

Purchaser to specify

As required

Head range

FIGURE 1-28 Cross-sectional view of a modified single-pedestal tank.

Capacity (US gal)

(m3 )

250,000 946 300,000 1,136 500,000 1,893 750,000 2,839 1,000,000 3,785 1,250,000 4,732 1,500,000 5,678 2,000,000 7,571 2,500,000 9,464 3,000,000 11,356

Range of Sizes Available Diameter Head (ft) Range (ft) 41–43 29–31 43–45 29–31 49–64 30–39 63–65 37–40 73–78 35–42 76–80 40–45 85–87 39–46 97–102 38–46 107–110 43–45 109–120 40–45

Diameter (m) 12.5–13.1 13.1–13.7 14.9–19.5 19.2–19.8 22.2–23.8 22.9–24.4 25.9–26.5 29.6–31.1 32.6–33.5 33.3–36.6

Head Range (m) 8.8–9.4 8.8–9.4 9.1–11.9 11.3–12.2 10.7–12.8 12.2–13.7 11.9–14.0 11.6–14.0 13.1–13.7 12.2–13.7

Source: AWWA Manual M42, Steel Water-Storage Tanks.

TABLE 1-9 Capacities and Sizes of Typical Modified Single-Pedestal Tanks

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

FIGURE 1-29 Composite elevated tank.

r Maintenance of the tank and piping after completion? r Abrasive blasting and painting of the tank now and in the future? Topography Does the site have—or can it be made to have—good drainage to ease construction operations and minimize standing water around the completed tank? Access to Site r Is the site accessible on public roads by concrete and large semitrailer tractor rigs?

r Is there an access road or temporary easement to the site?

Will permission be given to build a road? Who will pay for the road? Will it be a permanent or temporary road? If temporary, will it be necessary to remove it at the end of the project? Soil Conditions r Is the soil bearing strength at the bottom of the tank foundation adequate to support the tank without requiring expensive deep foundations?

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

30

Chapter One Upper roof cone

High water line

Steel bottom plate

Access tube

Low water line

Lower cone

Concrete support dome

Concrete column

Note: Not to scale.

FIGURE 1-30 Cross-sectional view of composite elevated tank.

r Where is the water table? Will the foundation need to be dewatered during construction?

r Is the earth firm enough to support construction equipment during normal weather conditions or will gravel, crane mats, and other earth-stabilizing methods be required? Hazards and Construction r Are there power lines or other obstructions above or beside the site or proposed access road that would interfere with the safety of site traffic, construction, painting, or maintenance operations? Will a power line be closer than 40 ft (12.2 m) from the tank?

r Are there underground obstructions such as gas lines, sewers, or buried electrical or telephone lines? Were there mines or burial grounds on this site?

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g r If pile driving is required, will it disturb or cause failure of or damage to neighboring foundations or other structures?

r Will pile driving, excavation, steel erection, or abrasive blasting cause noise unacceptable to a neighbor such as a school, hospital, or nursing home?

r Will the tank be in an area frequented by small children or vandals and, if so, could this be mitigated by site fencing?

Environmental Assessment r Has an environmental assessment been completed on the site?

r What agencies, forms, and permits may be required, and how long will approvals take?

NIMBY (Not in My Backyard) r Will the tank obstruct the view of historical landmarks or other items of concern to the citizens?

r How sensitive are the neighbors to having a tank in close proximity?

Determining answers to these questions can help you to better analyze and compare costs of alternate sites, so you can select the most desirable location for your new tank. Additionally, you will want to understand and consider the following criteria during your site selection.

Hydraulics Other issues that affect site selection include the required pressure at hydrants and residences, the required site elevation, compatibility with the distribution system, the geographic size and location of the demand area, and the tank’s proximity to the water supply. Rules of thumb for required water pressure are shown in Table 1-10. Check the local standards or codes for more specific requirements. One hundred sixty-two US and Canadian water utilities responded to an AWWA network modeling survey that requested the actual minimum and maximum distribution system pressures that

Location

Pressure (psi/kPa)

Comments

At hydrants during fire flow conditions

35/241

20-psi (0.138-kPa) minimum at other fire hydrants not directly serving the fire

Residential

50–75/ 0.345–0.517

Higher pressures may need to use a pressure-reducing valve

TABLE 1-10 Required Water Pressure

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32

Chapter One Min. psi 4% 10%

100

> 60 psi 50–59 psi

80

80 28%

60 55%

40

34%

30–39 psi

20

> 170 psi

10%

150–169 psi

12%

130–149 psi

25%

110–129 psi

18%

90–109 psi

15%

70–89 psi

40

20 23%

0

14%

60

Percent

62%

Max. psi

40–49 psi

Percent

100

20–29 psi

< 20 psi 1% Minimum pressure

6%

< 70 psi

0 Maximum pressure

FIGURE 1-31 Pressure ranges for utilities.

they provided. Figure 1-31 shows the percentage of utilities in each pressure range. If the pressures provided are more than 75 psi (0.517 kPa), it may be necessary to provide a pressure-reducing valve to prevent home appliances from being overpressurized. The required pressure can be provided through pumping, gravity flow, or a combination of the two. How pressure is provided depends on the sites available and the type of tank to be used.

Pumping with Ground Storage Tanks Pumping will be required if a ground storage tank is used where the topography is relatively flat throughout the service area and a higherelevation site is unavailable.

Gravity Flow with Ground Storage and Elevated Tanks The required pressure can also be obtained by building a ground storage tank on a hill or at higher elevation above the demand area so that gravity flow provides the pressure, much like a water cooler. An elevated tank provides the required pressure by raising the water storage height up to an elevation above the demand area so that gravity can provide the pressure. Costs can be lessened if the elevated tank is also constructed on a hill site or at higher elevation. This not only lessens the necessary height of the tank but also can reduce its cost.

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Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g

Gravity Flow and Pumping with a Standpipe Water could also be stored in a standpipe (a tall cylindrical tank) where the topography is relatively flat throughout the service area and a hill or higher-elevation site is unavailable. In a full standpipe, the uppermost one-third of water stored provides effective pressure for gravity flow. If the tank is two-thirds full, the upper half of the water would provide emergency pressure. In a tank only one-third full, the water provides little or no pressure (i.e., ineffective pressure) and would have to be pumped to be used. Much like with an elevated tank, costs can be saved if the standpipe is constructed on a higher-elevation site or hill. This not only lessens the necessary height of the tank but also can reduce its cost.

Gravity Flow Height Calculations Following is an example of how to calculate the minimum height at which to store water to provide an assumed minimum pressure for residential use through gravity flow. (Check your local standards or codes.) Height (for 50 psi [345 kPa] minimum) =

50 psi [345 kPa] (62.4 lb/ft3/144 sq in./sq ft)

= 115.4 ft [35.1 m] (≈ 115 ft [≈ 35 m]) or Height (for 50 psi [345 kPa] minimum) =

50 psi [345 kPa] = 115.4 ft [35.1 m] (62.4 lb/ft3/144 sq in./sq ft)

To this calculated height, add the additional height required to meet the friction loss of the water in the distribution piping. Alternatively, one can use a conversion chart to find the required height at which to store the water to provide the pressure needed. Figure 1-32 shows how various types of tanks provide this pressure using gravity flow.

Pumping Versus Gravity Flow Pumping If a site with an increased elevation of at least 115 ft (35 m) above the service area cannot be found, the only option with a ground storage tank is to use pumping to provide the required pressure. If you are going to pump, you should be aware that water demand varies throughout the day. As such, you will have to use a variable-speed pump. A typical water usage graph (Fig. 1-33) shows the filling of a tank during the night and early morning hours when demand is low. The

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

34

Chapter One

115 ft (35 m)

Reservoir

Elevated tank

Standpipe

FIGURE 1-32 Providing pressure using gravity flow.

tank is emptied during the day; water demand peaks sometime between 5 p.m. and 9 p.m. Electric utilities charge more for electricity during their peak demand period (see sample rates in Fig. 1-34). By overlaying the sample electric rates on the water usage graph (Fig. 1-35), one can see that the peak demands for electricity and water occur about the same time of day. Using these data, one can make the following calculations:

r Peak demand (5 p.m. to 9 p.m.) pumping costs: $0.1175/kWh average utility cost to pump half of the daily water demand to end users.

Peak demand

Emptying tank

Constant pumping rate Filling tank Usage rate

Midnight

6:00 A.M.

Noon

6:00 P.M.

Midnight

Time

FIGURE 1-33 Typical water usage.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

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Average cost per kilowatt-hour

$0.14 $0.12 $0.10 $0.08 $0.06 $0.04 $0.02 $0.00 Midnight

3:00 A.M. 6:00 A.M.

9:00 A.M.

Noon

3:00 P.M.

6:00 P.M. 9:00 P.M. Midnight

Time

FIGURE 1-34 Sample electric rates.

r Nonpeak demand pumping costs: $0.1080/kWh average utility cost to pump the other half of the daily water demand to end users.

r Tank filling costs: $0.0675/kWh average daily utility cost to fill the tank by pumping.

In this case, utility costs during peak demand are almost 75 percent more than the cost of the average rate used to fill the tank, while even nonpeak costs are about 60 percent more. These calculations should be modified for your system using your local daily water usage and utility rates. Regardless of the local factors, pumping during peak $0.14 Average cost per kilowatt-hour

Peak demand $0.12

Emptying tank

$0.10 Constant pumping rate $0.08 Filling tank $0.06 Usage rate $0.04 $0.02 $0.00 Midnight

6:00 A.M.

Noon

6:00 P.M.

Midnight

Time

FIGURE 1-35 Higher rates during peak demand.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

36

Chapter One electricity rates to meet peak water demand is usually more expensive than gravity flow and can become quite costly over time. Additionally, if you lack sites with hills or higher elevations and choose to pump to meet the pressure and water demand, incorporate the following initial and lifetime costs into your present-value analysis as follows:

r The additional daily costs of pumping over and above gravity flow (peak and nonpeak)

r The added cost of a variable-speed pump (usually required; larger than the constant-speed pump used at night for a gravity-flow tank)

r Cost of a backup pump or pumps r Cost for additional piping and controls for the backup pump(s)

r Cost of backup generator r Expense of enlarging the pump building to house the additional pumps and piping

r Cost to maintain and replace all of these as needed. Often, when these additional costs are considered, it is most likely that the extra initial costs to provide gravity flow may actually be a more cost-effective solution over time.

Gravity flow One can save these peak-demand electricity costs by peak shaving. To peak shave, start by locating a ground storage tank on the side of a hill, or build an elevated tank or standpipe. A smaller pump can then be used to pump the water up into the tank during the night and early morning at a constant rate when electricity rates are much lower. Then, during the demand period, water can be provided at the needed pressure by using gravity flow. This avoids the much higher electricity rates during this time period and allows use of a smaller, less costly pump. Because of these advantages, gravity flow is the preferred method of providing water pressure. If possible, place the tank on a hill or elevate it to take advantage of this method. The ideal location: For any type of storage tank, the ideal location is on a hill that is in the middle of the demand area and is owned by the community.

Proximity to Users When choosing a site for a new water-storage tank, the prospective tank owner should consider the growth in residential demand (singlefamily, multifamily, and high-rise structures) and commercial demand (industry, schools, and hospitals). A new residential development on

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

Ta n k H i s t o r y, C o n f i g u r a t i o n s , L o c a t i n g , S i z i n g , S e l e c t i n g the north side of the service area and a new tank on the south side would result in very little water pressure for residents of the new development. The ideal situation is to construct a new water-storage tank in the service area before the area experiences population growth and buildup. This way, you have a better chance to get the right piece of land at the right time and at the right price.

Acquiring Land When acquiring land, the prospective tank owner must consider the availability and suitability of the land for a tank project; the costs for the land, required support utilities, and the length of connections to the existing distribution system; and the surrounding conditions.

NIMBY (not in my back yard!) One of the biggest issues that a water utility can face when attempting to locate a new water-storage tank is the public concern of NIMBY! Despite these concerns, even the most appearance-conscious communities can agree to a mutually beneficial solution to this stumbling block. The following are some successful approaches to be used in overcoming public concern:

r Encourage community involvement: When choosing the style of the tank, let the citizens express their concerns and provide input. In some communities, citizen groups have used contests to select the color scheme of the tank exterior or the lettering and logo design.

r Educate the citizenry: Explain the reasons the new tank is

needed and the beneficial effect it will have on them personally (for example, improved water pressure and fire protection). Demonstrate how improved fire flow will affect insurance rates, assure them of the safety record of water-storage tanks, and explain the anticipated maintenance cycle.

r Help the public visualize the completed tank: Using an artist’s con-

ception, computerized renderings, and a digital photograph of the site, compile an image that shows the community what the finished water-storage tank will look like.

Zoning Regulations Once a site has been located, check on the zoning of the selected site to ensure that it is currently zoned for this use or can be rezoned. Obtaining proper zoning for a water tank is typically more difficult in a residential area than in an industrial area or in an area near public facilities such as schools, government property, and airports. Often, schools are built in the areas of population growth, and the school yard may make a good site for a tank. There are many aesthetically pleasing tank styles that limit access.

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Tank History, Typical Configurations, Locating, Sizing, and Selecting

38

Chapter One

FAA Considerations Forms must be completed and filed with the Federal Aviation Administration (FAA) to establish whether a tank can be built on the chosen site at the required height. The FAA is concerned about any obstruction to its airspace 200 ft (61 m) above ground level and any obstruction within an approach pattern to an airport runway. Lengths of approach pattern vary depending on the size of the airport, the length of the runway, and the direction of the runway, as follows:

r Large airport: No obstruction that exceeds a 100:1 surface within 20,000 ft (6.1 km) of an airport having at least one runway >3,200 ft (>975 m).

r Small airport: No obstruction that exceeds a 50:1 surface within

10,000 ft (3 km) of an airport whose longest runway is 35) have severe shrink/swell characteristics and require additional consideration in design, as is discussed later in the chapter.

Soil Consolidation All structures are subject to foundation settlement. Given the loading, footing size, and properties of soils, these settlements can be evaluated with reasonable accuracy. Consolidation is time-dependent settlement that can be significant in saturated soils. It occurs when the soil undergoes compressive deformation under the loading from the structure.

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Foundations

Foundations Water is extruded from the voids in the soil as the soil rearranges its grains to accommodate the increased pressure. The process occurs rapidly in granular soils because of their permeability, whereas in cohesive soils or fine-grained silty soils, consolidation can take a long time. Consolidation is also a function of footing size. The larger the footing, the greater the depth of the soil affected by the loads on the footing. The affected depth is also referred to as the effective bearing depth of the foundation. Depending on the thickness of the compressive layer or layers and the depth(s) at which they occur, the resulting settlements can be substantial and could create serious consequences for the structure. Therefore, to ensure structural stability, it is important to include consolidation testing in the work scope where deformationprone soil layers are present within the effective bearing depth of the foundation.

Total and Differential Settlement All water tank foundations undergo settlement. For reasons of stability and serviceability, it is necessary to minimize these settlements to tolerable limits. The extent or severity of the settlement depends on the foundation type, the magnitude and direction of loading, and the properties of the bearing and supporting soils. In addition to the overall settlement, a foundation undergoes relative or differential settlement, which, in essence, is the settlement of one part of the foundation with respect to another. Although all settlements must be evaluated for their effects on the system, differential settlements must be examined more closely, as they are critical to foundation strength and overall system stability. As noted in the section “Appropriate Foundation Type,” at the beginning of the chapter, for water tanks, it is preferable to limit the total and differential settlements to a maximum of 2 in. and 1 in. (5 cm and 2.5 cm), respectively. If the use of shallow foundations will cause excessive settlement, deep foundations can be used to further limit these settlements. Special piping and fittings are available that can offer flexibility in the system when high settlements are expected. Settlement is a major design consideration, and its effects on the entire water tank system must be carefully evaluated. In addition to the direct vertical settlements, the foundations are also subject to horizontal displacements under the influence of lateral loading. The extent of the horizontal displacement depends on the amount of foundation movement that is necessary to activate the surrounding soils’ passive resistance against the lateral loads. Therefore, geotechnical reports must fully address all settlement and lateral displacement considerations.

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Chapter Five

Required Design Information The subsurface investigation report must be prepared by qualified, registered geotechnical engineers who are experienced in designing water tank foundations. The number, location, and depth of the borings should be provided by the foundation designer and confirmed by the geotechnical engineer. All data supporting the recommendations for each feasible foundation type must be included in the geotechnical report, which should establish the following basic requirements: For all foundations r Site classification

r r r r r r r r r r r r r r r

Site topography and site preparation Description of the soil and its engineering properties Classification of soil strata Liquefaction potential and its consequences under dynamic loads Presence of rock, rock lenses, and boulders Potential for and consequence of shrink/swell Replacement or remediation of shrink/swell soils Anticipated total and differential settlements Drainage considerations Elevation of groundwater and dewatering requirements Minimum recommended bearing depth of foundation Excavation and backfill requirements Suitability of site soils for backfill Compaction and compaction testing requirements Seismic design parameters for American Water Works Association (AWWA) and/or other applicable codes

For shallow foundations r Soil ultimate and net allowable (FS = 3.0) bearing capacity

r Soil carrying capacity for lateral load based on soil passive resistance

r Extent of overexcavation, if necessary, and backfill recommendations For pile and caisson foundations r Anticipated pile/caisson type, size, and length

r Required pile/caisson spacing r Pile/caisson axial load capacity—compressive and pullout (include values for end bearing and skin friction separately, as appropriate)

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Foundations

Foundations r Minimum required reinforcement for caisson (as per local practice)

r Pile/caisson allowable lateral load capacity r Pile/caisson bending-moment diagram for appropriate end conditions

r Pile or caisson safety factor for long-term and transient loading

r Pile testing required and type of test r Special installation considerations r Appropriate uplift connection recommendations Typically, laboratory analysis of selected samples includes visual classification, cohesive shear strength tests, determination of Atterberg limits, grain-size analysis, determination of field moisture content, and the following additional parameters: Soil unit weight



Unit cohesion of soil

c

Coefficient of soil active pressure

Ka

Coefficient of soil passive pressure

Kp

Standard penetration resistance values

N

Angle of internal friction



Coefficient of friction, if different than tan 

f

Modulus of subgrade reaction

ks

All the required seismic parameters—including the mapped maximum considered earthquake acceleration at short period Ss and at 1-second period S1 —should be specified. Where required, site-specific geotechnical investigation and dynamic site response analysis should be performed to determine the appropriate values.

Problem Soils Experience indicates that certain types of soils pose special challenges in design and require remedial measures before they can support water tank foundation loads. Among these are expansive soils, which are prevalent in many areas of the United States and elsewhere in the world. The expansive clays with very high plasticity index values are not suited for shallow foundations unless remedial measures are taken that include lime mixing, prewatering, use of water barriers, or soil replacement. All of these measures are costly and require strict quality control. Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Chapter Five Foundation considerations in shrink/swell soils depend on the depth of the active zone and the swell potential of the soil. When a thin layer of these soils is present near the surface, it can be replaced with more suitable low-plasticity soils. When deeper layers are encountered, the expansive soils under the foundation can similarly be replaced and properly compacted. An approach that is often recommended is to place the shallow footing below the depth susceptible to shrink/swell and to replace or remediate the surrounding soils to make them suitable for backfill. However, the footing can be placed within the active zone as long as the uplift forces caused by the swelling of the soils are taken into account in design and as long as the structure can tolerate the resulting movements in the foundation. It is important to note, however, that as long as moisture is prevented from entering the soil, the shrink/swell volumetric changes cannot occur. Therefore, pouring a concrete slab over the footing area or placing waterproof barriers around the footing are alternate remedial options. Further detailed options are discussed later in the chapter. When materials such as organic soils, fills, or any other type of loose soil are encountered at the bearing level, they should be undercut and replaced with suitable soils. The replacement soils may be what is commonly known as select structural fill, sand, or crushed stone. Select structural fill consists of uniformly graded sands to silty or slightly clayey sands, free of organics and other deleterious material, with less than 30 percent passing through a no. 200 sieve. Select fill is also recommended for backfill around the footings and pile caps when unsuitable soils are present. Structural fills are commonly placed in thin (6- to 8-in. [15- to 20-cm]) lifts and compacted to 95 to 98 percent of the soils’ modified proctor maximum dry density (ASTM D1557) or other ASTM criteria. They may require some manipulation of the moisture content (wetting or drying) to achieve the required compaction. Flowable fill is another material that can be used for this purpose. Of course, replacing the undercut soils with low-strength concrete is always an option.

Structural Concrete Water tank foundations are primarily constructed of reinforced structural concrete. Concrete is a mixture of hydraulic cement with fine and coarse aggregates and water in appropriate proportions. Sand, gravel, crushed stone, and, in some cases, iron blast-furnace slag constitute the aggregates. The governing properties of hardened concrete are defined by the quality of the cement paste, ratio of water to cement, and the properties of the aggregates. Structural concrete is concrete mixed to a uniform distribution of materials on the basis of a precise mix/design and satisfactory quality control for required

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Foundations

Foundations durability and compressive strength. The design and construction of water tank concrete foundations follow the Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05) (see “Bibliography”).

r Building Code Requirements for Structural Concrete, ACI 318-05.

r Building Code Requirements for Structural Concrete, Commentary, ACI 318R-05. Concrete is strong in compressive strength, but its tensile strength is limited to a small fraction of its compressive strength. Therefore, in flexural design applications such as for foundations, steel reinforcement is provided on the tension side of a member to resist the tensile stresses. Similarly, as shrinkage and temperature reinforcement normal to flexural reinforcement, or wherever tensile stresses can develop, steel reinforcement is added to provide tensile resistance. Reinforcement bar sizes and details as well as criteria for determining the amount of reinforcement needed are all outlined in ACI 318-05.

Materials Structural concrete materials include cement, aggregates, water, and admixtures. The reinforcing steel used in water tank foundations comprises deformed bars ranging in diameter from 3/8 in. (9.5 mm) to about 13/8 in. (3.5 cm). Two larger bar diameters of 13/4 in. (4.5 cm) and 21/4 in. (5.5 cm) are also available but are seldom used in water tank foundations. Welded wire fabric is another form of reinforcement often used in floor slabs. Cement is a powdered substance produced from a burned mixture of clay or shale and limestone. Portland cement is the most common type, grayish in color, consisting chiefly of calcium and aluminum silicates. Portland cement is manufactured to various designations on the basis of the physical and chemical requirements as defined by ASTM C150. Type I designation represents the general-purpose cement for foundations subject to normal exposure. Where sulfate attack from soil or water is a concern, if high strengths at an early period are required, or if hydration heat needs to be minimized, other ASTM cement types would be better suited and should be specified. Aggregates are generally classified into fine and coarse categories on the basis of their particle size. Fine aggregates consist of sands that pass through a no. 4 sieve, meaning that their maximum particle size is less than 1/4 in. (6.4 mm). Some references include a particle size up to 3/8 in. (9.5 mm) in fine aggregates. Coarse aggregates constitute any material larger than 3/8 in. (9.5 mm). The most common aggregate size is about 3/4 in. (19 mm). However, the maximum coarse

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Chapter Five aggregate size is governed by the space limitations between the reinforcing bars. As the aggregates constitute up to 75 percent of the total volume of concrete, it is important not only that the proper size be selected carefully, but that the aggregates be properly graded and have the requisite strength, durability, and weather resistance for the exposure environment. Uniformity and workability of concrete are affected by the aggregate gradation or particle size distribution within the aggregate. A properly graded aggregate has a balanced distribution of particle size that remains consistent from batch to batch. Aggregates with smaller gradation minimize the number of air voids and result in more dense, stronger, and better concrete. Water is a necessary ingredient that initiates the hydration process of cement. The mixing water should be clean potable water that is free of oils, acids, alkalis, salts, and other organic materials that are detrimental to concrete or the reinforcement steel. Likewise, the water should be free of high concentrations of dissolved solids. Admixtures should be used only when required by design. They improve the workability of plastic concrete and enhance the properties of hardened concrete. The admixtures include air-entraining to increase resistance to freezing, water-reducing admixtures to reduce the quantity of water needed to maintain a certain slump, retarding agents to slow the setting of concrete, accelerators to hasten strength development at an early age, and fly ash and ground, granulated blastfurnace slag to improve the plastic or hardened properties of cement concrete.

Durability Because water tanks are erected in varied climates and locations, the environmental effects on their concrete foundations can be harsh and must be taken into consideration. Conditions that can profoundly affect the service life of the foundations include extreme temperature fluctuations, freeze/thaw cycles when exposed to water, and exposure to chemicals, salts, deicers, etc. Durability, in essence, refers to the capability of concrete to withstand these exposure conditions without damage, distress, or deterioration. ACI 318-05 provides detailed durability requirements for improving the performance of concrete. These requirements include air-entraining recommendations for concrete exposed to freezing/thawing or deicing chemicals, maximum water-to-cement ratios and minimum strength values for concrete exposed to special conditions, maximum percent of total cementitious material by weight for exposure to deicing chemicals, and criteria for resistance to sulfatecontaining solutions and soils. This reference also provides requirements for corrosion protection of the reinforcing steel.

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Foundations

Foundations

Quality Control, Mixing, Placing, Finishing, and Curing For reinforced-concrete foundations to perform and function as intended, it is essential to adhere strictly to specific requirements in each phase of construction. These requirements govern the strength, durability, mixing, conveying, depositing, workability, and curing of concrete. The quality of concrete is mostly defined by the quality of the cement paste, its proper mixing with the aggregates, and the aesthetic quality of the finish. Structural concrete is proportioned to achieve an average concrete strength based on a mix design and an anticipated exposure. The proportions are established to provide workability and consistency, resistance to special exposures, and conformance with strength test requirements as outlined in ACI 318-05. The mix design must be followed precisely to produce concrete that satisfies structural performance requirements. Placing and compacting are also important to the quality of concrete. Concrete must be placed and vibrated properly to avoid segregation, honeycombing, settling, and separation of the heavier aggregates from the rest of the mix. Concrete should be placed continuously in lifts or layers using multiple discharge locations. This eliminates aggregate separation caused by the horizontal flow of concrete within the formwork and the need for concrete to be moved into its final position. During placement, samples of the plastic concrete should be taken for field testing of unit weight, slump, and air content to ensure compliance with mix specification. The tests can be performed according to the appropriate ASTM specifications. Unless cured by accelerated curing techniques, poured-concrete foundations should be maintained at a temperature above 50◦ F (10◦ C) and in moist condition for at least 7 days after placement. Scheduledriven activities often dictate backfilling around the footings sooner than 7 days. In no case should backfilling be started before the concrete has gained sufficient strength to withstand the loading induced on the footing by the pressures resulting from the backfill and the construction equipment.

Required Strength, fc Structural concrete can be proportioned to a wide range of design strengths and characteristics. The design of water tank foundations is based on a specified design strength for concrete. The design strength refers to the compressive strength gained by concrete after 28 days of curing and is referred to as f c . For water tank foundations, a minimum design strength of 3,000 psi (20.7 MPa) is recommended. The preferred specified strength, however, is 4,000 psi (27.6 MPa). A common practice in the industry is that when concrete strength in excess of 3,000 psi (20.7 MPa) is required by the specifications, a design strength that is

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Chapter Five 500 psi (3.4 MPa) less than the specified strength is used in computations. This allows some flexibility in situations where strict quality control measures cannot be maintained due to the long hauls to remote job sites. Evaluation and acceptance of concrete for a given design mix can be made on the basis of actual testing. Adjustments to the mix design can be made to improve the resulting strength as necessary. To ensure that the concrete furnished meets the specified design strength requirements, fresh concrete specimens can be prepared at the job site for testing in the laboratory. ACI 318-05 provides criteria for concrete sampling and testing and for acceptance of concrete compressive strength: Concrete strength is considered acceptable when the average of any three consecutive strength tests equals or exceeds f c and no individual test (average of two cylinders) falls below f c by more than 500 psi (3.45 MPa). For the test results to be meaningful, it is critical that the samples be taken, handled, and cured in strict compliance with the applicable ASTM standard. Testing of cylinders that are mishandled or ignored at the job site may not be truly representative of the concrete furnished. Also, it is important that qualified personnel test all specimens, as the outcome of the tests determines the acceptability of the foundations. If the strength test results fail the acceptability criteria, hardened concrete can be tested by taking core bores in accordance with ASTM C42. Experience indicates that unless the requirements of ASTM C42 are strictly adhered to, the core bore test results will underestimate the true strength of the hardened concrete. ACI 318-05 also provides specific criteria for core drill testing.

Reinforcing Steel The reinforcement steel used in water tank foundations is generally deformed bars conforming to the ASTM A615 specification and having a minimum yield strength of 60 ksi (414 MPa). Where weldability is a requirement, low-alloy steel deformed bars conforming to ASTM A706 can be used. As noted previously, welded wire fabric reinforcement can be used in floor slabs. These fabrics conform to ASTM A185 for plain wire and ASTM A497 for deformed wire. Epoxy-coated bars or wires are not necessary for water tank foundations. ACI 318-05 provides detailed criteria for determining the amount of reinforcement necessary in design as well as requirements for the spacing, cover, development length, and splice length. Requirements for the development length of bars should be carefully reviewed, especially for the horizontal bars. If horizontal bars are so placed that more than 12 in. (30.5 cm) of concrete is cast in the member below the bar, the required development length is 1.3 times the normal development length.

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Foundations

Foundations

Nonconforming Concrete On occasion, it is possible that concrete mixed and furnished under a specification fails to meet the minimum design strength requirement. As a result, contractual obligations may require the foundation contractor to remove the nonconforming foundations and reinstall the foundations according to the stated requirements. This can significantly delay project completion and can be very costly. Therefore, it is important that serious attention be paid to quality control during the mixing, conveying, placing, and curing phases of the concrete foundation construction. Contractual obligations notwithstanding, not every nonconforming condition warrants the removal of the footings. If testing confirms low strength, calculations can be performed to check the adequacy of the furnished concrete with reduced strength and the actual reinforcement provided. The nonconforming condition may be accepted if the calculations confirm that the load-carrying capacity of the foundation is not significantly reduced and that the design intent has been met.

Formwork and Removal Construction of water tank foundations requires the use of formwork. Proper formwork ensures that the foundations conform to the shape and dimensions shown on the drawings. The formwork also prevents moisture loss from concrete, and it facilitates curing, especially when the top surface of concrete is kept moist. To be effective, the formwork must be strong enough to withstand the loads and pressures from pouring concrete and any other loads that are present. Leakage of plastic concrete must not occur. Formwork should remain in place for as long as possible, especially in cold weather. Formwork must not be removed until the concrete has gained sufficient strength to withstand its dead load and any other construction loads. When properly cured, general-purpose concrete gains about 500 psi (3.45 MPa) strength in 24 hours; within 1 week of placement, it reaches nearly 70 to 75 percent of its maximum compressive strength. Although many contractors are in a hurry to remove the formwork so that they can complete backfilling around the footings, it is best to keep the formwork in place based on achieving a defined minimum strength. To simplify formwork, some contractors take the liberty of adjusting footing thickness or other dimensions. This should not happen without the explicit consent of the engineer of record. The formwork required for water tank foundations is relatively simple and should easily facilitate the required shape and dimensions shown on the engineering drawings.

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Chapter Five Tank diameter

High water line

Head range

Overflow elevation

Low water line

Column (typ.)

Top of footing elevation

Center riser

Elevation

Centerline to centerline of foundation

at eter Diam rline of e cent ation d n u o f

Plan

FIGURE 5-1 Typical leg tank elevation and shallow foundation plan.

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Foundations

Foundations

Shallow Foundations The shallow foundation is the most cost-effective foundation for water storage tanks. Shallow foundations typically include isolated footings or mat or raft foundations placed just below the columns on the lowest part of the structure, as applicable. Footings can be placed as shallow as possible as long as the bottom of the footing is below the frost line, the resulting bearing pressures are within the allowable limits, the overall settlements are tolerable, and the stability requirements are met. However, for water-storage tanks, shallow foundations can be typically placed as shallow as 4.5 ft (1.4 m) and as deep as 10 ft (3.0 m) below grade. Shallow foundations transfer structural loads to the bearing soil or rock strata occurring below the base of the footing. Shallow foundations for multicolumn elevated storage tanks typically consist of isolated piers with footings (Fig. 5-1). For ground storage tanks and elevated single-pedestal tanks, the foundation may take the form of a ringwall, a ring-tee, or a ring-slab. These ring-type foundations are further discussed later in the chapter. Ground storage tanks may also be founded on a slab or a granular berm. Figures 5-2a, 5-2b, 5-2c, and 5-3 show several common types of shallow foundations. Based on tank geometry, site conditions, and specific environmental loading effects, various foundation alternatives should be evaluated. Typically a shallow foundation is the preferable option. If poor soil conditions, high settlement expectations, or low bearing capacities dictate, deep-foundation alternatives must be considered. Low bearing capacities generally result in large footings, causing the adjacent footings to encroach upon each other. As a result, the overlapping of the pressure bulbs from the individual footings can exacerbate the bearing stresses and magnify settlements. Therefore, when the net allowable bearing pressure falls below 2,000 psf (96 kPA), the deepfoundation alternative should be pursued.

Loads and Load Combinations Water-storage tanks are subjected to a variety of loads. The gravity forces consist of the weight of the tank metal, appurtenances, and the liquid. Common appurtenances include roof-mounted cellular antenna systems, platforms, floors, walkways, ladders, and piping. Snow loading consists of the weight of snow on the tank balcony and the tank roof where the roof slope with the horizontal axis is flat to moderate. The tank roof may also be subjected to live loading that is in excess of the snow loads. Lateral forces on the tank and tower consist of loads resulting from wind pressures or earthquake ground motion. The AWWA D100-05 Standard for Welded Carbon-Steel Tanks for Water Storage states that a unit weight of 62.4 lb/ft3 (1,000 kg/m3 ) for water, 490 lb/ft3 (7,850 kg/m3 ) for steel, and 144 lb/ft3 (2,310 kg/m3 )

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Chapter Five

Centerline of foundation

Centerline of anchor bolts

Radial centerline

Centerline of foundation

Diameter at centerline of anchor bolts

Top of footing elevation

Centerline of anchor bolts

Anchor bolts

Centerline of foundation

Plan

Dowels Hoops

Top of grade Exposure Bars

Bars

(a)

Elevation

FIGURE 5-2a Typical shallow foundations: sloped slab

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Foundations

Foundations

Centerline of foundation

Centerline of anchor bolts

Centerline of foundation

Radial centerline

Diameter at centerline of anchor bolts

Top of footing elevation

Top of grade

Centerline of anchor bolts

Anchor bolts

Centerline of foundation

Plan

Dowels Hoops

Exposure

Bars

(b)

Elevation

FIGURE 5-2b Flat slab

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Foundations

Chapter Five Centerline of foundation

176

Hole for inlet/outlet pipe

Centerline of foundation A

Anchor bolts on bolt circle

A

Top of footing elevation

Centerline of foundation

Plan Anchor bolts Slab reinforcing U-bars Dowels

Exposure

Dowels U-bars

Vault for piping

Bars

Bars

(c)

Elevation

FIGURE 5-2c Riser.

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Foundations

Foundations Centerline of tank and foundation Top of footing elevation Exposure

Mat top reinforcement bars Mat bottom reinforcement bars

(a) Extend 3–5 ft Tank plate (0.9–1.5 m) beyond tank Top of berm elevation Slope To drain down away

2 ft (0.6 m) minimum

Centerline of tank and foundation Compacted crushed stone, screenings, fine gravel, clean sand, or similar material

1 1.5 1

Coarse stone or coarse gravel

(b)

1

Thoroughly compacted fill of gravel, coarse sand, or other stable material

Coarse stone or coarse gravel

FIGURE 5-3 Examples of shallow mat and berm foundations: (a) typical square mat foundation and (b) typical granular berm foundation for flat-bottom tanks.

for concrete should be considered in the design of tank structures and foundations. The standard also recommends consideration of a minimum allowance of 25 lb/ft2 (1,205 N/m2 ) for the pressure resulting from the design snow load on the horizontal projection of the tank roof surfaces with slopes not exceeding 30 degrees. A reduction of this allowance is permitted in warmer regions where snow loading is smaller. However, D100-05 limits the minimum roof design load to 15 lb/ft2 (720 N/m2 ). AWWA D100-05 has adopted the American Society of Civil Engineers (ASCE) standard 7-05 for wind loading criteria. However, it retains the minimum design pressures to be 30 Cf lb/ft2 (1,436 Cf N/m2 ), with the force coefficient Cf being 1.0 for flat surfaces, 0.60 for cylindrical or conical surfaces with apex angle 30 in. (30 to >76 cm), with lengths from 50 to 100 ft (15 to 30 m) and compressive capacities that can exceed 125 tons (1 MN). Augercast piles are reinforced by inserting a single reinforcing bar and/or a reinforcing cage through the unset grout. The cage extends to a defined length based on the structural requirements of the pile in resisting tensile, compressive, and lateral loading. The single reinforcing bar at the center of the pile typically continues to the bottom end of the pile. The grout mostly consists of portland cement, sand, and water.

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Chapter Five Where vibrations due to pile driving can cause damage to other structures or entities in the near vicinity of the job site, auger-cast piles offer a better alternative. As with all other piles, pile axial load tests are performed in accordance with ASTM 1143. Pile installation records are maintained as required. More information on these piles can be found in the Augered Cast-in-Place Piles Manual prepared by the Deep Foundations Institute (DFI 1990).

Lateral Loads and Bending Moments In addition to the gravity loads, foundations for elevated water tanks must resist vertical and lateral loads caused by wind or seismic loading. These loads are transferred through the pile cap to the resisting piles. Depending on pile head fixity and the characteristics of the surrounding soils, the lateral shears can cause significant bending moments, in addition to the axial loads acting on the piles. Therefore, both the pile and the surrounding soils should be analyzed and investigated for strength and stability. Along with the allowable load capacity for the pile, geotechnical engineers must provide allowable lateral load capacities for various pile head fixity conditions. If the pile head extends into the pile cap and is anchored by uplift connections, there is very little, if any, pile head rotation. But due to the movement of the pile cap, lateral translation of the pile head is possible. For design purposes, however, it is helpful to define the pile head boundary condition to locate where the maximum bending moment occurs. With the pile head restrained against rotation, the maximum bending moment in the pile generally occurs at the restrained end, at the pile cap. Otherwise, it occurs at some distance below the pile cap. Geotechnical engineers often provide bending-moment curves as a function of the lateral loads and an assumed pile head fixity condition. These curves are very helpful in design and should be included in all geotechnical reports that recommend pile foundations. They should be carefully reviewed for assessing pile structural capacity as well as the effect of pile lateral displacement on the elevated water tank system.

Pile Caps and Uplift Connections Pile caps are reinforced-concrete structural elements that resist direct vertical and lateral forces and transfer them to the supporting piles. Pile caps are sized based on the number of piles required for a tower column loading. Depending on their thickness, pile caps can be rigid or flexible. Typically, pile caps join the pedestal from the top and the piles from the bottom. Piles usually extend 4 to 6 in. (10 to 15 cm) into the pile cap. The size of the pile cap is a function of the number and center-tocenter spacing of piles required for a footing. The larger and thicker the pile cap, the heavier it is. Thus, the dead weight of the pile cap Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

Foundations

Foundations

S

S

S

S

S

S

Four piles

Five piles

S

S

Six piles

S

S

S

S

S

S

S

S Seven piles

Eight piles

S

Nine piles

FIGURE 5-7 Typical pile group patterns for single foundations. (S = spacing between piles.)

itself can cause an increase in the number of required piles. Figure 5-7 provides typical pile layouts for four-, five-, six-, and sevenpile footings. Pile caps must accommodate pile spacing as well as edge distance requirements. The pile cap edge distance, measured from the centerline of the outer piles to the edge of the pile cap, is generally a function of the pile diameter. Typically, for piles with a diameter of about 1 ft (30 cm), a distance of 1 ft. 3 in. (38 cm) is used. The thickness of the pile cap should be checked for punching shear caused by piles exterior to the critical section. The critical section is taken to be at a distance of d/2 from the pedestal, where d is the current depth to centroid of tensile steel in the pile cap. The punching shear should also be checked around the individual piles at a critical section taken a distance d/2 from the face of the pile, and the pile cap thickness should be adjusted, if necessary. The pile cap thickness is also dependent on flexural shear both tangentially and radially at a distance d from the face of the pedestal. Flexural reinforcement, in both the tangential and radial directions, should also be checked at the faces of the pedestal. The flexural reinforcement determined should then be compared against the minimum flexural reinforcement requirements of ACI 318-05 Section 10.5.1 and adjusted, if necessary. IBC-2006 provides specific criteria for pile connection to the pile cap. For prestressed piles, uplift anchorage to pile cap can be achieved Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Chapter Five by exposing and developing the reinforcing strands at the top of the pile. This approach, however, is not permitted by other model codes in areas of high seismic risk. IBC-2006 permits the option, provided that the reinforcing strand results in a ductile connection. As an alternative, reinforcing dowels can be grouted into the top of the pile. In timber, steel, or pipe piles, a reinforcing bar can be inserted through the member and bent upward into the pile cap. The pile cap must be thick enough to accommodate the anchoring mechanism. Refer to Figures 5-8a and 5-8b for typical pile foundations. Centerline of foundation

Radial centerline

Prestressed concrete piles Pile layout plan view

Top of footing elevation

Anchor bolt

Centerline of foundation

Dowels

Projection Hoops

Exposure Aggregate

Radial bars

Typical uplift anchor

(a)

Tangential bars Prestressed concrete piles

Column foundation with piles elevation view

FIGURE 5-8a Typical pile foundations: typical pile foundation for a single column

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Foundations

Foundations

Anchor bolts on bolt circle

Prestressed, precast concrete piles

A

Centerline of tank and foundation

Outs ide d iame ter rin gwall

d

Ch

Ch

or

or d

Out side diam eter slab

ringwall b iameter la Inside d rs ete m dia ide s In

Radial bars Hoops A

Exposure

Plan

Anchor Top of bolts footing Expansion elevation joint

Hoops Hoops (special Verts. top bars) Typical uplift anchor Radial bars Hoops

(b)

Centerline of tank and foundation Concrete slab

Subgrade Verts. Radial bars Reinforced concrete Hoops thrust Prestressed block concrete piles

Section A-A

FIGURE 5-8b Typical ring-tee pile foundation). (Verts. = vertical reinforcement dowels.)

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Chapter Five The pedestals for pile foundations are usually compressive elements with a ratio of unsupported height to least lateral dimension not exceeding 3.0. As noted in the section on shallow foundations, although ACI 318-05 Section 15.8.2.1 recommends a minimum reinforcing ratio across the interface (between the pedestal and footing) of 0.5 percent of the gross area of supported member (i.e., the pedestal), a smaller reinforcement ratio can be justified for the tower column pedestals due to their large size. For taller pedestals, the 0.5 percent reinforcement ratio should be maintained. However, the reinforcement furnished must be sufficient to meet the requirements for uplift as well as flexural requirements necessitated by the lateral shears on the pedestal.

Pile Stability and Settlement Piles are required to be laterally braced in all directions. Piles interconnected by a rigid pile cap may be considered braced for lateral stability provided they are situated as defined in IBC-2006 Section 1808.2.5. Elsewhere, the surrounding soils furnish lateral stability along pile length. In regions where the piles extend vertically through voids or holes, the piles should be analyzed as columns. Settlement is an important aspect of design for all water tank foundations, including pile foundations. The geotechnical engineer should evaluate pile settlement as well as potential differential settlements for full consideration in design. As discussed previously in the section on soils and geotechnical investigations, settlement not only affects the structural behavior but also the piping systems and the interconnections among the various components and appurtenances.

Drilled-Pier (Caisson) Foundations Drilled piers, caissons, or shafts offer an alternative design option in deep foundations. Drilled piers are cast-in-place reinforced-concrete shafts with or without a bell at the bottom. They are installed by drilling a hole of predefined diameter and depth at the design location and then filling the excavation with concrete and reinforcement. Drilled-pier construction is relatively easy and can be accomplished with rotary drilling equipment. Depending on the soil conditions, casings or laggings may be needed to prevent the soils from falling or caving into the hole. Typical diameters of drilled piers for water tank foundations range from about 3 to 6 ft (0.9 to 1.8 m). Larger diameters may be needed for higher-capacity tanks, depending on availability of large-diameter drill bits. Otherwise, several smallerdiameter piers will be used, which would require a larger pier cap.

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Foundations

Foundations There are many advantages to using drilled piers for water tank foundations. Because of its size and capacity, a drilled pier can replace a group of piles and eliminate the need for a pile cap. Installation of drilled piers does not generate much noise or vibration, as pile driving does, and drilled piers can be set up in hard-to-access places. A primary advantage of drilled piers is that they can sustain large axial loads with minimal settlement when bearing on bedrock.

Pier Bearing Capacity Drilled piers draw their structural capacity from the reinforcedconcrete shaft. They develop their bearing capacity from side resistance, generated by skin friction, and base resistance, generated by end bearing. In equation form, the ultimate static load capacity of a pier can be expressed as

Qu = Qbu + Qsu − Wp

(5-18)

where Qu Qbu Qsu Wp

= pier ultimate resistance (kip [kN]) = pier ultimate end-bearing resistance (kip [kN]) = pier ultimate side friction resistance (kip [kN]) = pier dead weight (kip [kN])

An FS of 3.0 is applied for allowable-stress design application. Some references may apply an FS of only 2.0 on the ultimate resistance due to side friction Qsu in service-load design application. Others may also apply a load factor on the shaft dead weight Wp . Refer to Fig. 5-9 for a typical belled, drilled pier configuration. A very useful and relevant reference on drilled piers is a report entitled Drilled Shafts: Construction Procedures and Design Methods, issued by the US Department of Transportation, Federal Highway Administration (Reese and O’Neill 1988). This reference provides detailed analysis and design, fabrication, and quality control criteria for drilled-shafts foundations. The Bearing Capacity of Soils, prepared by the American Society of Civil Engineers (ASCE 1993a), is another source for criteria regarding analysis and design of drilled shafts.

Pier Side or Skin Friction Pier side resistance offered by the skin friction along the pier shaft is a function of the undrained shear strength of clay soils as determined by testing. Shear strength varies with depth and soil strata and is empirically related to the shaft load transfer in side resistance. The

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Chapter Five Axial load Lateral load

Additional reinforcement, if required Exposure

Hoops

Depth can vary per design

Diameter depends on loading and depth of excavation Qsu side resistance

Reinforcing steel WP

θ

Bell—may be used when required. Size varies—no larger than three times shaft diameter at base. Underream angle θ is 45° or 60° typically.

Q bu base resistance Base resistance

FIGURE 5-9 Typical drilled shaft. (Wp = pier dead weight; Qsu = pier ultimate side friction resistance; Qbu = pier ultimate end-bearing resistance) (Source: Reese and O’Neill 1988.)

shear resistance offered by sands or cohesionless silts, however, is a function of the soil angle of internal friction. The resistance capacity offered by side friction can be very significant. For piers socketed into bedrock, it is possible that the entire resistance capacity is emanating from the side resistance furnished by the socket. It is a recommended practice within the drilled-pier design community to ignore the contributions of side friction and passive resistance in the top 5 ft (1.5 m) or 1.5 diameters of the shaft when

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Foundations

Foundations evaluating lateral stability. The reason is that lateral movement by the pier causes a wedge of soil to move up and out, resulting in a loss of side friction and passive resistance. Similarly, in clay soils, the side friction in the periphery of the bell or the bottom of the straight pier within 1 diameter of shaft length is ignored in determining the resistance capacity of the pier. The reason is that movement of the base of the pier can result in the development of a tensile crack in the soil, which in turn can cause a lateral stress at the base of the pier and, consequently, a reduced load transfer in side friction (Reese and O’Neill 1988).

Design of Piers The diameter of the pier is a function of the soil characteristics within the profile, the location of the water table, and the presence of lateral loads and/or moments. The design of the concrete mix and its strength are also of critical importance. The geotechnical profile of the soil dictates not only how far down to extend the pier, but also the method of construction to be employed, the need for casing and/or dewatering, and the need for under-reaming. Special characteristics of soils—shrinking/swelling of plastic soils, occurrence of boulders, remains of abandoned footings, presence of debris or other unsafe materials, and so on—all require that certain measures be taken into full consideration. Aside from the basic structural design, the most important consideration is the amount of the expected settlement of the pier foundation and its effects on the elevated water tank system. Under-reaming, where possible, helps increase the pier bearing surface and consequently the bearing resistance. Under-reaming can also be used interchangeably with socketing where required. The longitudinal reinforcement for drilled shafts depends on the many factors noted previously, but as a minimum, industry practice has been to provide at least 0.5 percent of the cross-sectional area and at least six bars, forming a cage of equally spaced bars. This minimum is actually based on ACI 318 Sections 10.8.4 and 10.9.1. Section 10.9.1 requires a minimum reinforcement of 1 percent. For regions of low-to-moderate seismic risk, Section 10.8.4 states that, for compression members with cross-sectional areas larger than required by consideration of loading, it should be permissible to base the minimum on a reduced effective cross-sectional area not less than half the total area. Additional reinforcement may be required where heavy tensile loading or bending moments are present. The longitudinal reinforcement cage may extend a partial depth or the full length of the pier when required. Hoop or spiral reinforcement is also used for drilled shafts. Hoops are more economical, but from a performance standpoint spirals are preferred. Figure 5-10 illustrates pier reinforcement.

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Chapter Five

Ties—spacing as per ACI 318 Sections 7.10 and 7.11. Refer to ACI 318 Chapter 21 for special provisions for seismic design.

Length of cage depends on lateral design requirements and may not require extending to full length. Extend cage to full length in highly plastic (expansive) soils.

FIGURE 5-10 Drilled shaft reinforcing cage.

In regions of high seismic risk, special reinforcement requirements may also apply, including confinement steel near the interface regions with the pedestal or grade beams. Refer to ACI 318-05 Chapter 21 for special provisions in seismic design. Where belled bottoms are needed, the base diameter is generally limited to less than three times the shaft diameter. The under-ream angle is typically in the range of 45 degrees to 60 degrees, with 60 degrees often used for water tank foundations. Also, a toe height of about 1 ft (30 cm) is maintained at the base.

Lateral and Uplift Stability Lateral stability of the drilled pier depends on the shaft length and flexibility. It also depends on whether the pier is drilled in cohesive or cohesionless soils. Lateral stability of piers should be carefully examined, especially where the piers are not socketed into bedrock. Similarly, uplift stability is a function of pier length, geometry, and side friction. Where the uplift forces are relatively small, the resistance provided by the side friction along a straight shaft pier may

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Foundations

Foundations be adequate for uplift stability. But when the loads are severe, belled, or under-reamed, piers are necessary. Detailed criteria on the stability of drilled piers and a discussion on the potential collapse of the bell in loose soils during construction can be found in the study by Reese and O’Neill (1988).

Socketing into Bedrock Piers drilled into rock derive their load-bearing capacity from end bearing and side friction offered by the length of socket in the rock. A roughened socket length equal to 1 diameter into rock with higher modulus than the pier shaft enables the pier to carry 50 percent of the load by side friction, whereas a 4-diameter roughened socket length into the bedrock allows the shaft to transfer nearly all the load by side friction (Wyllie 1992). Hence, the depth of socketing should be a function of design requirements and not established arbitrarily. The socket drilled into bedrock also provides end fixity, allowing the pier to develop moment resistance at its base. Geotechnical engineers generally provide a simplified, uniform, unit side friction value along with the end bearing for design. This information may be presented as ultimate capacity or service load capacity. The references previously noted provide further information on the subject.

Design Considerations in Plastic Soils Plastic soils can be found in many parts of the world. In the United States, Texas, Oklahoma, and the upper Missouri Valley area have highly expansive soils. Increases in moisture cause swelling in these clayey soils, and, as a result, foundations are subjected to rather large uplift forces. These forces can be large enough to pull the drilled pier out of the ground unless it has been properly designed. Similarly, if the pier shaft is not adequately reinforced, it could break apart from the base because of the tensile forces caused by swelling soils along the shaft. Piers in shrink/swell soils should terminate in bells that bear deep in soil layers not in the zone of seasonal activity and movement. The reinforcement cage in these belled piers should extend the full height to allow the belled segment to anchor the uplift forces in the upper areas of the shaft. The tensile reinforcement needed is in addition to the reinforcement needed for normal tensile loading.

Load Testing A clear way of establishing the structural integrity of a drilled pier is by load testing. However, due to the high costs and logistical difficulties associated with the arrangement of reaction shafts, such testing is rarely performed. If it is absolutely necessary that a load test be

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Chapter Five performed, the pier (as with piles) must be able to sustain without excessive settlement a load that is at least twice the working load. The best way to ensure the structural integrity and intended performance of drilled piers is to follow a credible quality control program of inspection and installation procedures. Recent studies have shown that minor construction flaws that may not be detectable by common nondestructive evaluation methods can lead to significant capacity reduction in drilled piers. Such flaws include the presence of small voids, soil inclusions, misaligned cage or other reinforcement steel, weak concrete, or corroded reinforcing bars. Refer to Sarhan et al. (2004) for further details.

Settlement Settlement concerns associated with drilled piers are similar to those defined for piles. Typically, if the drilled pier is bearing on or socketed into the bedrock, settlement caused by direct loading is negligible. Under other installation conditions, it is important that a proper settlement analysis be performed by the geotechnical engineer to ensure that the expected settlements are tolerable from operations and performance perspectives and from the standpoint of structural design.

Reservoir and Standpipe Foundations Reservoirs and standpipes are considered flat-bottom tanks. The design of foundations for flat-bottom tanks follows the criteria defined previously for shallow and deep foundations. AWWA D100-05 provides detailed guidance on various foundation types for reservoirs and standpipes. The bearing pressure induced by the water at the base of a flatbottom tank is equivalent to the height of the high water line H times the density of water. For a flat-bottom tank to be supported on the ground, with or without a ringwall, the bearing soils must have an allowable bearing capacity of at least 62.4 H lb/ft2 (9.81 H kN/m2 ). Pile foundations may be necessary if the induced bearing is in excess of the allowable bearing capacity of the resisting soils. Therefore, it is important to reiterate that a formal geotechnical investigation must be performed to verify that the bearing soils can carry the resulting loads. AWWA D100-05 also provides criteria for grading the interface between the bottom of the tank and the supporting base and the use of oiled or clean sand, crushed rock, or asphalt road mix. The standard also provides information on granular berms, grout, foundation tolerances, anchor bolts, etc. Refer to Fig. 5-11 for a typical granular-berm foundation for a flat-bottom tank (Fig. 5-3b).

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Foundations

Foundations

Flat-bottom water-storage tank

Extend 3–5 ft (0.9–1.5 m) 2 ft (0.6 m) minimum beyond tank

To drain away

Compacted crushed stone, screenings, fine gravel, clean sand, or similar material

Slope down

Coarse stone or coarse gravel Subgrade

1 1

Thoroughly compacted fill of gravel, coarse sand, or other stable material

Berm

Coarse stone or coarse gravel

FIGURE 5-11 Typical granular berm foundation.

Slab Foundation Where the bearing soils are strong or when the water tank capacity is small, flat-bottom tanks can be supported by a mat or slab foundation. The slab is uniformly loaded by the pressure head in the tank. The resulting bearing stress under the slab is the pressure due to the weight of the tank and its contents added to the uniform pressure caused by the thickness of the concrete slab. Overturning moment resulting from wind or seismic loading also contributes to the bearing stress. The reinforcement requirements of the mat or slab foundation are based on the loading and deformation characteristics of the footing. Often the minimum reinforcement requirement defined by ACI 318-05 will control. Anchorage and stability requirements should be investigated when the tank is full and when it is empty. Slab exterior edges supporting the tank wall may be thickened, if necessary, to accommodate the additional bearing stress caused by wind or seismic overturning moments. Consideration should also be given to the frost depth in determining slab thickness and bearing elevation.

Ringwall Foundation As discussed in the section on shallow foundations, ringwall foundations are used when the bearing pressure under the tank shell exceeds the allowable bearing pressure of the soil near grade. The ringwall carries the loads deeper and distributes the pressure over a wider area. When the overturning moments are severe and anchor bolts are required for stability, a ringwall foundation best accommodates these anchorages. The design must consider hoop stresses caused by the internal soil pressure resulting from the weight of the tank and its contents.

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Chapter Five

Ring-Tee and Ring-Slab Foundations When the bearing soils are weak or when the tank is large, a ring-tee or a ring-slab foundation may be required. The foundation system is then designed for the direct bearing loads and the overturning moments caused by wind and seismic loading for the load combinations defined earlier. The ringwall requires vertical and horizontal reinforcement and full consideration of the hoop stresses. The tee or slab portion requires radial and tangential reinforcement. Uplift is a loading condition that must be considered for wind with the tank empty and for seismic with the tank full. Hence, for uplift stability, the dowels must be developed within the slab by hooks at the ends. It is possible that piping has been routed through the ringwall. At such locations, additional reinforcing must be provided to strengthen the opening periphery.

Deep Foundations Where the bearing soils are weak or the settlements are excessive, flat-bottom tanks require deep foundations. Driven piles and augercast piles are typically used under flat-bottom tanks. In regions of high seismic risk, special reinforcement requirements also apply, as is discussed subsequently in this chapter.

Anchor Bolts (Rods) Load combinations governing the design of foundations for elevated water tanks were defined in Equations (5-1) through (5-13). The load combination causing the maximum uplift and shear in the bolt generally governs the design of the anchor bolts (more recently also referred to as anchor rods). Because of the significant forces imposed on elevated water tanks, it is critical to properly design all anchor bolts to safely transmit these forces to the foundation. Flat-bottom tanks may or may not require anchor bolts. All elevated water tanks require anchor bolts. Cast-in-place anchors are the most common type of anchor bolt for water tank foundations, although post-installed anchors have their uses. For all bolt or anchor types, the embedment length, center-to-center spacing, edge distance, and group action should be evaluated for the design loads, with appropriate factors of safety. Of the many references addressing structural design of anchor bolts, Appendix D of ACI 318-05 is entirely devoted to anchors in concrete. This reference requires anchors and anchor groups to be designed for critical effects of factored loads as determined by elastic analysis.

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Foundations

Minimum Embedment and Projection The minimum embedment length should be such that the anchor bolt is capable of developing the required uplift strength and shear resistance for all loads in the design load combinations. Most manufacturers of post-installed anchors recommend an embedment length for their proprietary anchors. However, cast-in-place anchors require unique design based on the governing loads or as determined by testing or past evaluations. Bolt projection is especially important when settlements are expected to be large. In such cases, the projection should be long enough to accommodate shimming, as required. To provide for variations in foundation elevations, AWWA D100 further requires a projection of the anchor bolts’ threaded ends an additional 2 in. (5 cm) beyond the anchor nuts. Typically, a 7-in. (18-cm) projection above the top of concrete is sufficient.

Allowable Tension and Shear AWWA D100 recommends sizing the bolts for tension using the root area and a basic allowable tensile stress of 15 ksi (103 MPa), with a one-third increase for the wind load combination. The minimum bolt diameter specified is 1.25 in. (31.8 mm), and the maximum center-tocenter spacing required is 10 ft (3 m). For the wind load combination, tension–shear (linear) interaction is checked using a basic allowable tensile stress of 15 ksi (103 MPa) (as per AWWA D100-05) or 19.1 ksi (as per Table I-B in Allowable Stress Design, American Institute of Steel Construction [AISC 1989]) for A36 anchors in tension. For shear, AWWA D100 recommends 7.5 ksi (51.5 MPa) for unfinished bolts, and AISC (1989) recommends 9.9 ksi (68 MPa). An interaction value less than or equal to 1.33 renders the design acceptable. For the seismic load combination, AWWA D100-05, in Section 3.3.3.2, provides a higher allowable tensile stress for mild steel anchors based on the lesser value of 0.8Fy or 0.5Fu , where Fy and Fu refer to the anchor bolt yield and tensile stresses, respectively. For A36 steel, this means 28.8 ksi. For concurrent shear, the AWWA D10005 allowable stress of 10 ksi (i.e., 1.33 × 7.5 ksi) or the AISC allowable shear stress of 13.2 ksi (1.33 × 9.9 ksi) is used. An interaction value of 1.0 renders the design acceptable. Note that in this case, the tensile allowable stress is increased by a different multiplier than 1.33, so the increases are taken directly in the denominator of the interaction equation for the seismic load combinations. For single-pedestal and ground-supported flat-bottom tanks, the design tensile load in the anchors is calculated from Equations (3-41) and (3-42) in AWWA D100. For all styles of tanks, when checking bolt interaction under seismic loads, the resistance offered by friction

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Chapter Five forces may also be taken into consideration, in the authors’ view. Some codes specifically disallow this, but for water tanks it is justified since nearly the entire mass is considered effective in the formation of all seismic loads, including the seismic overturning moments. This must be done with attention to signs (load direction), since a column under uplift cannot generate frictional resistance.

Bolt Interaction Interaction can be checked by means of a simple equation. For the combined effects of tension and shear, the following linear interaction equations may be used in design: T Tallowable

+

V Vallowable

≤ 1.33

(wind)

V T + ≤ 1. Tseismic-allowable 1.33Vallowable

(seismic)

(5-19)

(5-20)

where T = tensile load V = concurrent shear load on bolt The allowable tensile and shear stresses are as defined previously for mild steel. For other types of steel anchors, refer to AWWA D100 or other applicable codes for all allowable stresses. If high-strength or stainless-steel bolts are required, D100 allowable tensile stress for these bolts is based on the lesser of 0.4 times the minimum published yield stress or 0.25 times the published tensile strength. The calculated bolt size may need to be adjusted when corrosion allowance is required in design. AWWA D100 discourages the use of J and L bolts because of their tendency to straighten out, as observed in pull-out tests. Quality control in placement of bolts is essential. Given the size and embedment length required, bolt relocation may not be possible, and remedial measures can be expensive. Therefore, proper bolt placement, including correct embedment and projected length, is critical to proper design.

Bearing Stress Under Base Plates The design bearing strength of concrete is defined in Section 10.17 of ACI 318-05. Typically, under service load conditions, the allowable bearing stress Fp is 0.35 fc when the entire area of concrete support is covered (AISC 1989). Otherwise, when the supporting √ surface is wider on all sides, the bearing stress is based on 0.35 f c A2/A1 ≤ 0.7 f c . An additional one-third increase may be taken for wind or seismic load

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Foundations

Foundations combinations. Refer to ACI 318-05 for determination of A2 and A1 areas.

Foundations in Regions of High Seismic Risk Seismic design has undergone a tremendous evolution over the past decade. For years, seismic design of elevated water tanks was primarily based on the fixed-percentage method, in which the total weight of water and structure was multiplied by a specified coefficient based on seismic risk zones. This was subsequently changed to the pseudodynamic approach, but design was still governed by seismic risk zones. Concurrently, some building codes required seismic design to be performed using the velocity-based acceleration Av and the effective peak acceleration Aa . AWWA D100-05 has entirely eliminated language regarding design on the basis of seismic zones (0, 1, 2, 3, and 4). Instead, the AWWA standard has essentially adopted the ASCE 7-05 criteria (based on NEHRP [2003]) with some variation with respect to the minimum design acceleration. These requirements are substantially different from the procedure thus far used by older AWWA standards. The International Building Code in Section 1613.1 invokes the requirements of ASCE 7-05 for the design and construction of elevated and flat-bottom water tanks to resist the effects of earthquake motions.

Special Design Provisions As per ASCE 7-05, seismic design involves a procedure in which spectral response acceleration parameters for the maximum considered earthquake ground motions are determined from figures and then modified for local site effects with site coefficients. The resulting accelerations are then scaled down to design values. ASCE 7-05 also permits the use of site-specific procedures in design and mandates this procedure where provisions specifically require it. ASCE 7-05 classifies sites based on shear wave velocity and other features. Depending on soil consistency ranging from hard rock to stiff soils, site classifications A, B, C, and D are defined. Site classification E involves any profile with more than 10 ft (3 m) of soil having high plasticity index or high moisture content, or low shear strength as defined in the reference. Site class F involves soils that are vulnerable to potential failure or collapse, highly organic soils, very high-plasticity soils, and very thick, soft/medium clays. Site class F soils require sitespecific evaluations. With the site classifications defined, the 5 percent damped design spectral acceleration at short period SDS and at 1-second period S D1 are determined. The elevated water tank system is then assigned to a seismic design category (SDC) based on these accelerations and on

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Chapter Five the appropriate seismic use group (I, II, or III). All structures having SDS ≥ 0.5g or SD1 ≥ 0.2g (where g is the acceleration of gravity) are assigned an SDC of D. Also, seismic use group III structures with 0.33g ≤ SDS < 0.5g or 0.133g ≤ SD1 < 0.2g are assigned an SDC of D. Seismic design category E is assigned to seismic use group I and II structures located on sites with mapped maximum considered earthquake spectral acceleration at 1-second period S1 equal to or greater than 0.75g. Similarly, seismic use group III structures at these accelerations are assigned an SDC of F. Structures assigned to categories C, D, E, or F require special attention to quality assurance during construction. Structures assigned to category E or F are prohibited from being sited where there is a known potential for an active fault. The seismic importance factor IE significantly affects seismic design. (This factor is defined in ASCE 7-05 Section 11.5.1, Table 11.5-1.) Values of the importance factor range from 1.0 to 1.5, depending on the seismic use group category assigned to the elevated water tank system. AWWA D100-05 assigns a default value of 1.5 to IE unless otherwise specified by the purchaser, but it allows the use of 1.0 for systems not supplying water for fire protection.

Reinforcement Criteria In regions of high seismic risk, ACI 318-05 requires structures to comply with requirements defined in Sections 21.2 through 21.10. These sections define maximum and minimum flexural and transverse reinforcement, maximum spacing for hoops and crossties, bar development length, and other requirements. Section 21.10 provides criteria for the design of foundations. Footings, mats, piles, pile caps, piers, and caissons are all required to be designed under this section. ASCE 7-05 refers to ACI 318-05 for design and construction of concrete foundations assigned to seismic design categories D, E, and F. ASCE 7-05 requires individual pile caps or drilled piers in these categories, as well as in category C, to be interconnected by ties. Likewise, spread footings founded on site class E and F soils are required to be interconnected by ties. The design strength for ties in tension or compression is required to be greater than 10 percent of SDS times the larger pile cap or column factored-dead plus factored-live load, with some exceptions. There are also rigorous requirements for the design of piles in site class E and F soils.

Precast Prestressed and Cast-in-Place Concrete Piles IBC-2006 provides detailed criteria for the design of foundations, piles (including precast prestressed piles), and pile cap connections. It specifies a 28-day compressive strength f c of 5,000 psi (34.5 MPa) and requires the prestressing strands to conform to ASTM A416. For prestressed piles, IBC-2006 also specifies a minimum volumetric ratio of

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Foundations

Foundations spiral reinforcement, defines the ductile region of the pile as a function of its length, and establishes bounds for the center-to-center spacing of the spirals or hoop reinforcement. Similarly, IBC-2006 also establishes criteria for the design and detailing of cast-in-place pile and pier foundations. The IBC-2006 requirements are very similar to the ASCE 7-05 requirements. Either reference can be used as required, individually or in conjunction with ACI 318, in designing elevated water tank foundations.

Foundation Stability Design for stability is critical in regions of high seismic activity. Foundations must be designed to withstand all design loads with adequate factors of safety. Foundations must also be stable against all forces causing uplift, lateral sliding, and overturning. The safety factors required for stability and strength are defined in various ways by different codes. It is important to appreciate the reasoning and philosophy associated with these factors to ensure structural integrity, safety, and stability. Lateral stability in saturated soils, settlement evaluation in saturated or high-moisture-content silty soils, and potential liquefaction in sandy soils are all conditions that require competent evaluation and assessment before elevated water tanks are built on sites with these characteristics. Piles designed for fixity at the pile head must be properly connected to or embedded deep into the pile cap to develop uplift and moment capacity. Backfill around spread footing and pile foundations must be consistent with the geotechnical engineer’s recommendations. Special recommendations made in terms of moisture content, maximum loose lifts, or soil remediation measures must be followed. All nonconforming conditions must be brought to the attention of the engineer of record for evaluation and disposition.

Special Considerations Design of elevated water tank foundations requires close coordination with the project geotechnical engineer and with the construction team at the job site. All parties must clearly understand the design requirements and must appreciate what is essential to quality design and construction. Structural engineers must not assume that all geotechnical requirements defined in the subsurface evaluation report will be routinely implemented during construction. In today’s fast-paced construction, it is not unusual to see the forms removed the following day, or long before the concrete has achieved its specified 28-day compressive strength f c . Backfill placement could be started immediately

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Chapter Five thereafter. Therefore, the engineer of record (EOR) must be specific in defining any special design and construction, including formwork, requirements. Thus, as the concrete strength and backfill procedures are critical to the various phases of construction, strict quality control measures must be put in place to ensure that the correct concrete strength is achieved before any backfill activities commence. Site inspection and testing will be necessary to ensure proper compaction or soil remediation. It is possible that the site soils are unsuited for backfill, and so suitable soils must be imported. It is also possible that topsoils containing organics and other deleterious material could get mixed with other site soils during backfilling around the footing. Some states have defined certain soils or soil mixtures as “select fills” that are recommended for specific structural fills or backfill. All of these issues are important in foundation design, and the necessary quality control steps should be taken before starting any construction activity.

Vertical Versus Sloped Excavations Some foundation contractors prefer making footing excavations no larger than is required to place the footings. Where possible, these excavations are vertical unless they are deep enough that OSHA regulations mandate them to be sloped. Unless backfill compaction is clearly specified and required by explicit notes on drawings, it is possible that backfill compaction in these excavations will not occur. If compaction of soils within the 25-degree to 45-degree wedge was also included in lateral stability consideration, those soils would not be compacted if the excavations are vertical. Therefore, it is crucial to clearly define all requirements for excavation, backfill, backfill compaction, moisture content, and dewatering where necessary. Curing procedures, minimum concrete strength before backfill can be placed and/or compacted, and the extent of compaction beyond the footings must be defined as well. These requirements must be delineated precisely by concise notes on the foundation drawings.

Backfill Compaction Geotechnical engineers generally specify compaction in terms of maximum thickness of loose lifts and standard proctor maximum dry density unit weights per ASTM D698 or modified proctor maximum dry density tests per ASTM D1557 (see Annual Book of ASTM Standards). These requirements are usually specified to be 95 to 98 percent of the maximum dry density; even higher percentages are specified for subgrade compaction. Soil compaction is accomplished by the use of hand tampers and sheepsfoot or pneumatic rollers.

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Foundations

Foundations Backfill compaction not only improves lateral and uplift stability by improving soil shear strength, but it also reduces permeability in cohesive soils. In cohesionless soils, surface drainage must be accomplished by proper grading to avoid basin or boat effects around foundations. A typical compaction note reads: Backfill material should be placed in 6 to 8 in. maximum loose lifts and compacted to at least 95% of the Standard Proctor maximum dry density (ASTM D698).

As a rule, for water tank foundations, a minimum unit weight of 100 pcf (15.7 kN/m3 ) should be achieved. Rock fragments and stones larger than 3 in. (7.5 cm) in diameter should not be used in the vicinity of the footings. Any soft or loose material should be removed from the bearing areas before concrete is placed. Thus, it is recommended that a geotechnical representative be present before and during the pouring of the foundations as well as during placement of backfill.

Water- and Moisture-Control Measures Design in areas of high water table should consider buoyant unit weights of soil and concrete for lateral and vertical stability. If the water table is likely to be encountered during excavation or construction, proper measures must be taken to dewater the bearing areas to a minimum depth of 3 ft (0.9 m) below the bottom of the foundation. When rainfall is imminent or when the excavation must remain open overnight, a 4- to 6-in. (10- to 15-cm) mud mat of lean concrete (2,000 psi [13.8 MPa]) should be poured over the bearing soils, with the top of concrete being at the required bearing elevation. If water does enter the excavations or if unsuitable soils are encountered, softened soils should be completely removed and excavation brought back to bearing grades with a mud mat or no. 57 stone. The geotechnical engineer should approve this activity. It is best to maintain soil moisture content as close to (within 2 to 3 percent of) the optimum moisture content. Plastic soils are not ideal for fill or backfill, but when the geotechnical engineer approves their use, they should be placed with a higher moisture content of ±5 percent of the optimum. These levels of moisture content facilitate compaction and help accomplish the desired unit weight.

Shrink/Swell Soils Expansive soils and challenges associated with shrink/swell soils were discussed earlier in the chapter in the section on design considerations in plastic soils. Expansive soils are generally plastic clays, also known as fat clays, that swell with increases in moisture content.

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Chapter Five

U

U

U

FIGURE 5-12 Typical foundation in shrink/swell soils. (Uw = vertical uplift due to wind; Us/w = vertical uplift due to shrink/swell.)

They are classified as CH clays in the ASTM’s Unified Soils Classification Chart. The depth of the expansive soil active zone can vary from a few feet or less than a meter to perhaps more than 15 ft (5 m). Foundations constructed in these soils can potentially be subjected to very large uplift and possibly destabilizing forces if they are not designed properly. Refer to Fig. 5-12 for a typical foundation in plastic zone with an effective active zone of 12 ft (3.7 m).

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Foundations

Foundations The plasticity index (PI) is generally used as a measure of swell potential in plastic soils. Soils with low swell potential are those with PI values below 25; a PI value of 25 to 35 indicates moderate swell potential; PI values exceeding 35 correspond to soils with very high swell potential. Foundations for elevated water tanks sited on soils with low swell potential can be constructed using the standard practices, but foundations sited on soils with PI values exceeding 25 (see ASCE 7-05 or IBC-2006) require precaution, remedial action, and special design considerations (Das 2006). Typically, when placing foundations in plastic soils, geotechnical engineers recommend bearing the footing deep in nonplastic soils or below the active zone. To mitigate the effects of the uplifting forces, the use of select structural fill and backfill, soil stabilization with lime, and/or inclusion of clearly defined uplift forces in the design have been recommended. Das (2006) provides recommendations for remedial measures as well as a procedure for estimating the uplift forces caused by the swelling forces. Typical remedial options often recommended for shallow footings include the following: 1. Bear the footing below the active zone and replace the backfill with select structural fill. 2. Bear the footing in the active zone, replace 3 to 5 ft (0.9 to 1.5 m) of soil below the footing with select structural fill, and use select structural fill for backfill. 3. Use site soils for backfill, but ensure that soil moisture content is greater than the plastic limit and that the moisture content is 3 to 5 percent above the optimum moisture. 4. Use a polyethylene or bitumen material on the vertical faces of the footing. 5. Same as point (2), but use site soils for backfill with lime mixing. 6. Same as point (1) or (2), but use site soils for backfill, and consider the uplifting forces in the design of the footing and in its stability. Use J-voids where necessary to allow room for the soil to swell without imposing any forces on the foundation element. 7. In soils where piers are recommended, geotechnical engineers should recommend belled piers, with emphasis on the reinforcement requirements for resisting the uplift forces at the junction of the shaft and the bell. In summary, when site soils involve clays prone to swelling, specific geotechnical guidance must be sought. The geotechnical report

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Chapter Five must clearly define the active zone, the potential for swell, uplift, or adhesion forces that need to be considered in design, the bearing depth at which the foundations are to be placed, and suitability of site soils for backfill or recommendations for imported soil. If imported soils are to be used for backfill, clear criteria must be provided regarding the nature of the soil, its Atterberg limits, compaction requirements, and guidance on local availability of the recommended soils.

Conclusion Foundations are critical to the design, construction, operation, and performance of welded-steel tanks for water storage. Therefore, foundation design and construction require attention to detail and proper understanding of all criteria and requirements. Sites that are relatively dry, level, and easily accessible and that have good soils properties are ideal locations for erecting elevated water tanks. The suitability of sites must always be established by a qualified geotechnical engineer. Grade elevations and site boundaries must be established carefully to achieve the proper overflow and foundation elevations. Geotechnical investigation reports must provide all the necessary information for design. This includes detailed soil properties and other characteristics defined in this chapter. Certain soils exhibit shrink/swell or other characteristics that require extra measures and precautions in design. All of these characteristics must be clearly defined and appropriately addressed in the report. Site classification and settlement evaluation must also be included in the geotechnical report. Generally, isolated spread footings or shallow foundations are the most economical foundation type when suitable to site conditions. Otherwise, deep foundations are necessary. Detailed criteria are provided herein to assist the designer in selecting the most suitable foundation type and to assist the designer with the design, be it shallow footings or deep foundations using piles and drilled piers. In regions of high seismic activity, special design requirements apply that must be incorporated into the foundation design. Both the logic and philosophy for these requirements are explained in this chapter, and further resources are provided in the bibliography at the end of the chapter. Requirements for the quality control, mixing, placing, finishing, and curing of concrete have also been defined here. These requirements are critical, as they govern the strength, durability, and workability of concrete foundations. Also, criteria have been introduced

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Foundations

Foundations for sizing anchor bolts and for allowable bearing capacity under base plates. Requirements for backfill compaction and lateral and uplift stability are defined. It is further emphasized that to ensure safety, all excavations must be performed in full compliance with the latest OSHA construction standards.

Foundation Design Example Problem Statement To illustrate the design of a shallow foundation, the loading resulting from the analysis of a typical 500,000-gal (1,893-m3 ) elevated torusbottom water tank will be considered. The tank has a diameter of 50 ft (15.24 m), head range of 37 ft (11.28 m), and a high water line of 116 ft (35.4 m). It is supported by six columns, similar to the tank shown in Fig. 5-1. The design service loads on the footing are as follows: Vertical Loads Dead load D = 37.0 kip

Horizontal Loads

Water load F = 520.0 kip Snow load S = 7.0 kip Wind load W = ±105.0 kip

Wind shear WS = 30 kip

Seismic load E = ±142.0 kip

Seismic shear ES = 28 kip

Assume the live load to be zero, and assume that the wind load has been reduced by a directionality factor so that the 1.6 load factor applies. Use a net allowable bearing pressure of 3,000 psf (144 kPA) at a minimum embedment depth of 5.5 ft (1.68 m) below existing grade and a concrete compressive strength f c of 4,000 psi (27.58 MPa). Assume the pedestal to be 4 ft × 4 ft (1.2 m × 1.2 m) with a 1-ft (0.3-m) projection above grade. Refer to Fig. 5-13.

Footing Design Step 1: Governing Load Combinations The load combinations were defined in Equations (5-1) through (5-7). A quick examination of these equations reveals that only load combinations (5-1), (5-4), (5-5), and (5-6) are governing. After simplification, these equations are as follows:

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Chapter Five U V

Embedment depth

Grade

Footing exposure

Pedestal width p w d Flexural shear d 2

t

Punching shear

d

Footing width B

FIGURE 5-13 Shallow footing example.

Vertical Loads U 1 = 1.4 (D + F ) U 4 = 1.2 (D + F ) + 1.6 W + 0.5 S U 5 = 1.2 (D + F ) + (1.4 × 1.0) E + 0.2 S U 6 = 0.9 D + 1.6 W

= = = =

780 kip 840 kip 869 kip −135 kip

(5-1) (5-4) (5-5) (5-6) (uplift)

Corresponding Horizontal Loads = 0.0 = 48 kip = 39 kip = 48 kip

V1 V 4 = 1.6 W S = 1.6 × 30 V 5 = (1.4 × 1.0) E S = (1.4 × 28) V 6 = 1.6 W S = V 4

(5-1) (5-4) (5-5) (5-6)

Step 2: Size Footing Using Service Loads Corresponding to the load combinations in Step 1, as per ASCE 7-05 Section 2.4.0, the service (vertical) loads are as follows: Service U 1

= (D + F )

= 557 kip (5-1)

Service U 4,Wind

= (D + F ) + W

= 662 kip (5-4) (governs wind)

Service U 5,Seismic = (D + F ) + E Service = 699 kip (5-5) (governs seismic) Service U 6,Wind

= 0.6D + W

= −83 kip (5-6) (uplift)

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Foundations

Foundations Required footing area: AWind =

662 kips = 220 ft2 3000 psf

ASeismic =

(governs)

699 kips = 175 ft2 1.33 × 3000 psf

AWind governs and a square footing of 15 ft × 15 ft (4.6 m × 4.6 m) provides the required bearing area. The net bearing pressure at the toe of the footing is f bearing-Wind =

662 kips

f bearing-Seismic =

2

+

30 kips × 6.5 ft

ft (15 × 152 /6) ft3 = 3,290 psf < 4,500 psf (using FS = 2.0) 152

699 kips 3

+

28 kips × 6.5 ft

ft (15 × 152 /6) ft3 = 3,430 psf < 4,500 psf (using FS = 2.0) 152

Although the increase in the net overbearing pressure resulting from the differential weight of concrete is not yet accounted for, the initial size selected is reasonable.

Step 3: Check Punching Shear at a Distance d/2 from the Pedestal Punching shear is checked at a distance d/2 from the face of the pedestal as shown in Figs. 5-13 and 5-14a . The maximum punching shear is caused by the load U 5 of 869 kip (3.87 MN) and the factored weight of the pedestal. Assuming a depth d of 17 in. (43.2 cm) for the slab, the critical perimeter is given by b 0 = 4( pw + d) = 4 × [4 ft × 12 (in./ft) + 17 in.] = 260 in. where p w is the width of the square pedestal. For a bearing depth of 5.5 ft (1.68 m) and a slab thickness t of approximately 20 in. (50.8 cm), the pedestal height will be 4.83 ft (1.47 m). The factored weight will be Dpedestal = 1.2 × (4 ft × 4 ft × 4.83 ft) × 0.144 (kip / ft3 ) = 13.36 kip, which results in a new factored U 5 of 882 kip.

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Chapter Five

2

Pedestal width pw1

d

2

d

2

Pedestal width pw2

d

2

d

Critical perimeter = 4(p w + d ) (square footing)

(a)

Footing width B

Footing width B

d

Footing width B

Pedestal width pw

(b)

(B – pw) –d 2

FIGURE 5-14 Design shear and bending moment evaluations: (a) punching shear, (b) flexural shear, and (Continued)

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Foundations

Foundations Footing width B

Footing width B

Pedestal width pw

(c)

(B – pw) 2

FIGURE 5-14 (Continued) (c) flexural bending.

The nominal punching shear capacity as per ACI 318-05 Section 11.12.2.1 is Vc = 4



f c .b 0 .d = 4 ×



3, 500 × 260 × 17 = 1,046 kip

Here, f c is reduced by 500 psi (3.45 MPa) for reasons described in the section on structural concrete. Assuming no contribution from slab reinforcement, using a shear reduction factor of 0.75 as per ACI 31805 Section 9.3.2.3, the nominal punching shear capacity is Vn = 0.75 × 1,046 = 784 kip The punching shear caused by U 5 is Vu =

882 kips 152 ft2

[152 ft2 − (4 + 1.42)2 ft2 ) = 767 kip

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Chapter Five Since V n is greater than V u kip, a slab depth d of 17 in. (43.2 cm) satisfies punching shear requirements.

Step 4: Check Flexural (Beam) Shear at a Distance d from Pedestal Flexural shear will be checked at a distance d from the face of the pedestal, as shown in Figs. 5-14b and 5-15. The bearing pressures for

pw

W3 (soil)

W1 (Concrete—pedestal)

d

4,370 psf (209 kPa)

4,125 psf (198 kPa)

4,040 psf (193 kPa)

3,920 psf (188 kPa)

3,740 psf (179 kPa)

W2 (Concrete—slab)

psf } 450 (22 kPa)

pw

2 d+

pw

2

(B – pw)

2 B/2

B

FIGURE 5-15 Flexural shear and bending moment evaluation. (psf = pounds per square foot, kPa = kilopascal.)

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Foundations

Foundations the factored wind and seismic load combinations are as follows: f bearing-Wind =

(840 + 13.36) kips 2

= 3,238 (psf)

±

48 kips × 6.5 ft

= 3.79 ± 0.55 ft (15 × 152 /6) ft3 = 4,347 (psf) maximum < 9, 000 psf (ultimate bearing) 152

minimum

39 kips × 6.5 ft + = 3.92 ± 0.45 152 ft3 (15 × 152 /6) ft3 = 4,370 (psf) maximum < 9,000 psf (ultimate bearing) = 3,470 (psf) minimum

f bearing-Seismic =

(869 + 13.36) kips

The bearing stress due to seismic loading governs. From Figs. 5-14b and 5-15, the bearing pressure at a distance d from the face of the pedestal is f bd = 3, 920 +

(2 + 17/12) (450) = 4, 125 psf (15/2)

The resulting flexural shear at the same location is   (15 − 4) 17 1 VFlex = (4, 125 + 4, 370)(15) − = 260 kip 2 2 12 The flexural shear capacity of the footing slab as per ACI 318-05 Section 11.3.1.1 is  Vcf = 2 3, 500(15 × 12)(17) = 362 kip Vnf = 0.75 × 362 = 272 kip Since Vnf > VFlex , the 17-in. (43.2-cm) depth selected is adequate for shear.

Step 5: Determine Required Flexural Reinforcement The bearing pressure at the face of the pedestal is (Figs. 5-14c and 5-15) f bp = 3, 920 +

(4/2) (450) = 4,040 psf (15/2)

The resulting ultimate bending moment at this location is       15 − 4 2 1 1 Mu = (4, 040) (4, 370 − 4, 040) (15)+ 2 2 2      2 15 − 4 2 (15) 3 2 Mu = 967 ft · kip

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Chapter Five Determine the reinforcement needed to carry this bending moment. The minimum reinforcement required as per ACI 318-05 Section 10.5.1 is (200)(15 × 12)(17) A s, min = = 10.20 in.2 (60,000) Try a steel area of 14.0 in.2 (PCA 1999):   14  = = 4.58 × 10−3 (15 × 12)(17)   fy Mn = A s f y d 1 − 0.59 ·   fc   60 Mn = (0.9)(14)(60)(17) 1 − 0.59(4.58 × 10−3 ) 3.5 Mn = 1,021 ft · kip Mn > Mu Therefore, fourteen no. 9 bars each way, 14.5 ft (4.42 m) long, will suffice.

Step 6: Reinforcement for the Pedestal ACI 318-05 Section 15.8.2.1 requires a minimum pier reinforcement of 0.005Ag , where Ag is the gross area of the pier across the interface. Thus, Aspedestal = (0.005)(48 × 48) = 11.52 in.2 As noted in the section on design of isolated spread footing, for the short pedestal, this reinforcement area can be reduced per industry practice. But this is a matter of decision by the designer. If this reinforcement is to be maintained, sixteen no. 8 bars will provide 12.64 in.2 (81.55 cm2 ). Under the combined action of the 135-kip uplift U 6 and the 48-kip shear V 6 , the resulting stress in the concrete pedestal will be ped =

135 (48 × 4.83) + = 210 psi (4 × 4) (4 × 42 /6)

This stress is less than the concrete modulus of rupture f r where, as per ACI 318-05 Section 9.5.2.3, √ f r = 7.5 3500 = 444 psi As per ACI 318-05 Section 11.5.6.1, assuming a 3-in. (7.62-cm) cover for the pedestal dowels, since √ 0.5Vc = 0.5(0.75)[2 × 3500(48)(45)] = 96 kip > U6 = 48 kip,

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Foundations

Foundations no shear reinforcement is needed. However, since the dowels will be in compression due to the other load combinations, No. 4 ties at a spacing of 12 in. (30.5 cm) on center are recommended.

Step 7: Check Uplift Stability AWWA D100 requires the concrete weight along with the weight of the soils directly above the footing to be greater than the service load uplift. The weight of the concrete (W1 + W2 = 65 kip) and the soil (W3 = 80 kip) amounts to 145 kip total, which is in excess of the 105-kip wind uplift. Refer to Fig. 5-15. Therefore, uplift stability is maintained.

Other Steps The anchor bolts should be designed for tension and shear interaction. Lateral stability should be checked on the basis of the active and passive pressures and the cohesion, if any, of the backfill soils. Backfill compaction is a function of the stability requirements. A compaction to 95 percent standard proctor maximum dry density, as discussed in this chapter, may be recommended. The embedment depth can be adjusted, if necessary, to achieve additional passive resistance. Refer to Fig. 5-16 for the reinforcing details. Note that, in general, strain compatibility must be checked to ensure that a balanced condition prevails and that the footings are not over-reinforced. Also, note that a nominal top mat reinforcement can be added as required for uplift or shrinkage control.

4 ft (1.2 m) Anchor bolt

#4 ties @ 12 in. (305 mm) centerline to centerline

16 #8 dowels No. 5'S–nominal–each way (only if required for uplift or shrinkage control) 20 in. (508 mm)

5.5 ft (1.65 m)

1 ft (0.3 m)

14 #9 each way 15 ft (4.5 m)

FIGURE 5-16 Shallow footing design example.

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Chapter Five

Bibliography American Institute of Steel Construction (AISC). 1989. Manual of Steel Construction, Allowable Stress Design. 9th ed. Chicago: AISC. American Petroleum Institute (API). 2008. Design and Construction of Large, Welded, Low-Pressure Storage Tanks, 11th ed. API Standard 620. Washington, D.C.: API. American Society of Civil Engineers (ASCE). 1993a. Bearing Capacity of Soils. Technical Engineering and Design Guides as Adopted From the U.S. Army Corps of Engineers, No. 7. New York: ASCE Press. American Society of Civil Engineers (ASCE). 1993b. Design of Pile Foundations. Technical Engineering and Design Guides as Adopted From the U.S. Army Corps of Engineers, No. 1. New York: ASCE Press. Bowles, J. E. 1995. Foundation Analysis and Design. 5th ed. New York: McGrawHill. Das, B. M. 2006. Principles of Foundation Engineering. 6th ed. Florence, KY: CL Engineering. Deep Foundations Institute (DFI). 1990. Augered Cast-in-Place Piles Manual. 1st ed. Englewood Cliffs, NJ: DFI. Kosmatka, S. H., and W. C. Panarese. 1990. Design and Control of Concrete Mixtures. 13th ed. Skokie, IL: Portland Cement Association. Liu, C., and J. B. Evett. 1987. Soils and Foundations. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall. National Earthquake Hazards Reduction Program (NEHRP). 2003. NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 450). Part 1: Provisions. Washington, D.C.: NEHRP. Nilson, A. H., D. Darwin, and C. Dolan. 2004. Design of Concrete Structures. New York: McGraw-Hill. Peck, R. B., W. E. Hanson, and T. H. Thornburn. 1974. Foundation Engineering. 2nd ed. New York: John Wiley & Sons. Portland Cement Association (PCA). 1999. Notes on ACI 318–99 Building Code Requirements for Structural Concrete, with Design Application. 7th ed. Skokie, IL: PCA. Prakash, S., and H. D. Sharma. 1990. Pile Foundations in Engineering Practice. New York: John Wiley & Sons. Reese, L. C., and M. W. O’Neill. 1988. Drilled Shafts: Construction Procedures and Design Methods. US Department of Transportation, Federal Highway Administration FHWA-HI-88-042, ADSC-TL-4. McLean, VA: US Department of Transportation Federal Highway Administration; and Dallas, TX: ADSC, the International Association of Foundation Drilling. Sarhan, H. A., M. W. O’Neill, and S. W. Tabsh. 2004. Structural Capacity Reduction for Drilled Shafts with Minor Flaws. ACI Structural Journal 101(3):291– 297, May/June. Smith, G. N., and E. L. Pole. 1981. Elements of Foundation Design. New York: Garland STPM Press. Terzaghi, K., and R. B. Peck. 1967. Soil Mechanics in Engineering Practice. New York: John Wiley & Sons. Woodward, R. J., W. S. Gardner, and D. M. Greer. 1972. Drilled Pier Foundations. New York: McGraw-Hill. Wyllie, D. C. 1992. Foundations on Rock. New York: E. and F. N. Spon, an imprint of Chapman and Hall.

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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

CHAPTER

6

Construction of Welded-Steel Water-Storage Tanks Jim Noren, P.E Advance Tank Construction

Donita Fredricks, P.E. CB&I Constructors

Steel Fabrication Tank constructors have developed specialized equipment, tools, and procedures for the construction of ground storage tanks and elevated water tanks. In the construction of nearly all steel-welded tanks, the steel components are fabricated in a shop environment and shipped to the tank site, where the fabricated components are fit and welded into the finished tank by field construction crews. Steel plate layouts are developed by the constructor, which minimizes welding and maximizes the use of the ordered plate, with consideration to the size and weight restrictions for shipping. For the composite elevated tank, specialized forms and equipment have been developed and are used in the construction of the concrete components.

Material Plate material may be purchased from a steel warehouse or directly from a steel mill. Steel warehouses stock plate material in most of the

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Chapter Six grades used in welded-steel tanks. Material that is required to conform to supplementary requirements (e.g., silicon-killed, fine-grain practice, normalized, or ultrasonically inspected material) is generally not available from a warehouse.

Material Purchased from Steel Warehouse Steel plate from a warehouse is typically available in standard plate widths ranging from 48 to 96 in. (1.2 to 2.4 m) in 12-in. (0.3-m) increments. Plates are normally stocked in 20-ft (8.8-m) lengths, though some warehouses have the capability to cut coiled plate to length. Stock plates commonly used in welded-steel water-storage tanks are available in the following thicknesses: 3/16 in. (4.8 mm), 1/4 in. (6.3 mm), 3/8 in. (9.5 mm), and in 1/4-in. (6.3-mm) increments for thicknesses between 1/2 in. (12.7 mm) and 2 in. (51 mm). Stock material grades, sizes, and thicknesses vary from warehouse to warehouse, so availability of a specific plate size and thickness must be verified in the design phase of the project. Delivery time for warehouse material is shorter and minimum tonnages normally do not apply, but the cost is higher than for material purchased directly from the mill.

Material Purchased from a Steel Mill Plate material purchased from a mill may be ordered to the customerspecified width, length, and thickness. Material conforming to specific supplementary requirements is available from most mills. Steel mills typically require a minimum order, and delivery times are significantly longer than for warehouse-purchased material. Regardless of whether a plate is purchased from the warehouse or mill, conformance to the American Society for Testing and Materials (ASTM) requirements for the ordered plate should be confirmed by the constructor on receipt of the material. This can be accomplished by reviewing material test reports or certificates of compliance furnished by the supplier. If the plate cannot be traced to a material test report or a certificate of compliance, testing by a qualified testing laboratory may be used to verify that the plate complies with the chemical and mechanical requirements of the specified ASTM standard. On receipt of the plate, measurements should be taken to verify that its width, length, and thickness are consistent with the ordered plate size. Permitted variations in dimensions are outlined in ASTM A6. Visual examination of the plate should be performed to verify that the material is free from injurious defects and has a workable finish. Thick plate should also be checked along the edge for lines that would indicate a possible lamination.

Cutting Several methods are available for cutting plates to size in the shop, including thermal cutting by either oxy-fuel gas torches or plasma

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FIGURE 6-1 Cutting plate by oxy-fuel torches.

arc. Thermal cutting methods are commonly used in the shop for irregularly shaped plates (Fig. 6-1). Oxy-fuel gas torches may be automated by either setting up a track burner or by use in a numerically controlled burning bed. Plasma arc cutters are used in a numerically controlled burning bed. Using either method, the plate may be simultaneously cut to size and the edges tapered and beveled for welding by using multiple burning heads. The finished edges should closely follow the detailed plate dimensions to ensure good fit-up in the field. The edges should be uniform and smooth and cleaned of slag accumulation when necessary. Machining and shearing are other methods of cutting plates to size. Rectangular plates that are ordered with minimal trim allowance may be trimmed and squared by machining the edges using an edge planer. Shearing is another option for straight edges that are less than the width of the constructor’s plate shear. American Water Works Association (AWWA) Standard D100 limits plate thicknesses for shearing to 1/2 in. (12.7 mm) or less if the joint is to be butt welded. Edges that will be lap welded are limited in thickness only by the capacity of the plate shear. Sheared edges should be square and burrs removed before welding.

Forming Single-curvature plates for welded-steel tanks are typically cold rolled in the shop to the appropriate curvature using a plate roll. Plate widths

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Chapter Six Plate Thickness Minimum (in.[mm])

Maximum (in.[mm])

>3/8 [>9.5]

≤3/8[≤9.5]

40 [12]

>1/2 [>12.7]

1/2 [12.7]

60 [18.2]

>5/8 [>15.8]

5/8 [15.8]

120 [36.6]

Minimum Plate Diameter (ft[m])

Must be rolled for all diameters TABLE 6-1 Minimum Diameter for Plates Not Rolled

may be limited by the fabricator’s plate roll capacity. AWWA D100 makes provisions for plates that need not be rolled on the basis of the minimum diameter and plate thickness as outlined in Table 6-1. Singlecurvature plates are frequently used to construct a double-curvature surface if the radius is large enough. One example of this is pie-shaped plates in a dome roof. Double-curvature plates are cold pressed using repeated blows with a mortar-and-pestle-shaped die (Fig. 6-2). Typical examples of a double-curvature plate include the flare and ball of a pedestal tank. Press breaks are used to form sharp bends in a plate—for example, the fluted plate in a fluted-column-style tank. Press breaks and presses can also be used to simulate a rolled plate by repeatedly hitting the plate with a straight die, allowing for short spaces between hits (Fig. 6-3). This method can also be used to form cone-shaped plates and is particularly useful for thick plate. Angle rolls are commonly used to roll structural angles for weldedsteel tanks. With all forming operations, it is important to have

FIGURE 6-2 Pressing double-curvature plate.

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FIGURE 6-3 Forming fluted plates in a press break.

adequate dimensions on the shop drawings to verify the accuracy of the formed plate.

Shop Subassemblies Fabricated plate may be subassembled and welded in the shop (Fig. 6-4). This is done to maximize the welding that can be performed

FIGURE 6-4 Shop assembly of cut and formed plates for a dome roof.

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Chapter Six in the controlled environment of the shop using optimal welding processes and configurations. Shipping restrictions dictate the extent of shop subassembly that can be executed. When plate sections are sized such that the shop-welded subassemblies are not an option, complete or partial shop assembly may still be warranted to ensure proper fit-up in the field. This is especially useful for complex geometries with double-curved surfaces.

Blast and Prime The life of a coating system depends on the surface preparation. A smooth, regular surface with the proper steel profile will provide a good basis for the application of a protective coating system. The fabricator should ensure that weld contours are smooth and that any unacceptable weld undercutting is eliminated. Weld flux and weld spatter should be removed and the sharp edges ground smooth. Most tank constructors recommend that tank components be abrasive blast cleaned and primed in the climate-controlled atmosphere of the shop. Exterior surfaces and interior dry surfaces should be cleaned to a commercial finish per Society for Protective Coatings (SSPC) SP6 as a minimum. Inside wet surfaces should be cleaned to a Near White Blast finish per SSPC SP10. Blast-cleaned surfaces should have a surface profile that is appropriate for the specified primer and coating system per recommendations of the coating manufacturer. Blasting may reveal small laminations or pitting in the plate surface not previously apparent. If these imperfections are large enough to produce holidays in the coating system, they should be removed by grinding. Occasionally, deeper laminations may require welding or further testing. The prime coat should be applied immediately after surface preparation, before the occurrence of any surface rusting or accumulation of dust or moisture. The type and thickness of primer should be defined in the customer’s specifications. AWWA D102 Coating Steel WaterStorage Tanks may be referenced for interior and exterior coating systems. Prime coats may be applied using any method recommended by the coating manufacturer, except that rolling should not be used for the prime coat on interior wetted surfaces unless required for rough pitted surfaces. An unpainted margin approximately 4 in. (102 mm) wide should be provided around all plate edges that will be field welded.

Shipping Shipping from the shop to the job site is almost exclusively by truck (Fig. 6-5). The current weight limit is 80,000 lb (36,287 kg) gross for the truck, trailer, and load, resulting in a net load capacity of approximately 45,000 lb (20,412 kg), depending on the weight of the truck

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FIGURE 6-5 Shipping formed plates by truck.

and trailer. Rules for oversized loads vary from state to state. Special permitting, routing, and escorts may be an option when oversized loads cannot be avoided or are deemed to be more economical. Plate layouts are often dictated by shipping limitations. Site access should also be considered in planning shipping loads. Material should be sufficiently blocked, braced, and tied down to secure the components to the trailer and maintain the fabricated shape during shipping.

Welding In the 1950s, welding replaced riveting as means of connecting tank joints. Welding can be performed in all climates and in a variety of positions. Over time, the technology has improved, leading to increases in productivity. To convey the correct welding information from the tank designer, weld symbols in accordance with AWS Standard Symbols for Welding, Brazing, and Nondestructive Examination should be used on the fabrication and erection drawings.

Welding Processes The primary welding processes used in the shop and field are shielded metal arc welding (SMAW), submerged arc welding (SAW), and flux cored arc welding (FCAW). All are arc welding processes that use an electric arc generated by an electric current between the tip of the electrode and the base metal. Heat from the arc melts the electrode

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Chapter Six and adjacent base metal which then combines, cools, and solidifies to form the weld bead. Welding may be performed manually, semiautomatically, automatically, or by machine welding. Manual welding requires the welder to manually maintain the proper positioning and arc length and replace the electrode as it is consumed. Semiautomatic welding is performed with a handheld gun that continuously feeds the electrode and flux. Automatic welding is accomplished with equipment that is capable of performing the welding operation without a welding operator. This type of welding is more commonly used in assembly line operations. In machine welding, specialized equipment performs the complete welding operation; however, the welding equipment must be monitored by a qualified operator who is responsible for positioning the steel components, starting and stopping the weld, setting the speed, and adjusting the controls.

Shielded Metal Arc Welding SMAW, also referred to as stick welding, utilizes a stick electrode— typically 9 to 18 in. (229 to 457 mm) long—a solid metal wire core that conducts electric current and provides filler metal for the joint. The metal core is coated with a material that provides arc stability and a shielding gas or a flux coating as the electrode is consumed. Shielding gases are needed to eliminate oxygen from the molten weld metal. The fluxing agents allow the molten metal to wet the surfaces of the base metal and remove impurities from the weld metal. SMAW is one of the most versatile weld processes and is widely used in tank construction. The equipment is relatively simple and portable. SMAW can be used in a wide range of positions and in areas with limited access. It is also less sensitive to wind than either SAW or FCAW. Shielded metal arc welding is limited to manual welding and, consequently, has one of the lowest deposit rates. The electrodes are relatively short and frequent stops are required to replace them. When present, slag must be removed before restarting. As a result, SMAW is the least efficient welding process for long production welds.

Submerged Arc Welding The electrode for SAW is a continuous bare wire inserted into a wirefeeding mechanism that automatically feeds the electrode toward the joint at a controlled rate. The weld is submerged in a blanket of granular flux that is continuously deposited ahead of and around the electrode. During welding, some of the granular flux is melted and serves the same purpose as the electrode coating in SMAW welding. This weld process can be used in a semiautomatic, automatic, or machine mode. SAW has one of the highest deposit rates because of the continuous wire feed. The process is limited by joint position and accessibility.

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C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s SAW is typically limited to the flat position for butt welds and the flat and horizontal positions for fillet welds. With specialized equipment to contain the flux, SAW may be used for lap and butt joints in the horizontal position. Moving the bulky wire-feeding mechanism for the continuous-feed electrode may make SAW a less desirable option for inaccessible areas. Good joint fit-up is also critical for SAW.

Flux Cored Arc Welding Like SAW, flux cored arc welding is a continuous-feed wire welding process. The electrode has a center core of flux encased in a tubular metal sheath. Two types of FCAW exist: gas shielded and self shielded (Fig. 6-6). Gas-shielded FCAW uses a gas envelope, usually CO2 or argon/CO2 , to protect the molten metal from the air. This method is not suitable for use when the weld cannot be protected from the wind. For self-shielded FCAW, shielding is provided by gas emitted by the flux as it vaporizes and by a slag blanket that covers the molten metal. Self-shielded FCAW is no more sensitive to wind than SMAW, so it is commonly used in the field. FCAW is a semiautomatic process in which a handheld weld gun is used. Deposition rates are typically higher than for SMAW but lower than for SAW. FCAW is versatile, in that it can be used in all positions for all the basic joint types. Like SAW, FCAW requires moving the wire-feeding mechanism; consequently, it may not be the best option for inaccessible areas in the field.

FIGURE 6-6 Shop flux cored arc welding.

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Chapter Six

Weld Procedure Specification and Procedure Qualification Record Constructors are required to develop weld procedure specifications (WPS) that define the welding parameters to be used in the fabrication and erection of the tank. Each WPS must be qualified in accordance with the rules in American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section IX, or American National Standards Institute (ANSI)/AWS B2.1, Standard for Welding Procedure and Performance Qualification. To qualify a WPS, the constructor welds test coupons and tests the specimens per ASME Section IX or ANSI/AWS B2.1. The weld parameters and test results are recorded in a document known as a procedure qualification record (PQR). The constructor is required to certify that he or she has qualified each WPS with a PQR. Constructors maintain standard WPSs and PQRs for weld parameters that are routinely used. The constructor may elect to use an ANSI/AWS standard welding procedure to justify a weld in lieu of performing an independent qualification. If this option is selected, it is necessary to comply with all the rules in AWS B2.1 that govern the use of the ANSI/AWS standard welding procedure.

Welder Qualification and Identification Welders in both the shop and the field are required to demonstrate their ability to perform acceptable welds. Weld testing shall be in accordance with ASME Boiler and Pressure Vessel Code, Section IX, or American Welding Society (AWS) B2.1 Standard for Welding Procedure and Performance Qualification. The tank constructor is responsible for testing all welders for the specific weld processes that the individual welder will use. Records of the testing dates and test results must be maintained by the tank builder. Each qualified welder is assigned a number, letter, or symbol that is stamped on the tank to identify the weld operator employed for each joint. The stamp is placed adjacent to and at intervals not exceeding 3 ft (0.9 m) along the weld. Alternately, the tank constructor may keep a written record of the welders employed on each joint and omit the stamping. This record must be certified by the tank constructor and included in the inspection report when specified by the purchaser.

Grinding Some grinding of welds may be required to ensure that the finished weld contour is suitable for cleaning and painting and will not be detrimental to the life of the coating. Grinding should be used to remove weld slag, weld spatter, burrs, and any sharp surfaces along welds. If the purchaser requires special grinding, it should be noted in the

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C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s contract specifications, and a recognized standard addressing weld profiles should be referenced to clearly define the extent of grinding required. When lapped plates are joined with fillet welds that are less than the full thickness of the plate, the exposed sharp edge of the plate should be removed by grinding. This will minimize the potential for a paint failure at the edge of the plate.

Construction Scheduling Issues In addition to the production capacity and workload of the tank constructor, construction schedules are affected by the style and size of the tank, the availability of material, the time of year, and daily weather conditions. The purchaser may specify either the number of weeks to completion after award of contract or a set calendar date for completion, or the purchaser may allow the tank builder to propose a scheduled completion. If a specified completion date is critical to the owner, the purchaser may specify liquidated damages to be charged on a daily basis if the work is not completed on time. If the purchaser chooses to set a construction schedule, consideration should be given to mill delivery time to allow the constructor to use material from the mill. In northern climates, a schedule should be specified that allows painting to be performed at an appropriate time of year. Tanks with very short construction schedules are typically purchased at a premium. Weather can significantly influence the field schedule. Wind, extreme temperatures, rain, snow, and sleet can affect what work can be performed and how productive the crew is. Even moderate winds can make it unsafe to pick up and place steel plate. Weather and temperature conditions must also be appropriate for welding. Welding is not permitted when the parts to be welded are wet from rain, snow, or ice, or during periods of high wind, unless the welder and the work are properly protected. No welding is allowed when steel is wet. The protection is typically an enclosure to block the wind. Welding is not allowed if the base metal temperature is lower than 32◦ F (0◦ C) unless the base metal is preheated to at least 100◦ F (38◦ C) through the thickness and maintained for a distance along the weld of four times the thickness of the parts to be welded. If base metal temperatures fall below 0◦ F (−18◦ C), welding is not recommended. If welding is performed, low-hydrogen electrodes or low-hydrogen processes must be used, and the base metal must be preheated to 200◦ F (93◦ C) in accordance with AWWA D100.

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Chapter Six Unless protection is provided, concrete should not be placed in rain, sleet, snow, or extreme temperatures. American Concrete Institute (ACI) 306.1 and 305.1 provide guidelines for concrete placed in cold and hot weather.

Site Issues The engineer’s drawings typically show the site layout superimposed on a topographic drawing. This gives some indication of the relative slope of the site and of potential access problems. If, during the bid stage, questions arise regarding the site, a site visit may be warranted.

Access The owner should provide a suitable right-of-way for access from the nearest public road to the tank site. The access should be able to handle a semitrailer tractor rig with a trailer that is 53 ft (16 m) long and that weighs 80,000 lb (36,287 kg) under ordinary weather conditions. Side clearance needs to be adequate to accommodate the maximum shipping width for the job. The access road should be free from underground and overhead obstructions that could be damaged by the truck traffic. A minimum vertical clearance of 14 ft (4.2 m) is required.

Site Size Final property lines should be located sufficiently far from the foundation footprint to permit construction operations. During construction, additional clearance is required for steel delivery, storage, staging, and subassembly. If a permanent site of adequate size is not available, the owner should provide an adequate temporary construction easement. As a minimum for the construction operations, a site clearance from the center of an elevated tank to the site limits should be equal to the height of the tank. For a ground tank, it is preferable to have at least 20 ft (6 m) clear around the entire tank so that a crane can be used around the full circumference of the tank. The site should also be big enough to permit abrasive blasting and painting without impacting neighboring property, both after initial construction and during future recoating operations. Clearance requirements between the tank and the neighboring property vary with the prevailing wind conditions, type of paint application, and consequence of damage. Sites should be evaluated on a case-by-case basis, but as a general rule, a clearance of approximately 100 yd (91.5 m) is suggested. If adequate clearance cannot be provided, it may be necessary to shroud the tank during initial and future painting operations. Shrouding the tank is costly and should be avoided if possible.

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Drainage The tank site should have good drainage during construction. Storage, staging, and subassembly areas should be free from standing water. For sites with poorly draining soils, the bearing surface for a shallow foundation should be protected from becoming saturated prior to concrete placement.

Power Lines Overhead or buried power lines present a significant safety risk for tank construction. Sites having power lines within 40 ft (12 m) of the tank or tank foundation are unacceptable.

Security Access to the tank should be blocked when the tank is left unattended. If the location is remote or subject to frequent vandalism, additional measures to ensure site security may be required. The additional measures may include fencing and full- or part-time security.

Power During Construction Power Supply The purchaser should indicate whether electrical power is available at the site. If power is available, the purchaser should indicate the voltage and whether it is direct current or alternating current (if alternating current, what cycle and phase). The specifications should define who will furnish the power to the site and who is responsible for the associated costs. Tank constructors frequently provide their own power supply in the form of generators.

Power Requirements Power requirements in the field vary depending on the types of tools that will be used on the job. If the purchaser is furnishing power to the site, the power requirements should be coordinated with the tank constructor.

Construction of Welded-Steel Ground Water-Storage Tanks Anchorage and Grout If the tank is not anchored, the interface between the tank bottom and the concrete foundation can be either grouted or filled with the placement of asphalt-impregnated cane fiberboard. If the tank is anchored, the use of grout is recommended, since the fiberboard may deteriorate over time. This deterioration would cause vertical movement of the tank, which would require subsequent tightening of the anchor bolts.

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Chapter Six

Tank-Bottom Construction The bottom of a ground-supported reservoir or standpipe is essentially a nonstressed membrane, the purpose of which is to contain the product inside the tank and transmit the water-bearing load directly to the foundation. The minimum thickness of the bottom plate is 1/4 in. (6.3 mm); it may be thicker if a corrosion allowance is specified. The tank bottom should be crowned up from the shell to the center with a minimum slope of 1 in. (25.4 mm) vertical to 10 ft (3 m) horizontal.

Layout The typical plate layout for bottom plates is a “rect-andsketch” layout, which refers to rectangular plates with sketch plates at the outside cut to a radius. The outside radius must be a minimum of 2 in. (50 mm) outside of the shell. Additional projection may be provided to compensate for shell out-of-roundness and weld shrinkage in the bottom welds. With a lap-welded bottom, there will be three plate laps at the corners of the rectangular plates. Three plate laps must be at least 1 ft (0.3 m) from the shell. Refer to Fig. 6-7 for an example of a rect-and-sketch bottom layout. Annular ring An annular ring may be required under two possible scenarios. First, if the shell uplift due to seismic overturning is large, a thickened annular ring will increase the uplift resistance of the shell. This strategy is used to eliminate tank anchorage. For this circumstance, butt-welded sketch plates may be substituted for an annular

1 ft. (0.3 m)

s

Cut

iu rad

minimum

1.5 in. (38 mm) typical

1 ft. (0 .3 m ) minim um

(1 in. [25.4 mm] minimum)

Inside shell

FIGURE 6-7 Typical rect-and-sketch layout for bottom plates.

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) 6m . (0. 2 ft nimum ll mi e she d insi 1.5 in. (38 mm) typical (1 in. [25.4 mm] minimum)

Shell ) m n m ctio 1 e j (5 o n. pr 2 i ide s t ou

Annular plate

FIGURE 6-8 Tank bottom with annular ring.

ring. Second, if the tank is designed in accordance with AWWA D100 Section 14 and is greater than 150 ft (45.7 m) in diameter, an annular ring is required. If an annular ring is provided, the minimum inside projection shall be 2 ft (0.6 m) or the minimum width required for seismic uplift resistance, whichever is less. The bottom plate is lap welded to the annular ring and has a rect-and-sketch layout. Refer to Fig. 6-8 for an example of a tank bottom with an annular ring.

Welding sequence The welding sequence for the bottom plates shall minimize out-of-plane distortion. A general sequence for bottom welding is described as follows:

r r r r

Weld the sketch plate to sketch plate joints. Weld the rectangular short side joints. Weld the rectangular long side joints. Weld the rectangular plate to sketch plate welds.

Lap welded versus butt welded Bottom plates can be welded by either lap welds or butt welds. For bottom plate thicknesses up to 3/8 in. (9.5 mm), the plates are typically lap welded from the top side only. The weld is a full-thickness fillet weld and, for thicknesses equal to or greater than 5/16 in. (8 mm), the fillet is typically a two-pass weld. If it is necessary to seal the underside of the bottom or if the bottom

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Chapter Six FIGURE 6-9 Breakdown of lapped area beneath shell and projected outside of tank.

Shell plate

Bottom plate

plate thickness is greater than 3/8 in. (9.5 mm), butt welding of the bottom plates is appropriate. For lap-welded bottoms, the lapped area that is beneath the shell and projected outside of the tank must be “broken down.” The purpose of the breakdown is to provide a smooth transition at the lap on the top side so that there will be no gap at the shell-to-bottom connection in the region of the lap. Refer to Fig. 6-9 for an illustration of a breakdown. Annular ring splice welds must be butt welded. The welds may be either single butt welds with backup bars or double butt welds. Commonly, the fabrication shop will subassemble annular ring sections with double butt welds, and the subassemblies will be welded in the field with single butt welds. Butt-welded bottom plates can be either a one-sided weld with a backup bar or a double butt welded. Single butt welding is the preferred method since the bottom can be laid out and the welding performed from the top side. Double butt welding is difficult for large tanks due to the inaccessibility of the underside of the bottom. For small tanks, however, the initial weld pass can be performed downhand and the bottom can be flipped over so that the welding can be completed down-hand.

Shell-to-bottom junction The connection of the shell to the bottom plate shall be a continuous fillet weld on both sides of the shell. Table 18 in AWWA D100-05 gives the minimum size of the fillet welds to be used on the basis of the thickness of the shell plate. If the fillet weld is 5/16 in. (7.9 mm) or larger, the weld shall be two-pass minimum. The weld should be inspected for watertightness using dye penetrant, penetrating oil, or diesel fuel. The inside fillet weld is completed first, and indicator is sprayed on the weld. If any indicator is visible outside of the shell after a wait period, a leak is indicated and should be repaired. Once there are no indications of leakage, the outside weld can be completed.

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C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s

Shell Construction Crane versus jacking Normally, there are two basic methods for erecting the tank shell. The more common method is to start from the bottom ring and use a crane to place each individual shell plate for each successive ring until the shell is complete. However, for tall tanks a method using hydraulic jacks may be more economical. Using this method, the top two shell rings and roof are erected on temporary jack stands. Once this is complete, the shell is jacked using hydraulics, and the next shell ring is placed. This process is repeated until the shell is complete. This method reduces the crane requirements since there are no high picks. Also, temporary scaffolding for the shell is not required since all shell erection and welding activities are performed at ground level. The shell is the critical component of a storage tank. It is the primary stressed membrane that contains the liquid. Therefore, great care must be exercised in laying out, fitting, and welding the shell. Listed here are general steps for layout and fit-up of a tank shell. This procedure varies among contractors; however, the general steps are the same.

r Check the elevation of the tank bottom. r Scribe the radius of the inside of the shell on the bottom. This will be used as a guide for setting the first ring.

r Mark the chord dimensions for the first ring. r Install erection nuts or lugs on the bottom that follow the outside radius of the first ring.

r Set the first plate starting at the first chord mark and following the circular scribe mark.

r Set the second plate and use fit-up gear to align the vertical r r r r

seam for welding. Set the remainder of the first ring plates and check for level. Weld the vertical seams. Weld the first ring to the bottom. Hang the second and subsequent rings using fit-up gear on the horizontal and vertical seams.

Shimming and the tub ring After the bottom plate is laid, the first shell ring, or “tub ring,” is set. Care must be taken in setting the tub ring, since the roundness of the tub ring is the basis of whether the rest of the shell will be round. An essential part of making the shell round is that the tub ring must be level. If the tub ring is not level, tank roundness is difficult to achieve. The tub ring is leveled by using shims between the bottom and the foundation.

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Chapter Six Wind stability During shell erection, the shell is susceptible to wind loads. The shell is designed to be stable and resist buckling due to wind when the tank is complete. However, during construction the shell can buckle easily, even during moderate winds. Therefore, the shell must be braced to prevent a “blow-in” incident. One method of bracing the shell is with the erection scaffolding. The scaffolding is normally set 3 to 4 ft (0.9 to 1.2 m) below the top of the ring being erected and consists of brackets and scaffold boards or planks spanning between the brackets. If the boards overlap at the brackets and are tied down securely, the scaffolding itself acts as a ring stiffener on the shell. Because of this phenomenon, incomplete scaffolding is normally not allowed to be left overnight. Partial- versus complete-penetration welding The shell vertical welds are always complete-penetration welds. The horizontal welds may be either complete-penetration or partial-penetration welds. When the shell thickness of the thinner of the two plates being joined is greater than 3/8 in. (9.5 mm), the horizontal weld can be a partial penetration. The finished weld must have at least two-thirds the strength of a complete-penetration weld. Partial-penetration welds are not allowed in the shell plates for Section 14 designs. Weld clearances Weld clearances for shell vertical joint offset, permanent attachments, and shell penetrations should meet the requirements of AWWA D100 and good industry practice. Section 14 of AWWA D100 prescribes the requirements for weld clearances. The base code does not describe any weld clearance requirements; however, good practice indicates the following weld clearances:

r Vertical shell plate offset = 12 in. (305 mm) minimum r Permanent attachments = 3 in. (76 mm) (horizontal) and 6 in. (152 mm) (vertical)

r Shell penetrations = 3 in. (76 mm) Construction openings A construction opening in the shell is normally provided to allow easy access to the interior of the tank. This opening is usually in the form of a short plate that is removed in the first shell ring. The short plate left out of the shell, called a door sheet, is 6 to 12 ft (1.5 to 3.6 m) wide and has a height equal to the width of the first shell ring. If a crane must be driven inside of the tank for roof erection, a taller opening is usually required. This is accomplished with a first and second ring door sheet, which may or may not include the full height of the second ring. Temporary stiffening must be provided around the door sheet to bridge the vertical loads during tank erection.

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C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s Shell penetrations Shell penetrations are required for access to the interior of the tank and for an overflow to prevent overfilling. The inlet and outlet piping may go through a shell penetration but more commonly goes through the tank bottom. A minimum of two manholes are required for access. The penetrations have a neck plate that is welded to the shell, and additional reinforcing in the form of a circular reinforcement plate may be required. Post-weld heat treatment Post-weld heat treatment is only required for shell penetrations that are 12 in. (305 mm) in diameter or greater in shell plate that is thicker than 1 in. (25.4 mm). The penetrations should be prefabricated in the shell and stress relieved before shipment. This requirement applies only to Section 14 designs.

Roof Construction Roof configuration can be either supported on structural framing or self-supporting. The self-supporting roofs can be unstiffened, or stiffeners can be welded to the roof plate.

Method of roof erection—crane versus air raised For tanks with structurally supported roofs, the typical method of construction is to use a crane to lift the various components into place. The roof framing is erected after the bottom and shell are in place, and the roof plate is placed after the framing is complete. For tanks with self-supporting roofs, there are more options. The roof can be built in place using a crane and temporary support for the roof or the roof can be built on temporary supports outside of the tank and the entire roof can be lifted into place. The latter method is advantageous for tall tanks, roofs requiring seal welding, and in situations where a crane with enough capacity can be used economically. Another option is to have the roof erected on the floor of the tank and to lift it into place after the shell erection is complete. This is accomplished by sealing the outside edge of the roof to the shell and pressurizing the underside of the roof to lift it to its final position. This method is economical for large-diameter tanks that are relatively tall. Surprisingly, the pressure required to air-raise a roof is on the order of a water column of 3 to 6 in. (76 to 152 mm). The roof can be raised using high-velocity fans bolted to the shell manholes. Subassembly For self-supported roofs, to minimize the number of crane picks and reduce the need for welding in place, the field crew may elect to subassemble some of the roof sections. This may also decrease the amount of time the crane needs to be on-site, therefore reducing costs. Roof-to-shell junction The roof-to-shell junction can be configured in several ways. For cone- and dome-type roofs, an angle can be either

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Chapter Six butt or lap welded to the top of the shell. The roof plate laps onto the top of the angle. This arrangement can be advantageous since fabrication and erection variances can be tolerated. The top angle is used to aid in keeping the tank shell round. As an alternative to using an angle, a bar may be used. A double-curved transition may be used for either a supported or unsupported roof. This type of transition can be more visually pleasing for taller tanks and any tank for which aesthetics are important.

Seal welding Seal welding may be specified to reduce rust bleeding from the inaccessible plate lap areas. If seal welding is required, the type of roof must be considered. If the roof plate is supported on structural members, the surfaces mating the tops of the structural members to the underside of the roof plate will also be inaccessible. Seal welding the roof plate lap welds will solve only part of the problem. However, if the roof framing is to be seal welded to the roof plate, the tank designer must consider the effects of thermal expansion and contraction caused by temperature differentials on the roof framing. Also, the amount of welding required to seal all of the roof framing might induce additional weld distortion in the roof plate. If the roof is self-supported, any framing will be welded to the roof plates by design. Ponding For supported cone roofs, the minimum roof slope is a 3/4in. (19-mm) rise in a 12-in. (305-mm) run. This is a very shallow roof slope and it may therefore produce ponding if the roof plate is built with excessive distortion. In many local jurisdictions, ponding is not allowed by law. The easiest solution to potential ponding is to increase the roof slope.

Construction of Elevated Steel Water-Storage Tanks Method of Erection—Crane Versus Derrick The constructor should select the type of crane to be used to construct an elevated water tank, basing the decision on the tank geometry, schedule, equipment availability, and cost. Elevated tanks are frequently erected using a derrick. A derrick is a fixed-mast, guyed crane that is positioned at the center of the tank. Placement of a derrick is optimal for construction of circular tanks, because the boom is capable of a full 360-degree swing. On the basis of years of construction experience, tank constructors have developed guyed derricks specifically designed for tanks. Mobile or tower cranes are also options for erecting an elevated tank in the field. Because of rental expenses, mobile or tower cranes are usually limited to use on a smaller-capacity tank of limited height

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Construction of Welded-Steel Water-Storage Tanks

C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s with a short field-construction schedule. Use of a mobile crane requires more site clearance around the tank. Whether using a derrick, tower crane, or mobile crane, adequate support is critical. For either a derrick or stationary tower crane, supplemental groundwork or a support pad should be provided if required before setting the crane. A mobile crane requires a reasonably level surface around the tank and site conditions capable of supporting the loaded crane.

General Requirements Field subassemblies Shipped plates are frequently subassembled on the ground in the field. The subassemblies are planned on the basis of the maximum weight and size feasible to lift and fit into place. This erection practice allows the welding to be performed close to the ground in more favorable positions. Construction aids Specialized erection equipment developed by the tank constructor is used to aid in lifting, fitting, aligning, and spacing plates with the appropriate weld gaps. Maintaining the proper gaps, alignment, and overall dimensional accuracy is critical for subsequent plate placement. Some construction aids may be permanently left in place, while others are temporary and are removed after the plate is secured. Temporary attachments need to be removed without damaging the plates, and the remaining weld should be chipped or ground smooth before painting. Dimensional accuracy is maintained by consistently checking dimensions. Levels and transit levels may be used to verify elevations, check angles, and to verify that a component is plumb. Access to the tank Safe access to the tank and tower is required for welders, inspectors, and painters. Temporary scaffolding is commonly used in conjunction with permanent and temporary ladders for access. Aerial lifts such as a boom lift or scissor lift may also be used. A work basket or chair hung from a spider line is another frequently used option. This option requires a secure anchor point above the area to be accessed. Regardless of the method of access, fall protection needs to be considered and special measures taken to ensure the safety of the workers.

Fluted-Column-Style Tank The fluted tower rests on butt-welded base plates that are seated on shims and fixed to the foundation by the anchor bolts. It is critical that the base plate is level before erecting the fluted plates. After the tank has been erected, but before it is filled with water, the space between the base plate and foundation is grouted.

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Chapter Six FIGURE 6-10 Erection of tower, fluted-column-style tank.

The vertical joints in the fluted tower are lap welded, and the horizontal joints are butt welded. Water-bearing plates in the tank are welded with full-penetration butt welds. The roof is typically lap welded on the top side only. When specified by the purchaser, the overhead laps in the roof are also seal welded (Figs. 6-10 to 6-12). Access to the inside of the fluted tower is required at all times and is typically provided at the opening for the overhead door. A bottom manhole provides access to the inside of the tank. Tank constructors have developed specialized equipment to enable safe access to difficult areas such as the outside of the cone or the underside of the roof. The constructor must be cognizant of the stability of the structure at all times, but especially when the structure is left overnight. Regardless of what component the crew is erecting, the crew should not leave the tank unattended until all the plates in a given ring are in place and adequately secured. Provisions should be taken to stiffen unfinished sections of the tank in case of high winds. This may include providing stiffening or continuous scaffolding at the upper limits of construction or guying the structure to the ground.

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FIGURE 6-11 Erection of cone plate, flutedcolumn-style tank.

FIGURE 6-12 Erection of cylindrical shell plate, flutedcolumn-style tank.

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Chapter Six

FIGURE 6-13 Erection of spherical plate, pedestal-style tank.

Pedestal-Style Tank Complete-penetration butt-welded construction is used for all the components of the pedestal tank and tower except the roof. The roof is usually lap welded on the top side. At the request of the purchaser, the underside may also be seal welded or, alternately, the roof may be butt welded (Figs. 6-13 and 6-14). The base cone of the tower of a pedestal-style tank sits on a thick base plate that is welded with complete-penetration butt welds, set on shims, and fixed to the foundation by the anchor bolts. As with the fluted-column-style tank, it is critical that the base plate be level before erecting and welding the base cone. The base cone is fillet welded to the base plate. As with the fluted-column-style tank, grout is placed under the base plate after the tank is completely erected but before it is filled with water.

Multicolumn-Style Tank Erection for a multicolumn-style tank typically begins with one bent in the first panel, consisting of a pair of columns, one bolted strut, and loosely connected cross-bracing that will either be welded or

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Construction of Welded-Steel Water-Storage Tanks

C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s

FIGURE 6-14 Erection of roof plate, pedestal-style tank.

bolted to the columns. Base plates are welded to the bottoms of the columns in the first panel. The base plates of the first bent are set on shims placed on the foundation pedestals and are fixed using the anchor bolts. Additional bents are erected by sequentially adding a column, strut, and cross bracing around the tower. After all the bents are in place in the first panel, the cross braces are adjusted to length, as required, to ensure that the panel is square and true before proceeding to the next panel. Subsequent panels are erected in a similar manner with the columns of the upper panel welded to the lower columns. Typically, the portion of the tank that is welded to the upper column is welded to the column before the tank is erected. After the tower is complete, the intermediate plates are fit-up and welded with complete-penetration butt welds. It is critical to maintain dimensional accuracy of the tower for proper fit-up of these plates. The tank joints between water-bearing plates are joined with complete-penetration butt welds. The roof plates may be lap welded with or without seal welding, or they may be butt welded. An alternate construction sequence is to construct the upper bents and tank without the lower columns in place. The advantage of this construction method is that the tank and upper tower can be constructed and painted when closer to the ground. After this portion of the tank is complete, it is lifted by cranes and the lower columns are

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Chapter Six FIGURE 6-15 Multicolumn-style tank.

set underneath (Fig. 6-15). The structure is then lowered to its proper height and seated on the columns. After the tank is complete, but before it is filled, the final tightening or welding of the cross-braces is done. At this stage, the grout is also placed under the base plates.

Construction of Composite Elevated Tanks Concrete Support Structure The concrete support structure for a composite elevated tank is cast in place. Jump forms are commonly used. Constructors of this style of elevated tank have specialized forms conforming to their standard geometries. Forms have horizontal and vertical rustications built into the exterior face to provide architectural relief and help mask form panel joints and construction joints (Fig. 6-16). Wall reinforcing is placed and tied before the forms are installed. Special reinforcing is required around the overhead, mandoor, and other significant openings. After the rebar is placed, the forms are set and prepared to receive concrete. Large openings in the tower are blocked out, while smaller openings may be cut or drilled after concrete placement. The concrete is delivered to the tops of the forms

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Construction of Welded-Steel Water-Storage Tanks

C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s FIGURE 6-16 Tower construction, composite elevated tank.

either by pumping or by bucket. Hand compaction and power vibration are performed in accordance with ACI 309 to ensure proper compaction, minimize segregation, eliminate air voids, and to ensure close contact with the reinforcement and forms. After the first ring is poured and has had time to cure, the forms are removed. The sequence of placing reinforcement, jumping the forms, and pouring the concrete is repeated until the support tower is complete. Forms are set for the dome and ringbeam or the flat slab. Similar to the tower forms, the tank constructor will have developed specialized forms for their geometry and construction practice. Reinforcement is placed and tied and embedments secured before the concrete is poured.

Concrete Mix The concrete mix should be suitable for the method of placement and the weather conditions. The proportions of the mix should be adjusted to provide adequate workability and the proper consistency for placement. For each tank, the material should be from a consistent source and the mix design number verified upon delivery. The arrival of trucks should be sequenced to sustain a pour without long delays. Retempering of the concrete should be controlled to maintain the mix

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Chapter Six FIGURE 6-17 Roof erection, composite elevated tank.

parameters. Concrete testing should be in conformance with ACI 318, Building Code Requirements for Structural Concrete.

Welded-Steel Container for the Composite Elevated Tank Welding for the cone, shell, and roof of a composite elevated tank is similar to that for a fluted-column tank (Fig. 6-17).

Method of Erection—Hoisted Versus Crane or Derrick Construction methods for the steel container on the composite elevated tank and the fluted-column tank are similar if a crane or derrick is used. One additional construction technique, hoisting, has been successfully employed for composite tanks. This method allows the constructor to erect and weld the cone, shell, and upper cone roof transition as a complete unit around the concrete shaft at grade. After the welding is complete, the container is hoisted into position using a series of cables and hoists. It is welded into place, and the roof is installed. (Fig. 6-18a and b).

Liner Plate An interior liner plate is placed over the dome or flat slab. The liner is lap welded on the top side only. For tanks with a dome, formed liner plates may be used and constructed so that the liner lies directly on

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Construction of Welded-Steel Water-Storage Tanks

(a)

(b) FIGURE 6-18 (a) CET hoisted tank erection as the tank is being raised. (b) CET hoisted tank erection with the tank in the final position.

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Chapter Six FIGURE 6-19 CET liner plate formed to fit dome with derrick-erected cone and shell plates.

the dome. Alternately, unformed steel liner plates that do not match the shape of the dome may be used if the space between the plate and the dome is completely filled with flowable grout after welding (Fig. 6-19).

Inspection and Testing Foundation Tolerances Before construction is started of the tank’s support structure, the foundation should be checked to verify that it is within specified tolerances. AWWA D100 provides minimum requirements for foundation tolerances for multicolumn tanks, single-pedestal tanks, and groundsupported flat-bottom tanks. Minimum tolerances for anchor bolts need to be maintained for installation of the base plates for elevated tanks and anchor chairs for ground storage tanks. AWWA D100 anchor tolerances for all tank styles are as follows: Anchors should be within ±1/4 in. (±6.3 mm) of the theoretical location and plumb within 1/8 in. in 12 in. (19 mm in 305 mm). The anchor projection above the top of the foundation should be within ±1/4 in. (±6.3 mm).

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Construction of Welded-Steel Water-Storage Tanks

C o n s t r u c t i o n o f We l d e d - S t e e l Wa t e r - S t o r a g e Ta n k s Foundations and anchors that are not within tolerances should be identified and addressed before the tank is erected. A design professional representing the tank constructor should evaluate whether the structural capacity of the foundation has been compromised and should provide details for remedial action when required.

Tolerances for Concrete Support Structure Dimensional tolerances for the concrete support structure of a composite tank are outlined in ACI 371R and repeated here. Fabrication and placement tolerances for rebar should be in compliance with ACI 117. Dimensional tolerances for the concrete support structure are as follows: Variation in thickness r Wall: −3.0 percent, +5.0 percent

r Dome: −6.0 percent, +10.0 percent

Support wall variation from plumb r In any 5 ft (1.5 m) of height: 3/8 in. (9.5 mm)

r In any 50 ft (15 m) of height: 1.5 in. (38 mm) r Maximum for total height: 3.0 in. (76 mm)

Support wall diameter variation r 0.4 percent (not to exceed 3.0 in. [76 mm]) Dome tank floor radius variation r 1.0 percent Level alignment variation r From specified elevation: 1.0 in. (25.4 mm)

r From horizontal plane: 1/2 in. (12.7 mm)

The offset between adjacent pieces for formwork facing material should not exceed the following:

r Exterior exposed surfaces: 1/8 in. (3 mm) r Interior exposed surfaces: 1/4 in. (6.3 mm) r Unexposed surfaces: 1/4 in. (6.3 mm)

The finish tolerance of troweled surfaces should not exceed the following when measured with a 10-ft (3-m) straightedge or sweep board:

r Exposed floor slab: 3/8 in. (9.5 mm) r Tank floors: 3/4 in. (19 mm) r Concrete support for suspended steel floor tank: 1/4 in. (6.3 mm)

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Chapter Six Maximum Diameter (ft[m])

Radius Tolerance (in. [mm])

40 [12]

± 0.5 [±12.7]

150 [45.7]

± 0.75 [±19]

ambient velocity ua ). Regime (5) is usually—but not always—the final regime (Jirka 1999). Figure 9-21 shows a model of a positively buoyant turbulent jet. The buoyant jet is a flow phenomenon with free turbulence. It represents a gradually evolving flow along its axis and thus exhibits boundary-layer characteristics with its possibilities for mathematical simplification including self-similarity techniques (Schlichting 1968). However, because of the variety of forcing elements, buoyant jet motions are in general not self-similar (Jirka 1999). They are self-similar

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353

Operation

354

Chapter Nine

FIGURE 9-21 Model of positively buoyant turbulent jet.

only in five possible asymptotic regimes in which they have an invariant internal force balance, invariant turbulence, and entrainment properties. In between these regimes, the buoyant jet properties are variable and cannot be scaled uniquely by local jet parameters (Jirka 1999). The local Reynolds number (Re) in the case of a turbulent jet in a quiescent reservoir is approximately equal to the jet exit Reynolds number Rej = d j U j / (d j is the jet diameter, U j is the jet velocity, and  is the kinematic viscosity of water). If the jet momentum flux Mj U j =  j (d j /2)2 U j 2 ( j is water density and Mj is the mass of water in the jet) is held constant as the jet diameter decreases and its velocity increases, the jet approaches a point source of normal momentum, which generates a counterrotating vortex pair (Shan and Dimotakis 2001). At low Reynolds numbers, in particular, tertiary and sometimes quaternary vortices are formed such that the vortices no longer have equal circulation, and each vortex could have a different induced velocity. In that case, the induced vertical velocities would be substantially smaller, and the overall mean trajectory would be shallower. At high-Re transverse jets, the counterrotating vortex pair is the dominant structure and the primary mechanism for entrainment of free stream fluid. The vortex circulation is a decreasing function of downstream distance decaying through viscous diffusion (Shan and Dimotakis 2001). In other words, high-Re (more turbulent) inlet sources would result in a flow with higher vertical velocities and a vortex pair as the main source of water entrainment. As will be explained later, entrainment is the major mixing mechanism. The higher vertical velocity means that the influent would reach the water surface faster, resulting in even less entrainment of the ambient water in the tank. Hence, the high level

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Operation

Operation of micromixing caused by the high Reynolds number results in more homogenous mixing in the vicinity of the inlet, but less water is mixed. Experimental data indicate that the scalar gradients are steeper in the horizontal directions than in the vertical directions. Anisotropy of the transverse jet’s scalar field is in contrast to the far field isotropy for axisymmetric jets discharging into a tank (Shan and Dimotakis 2001). Scalar mixing in the transverse jet is enhanced by increasing the Reynolds number. In the case of turbulent jets discharging into a tank, the concentration’s probability distribution functions lose their peak at the highest Reynolds number (Shan and Dimotakis 2001). This means that when discharging into a reservoir or tank in which there isn’t a crossflow, mixing is decreased in the far field at high Reynolds numbers (or influent velocities). The minimum Reynolds number for which turbulent mixing can be considered as fully developed is approximately 10,000 (Nathman et al. 2004). On the basis of the previous analysis, tank mixing systems need to reduce the Reynolds number to slightly above 10,000 by reducing the velocity through each inlet. An efficient way to accomplish this is to have more inlets to spread the flow. Having more inlets results in a larger volume of water involved in near-field micromixing, and it introduces the influent at various locations in the tank, further enhancing the distribution of the micromixing effect. Flow-dependent mixing is explained by noting that turbulent mixing is essentially a three-stage process (Shan and Dimotakis 2001): Entrainment: Engulfment of irrotational (ambient) flow into the turbulent flow region, or macromixing. Stirring: Kinematic motion responsible for creating interfacial area between species. Molecular mixing: Diffusive mixing on the molecular scale, or micromixing. The balance among these three stages determines the probability distribution function of the mixed water. Nevertheless, the mean concentrations are a measure of entrainment rather than of molecular mixing. Hence, the transverse jet entrains less ambient fluid than the ordinary turbulent jet. Transverse jets homogenize the entrained fluid more thoroughly. This indicates that for transverse jets, there is more stirring and molecular mixing; for ordinary jets, there is more entrainment (Shan and Dimotakis 2001). Increases in water elevation in tanks with mixing systems result in little transverse motion. Transverse motion is especially negligible in a large tank unless it is a pass-through tank with considerable and independent inflow and outflow rates. Therefore, the influent jets need to entrain larger volumes to mix the whole tank. To maximize entrainment, the inlets should be located such that the inflow engulfs

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Chapter Nine the maximum amount of volume possible. This is done by creating sheet flows at different elevations that intersect and or interact in the far field. This is further enhanced if the influent is buoyant, as will be shown later, whether positively or negatively due to temperature difference between influent and ambient water.

Turbulent Jet Mixing Efficiency Turbulent jet mixing efficiency is determined by the behavior of the mixed influent/ambient water interfaces. Knowledge of the dynamics of the interfaces is crucial for physical descriptions, predictions, and control of the mixing efficiency (Nathman et al. 2004). Although it has long been recognized that large-scale entrainment is important for mixing, one must understand that entrainment alone sets an upper bound only on mixing efficiency. In other words, knowledge of the growth rate of a turbulent shear flow is not sufficient to deduce mixing efficiency (Nathman et al. 2004). The crucial point is that the volume enclosed by the outer interfaces (i.e., the interfaces between the influent water and the ambient tank water)—rather than the interfacial surface area—determines mixing efficiency. Hence, it is the large-scale dynamics of the outer interface that provide the dominant contributions to the mixing efficiency (Nathman et al. 2004). On the basis of high-resolution measurements in the far field of fully developed round jets, it was found that large-scale folding of the interfaces, as opposed to the small-scale wrinkling of the interfaces, provides the dominant contribution to mixing efficiency (Nathman et al. 2004). A discharge with no buoyancy is referred to as a nonbuoyant jet or pure jet. A release of buoyancy only (no initial momentum) is called a pure plume. A discharge with both momentum and buoyancy is called a buoyant jet or forced plume. Positively buoyant flows are defined where the buoyancy force acts vertically upward against the gravity force; negative buoyancy is defined as acting downward in the direction of the gravity force (MixZone 2005). Dilution in turbulent buoyant jets is caused by entrainment of surrounding ambient water into the influent water jet. Entrainment is a turbulent process caused by shear stress between the discharge flow and the surrounding ambient water (MixZone 2005). As briefly stated earlier, to improve mixing efficiency, a tank mixing system would have to engulf or entrain as large a volume of water as possible during inflow. To take maximum advantage of this mixing phenomenon, a tank mixing system must have inlets creating sheets of flow at different elevations. The interfaces should then fold over each other for optimum mixing efficiency. The directed momentum of the created sheets of flow would create more folding if the inlet orientations and design allow for the energy to be converted to directed momentum rather than excessive stirring turbulence. This process

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Operation requires less throttling at each inlet and multiple inlets at the proper elevations. The locations of the inlets should be such that boundary-layer attachments occur only as part of the entrainment scheme and do not disrupt it.

Mixing Regions The hydrodynamics of an influent continuously discharging can be conceptualized as a mixing process occurring in two separate areas. In the first area, the initial jet characteristics of momentum flux, buoyancy flux, and outfall geometry influence the jet trajectory and mixing. This area, called the near field, encompasses the jet subsurface flow and any surface or bottom interaction (or, in the case of a stratified ambient, terminal–layer interaction). The mixing zone is the part of the near-field area in which the initial dilution of a discharge occurs. Many hydrodynamic definitions of mixing zones include both nearfield mixing and boundary-interaction processes (MixZone 2005). In this area, mixing system design can usually affect the initial mixing characteristics through appropriate manipulation of design variables. In particular, designs with dynamic bottom attachments should be avoided (MixZone 2005). Dynamic plume attachments occur when the discharge flow interacts strongly with a boundary in the near field. Such near-field boundary interactions present the possibility of high influent concentrations near the discharge (MixZone 2005). Often, near-field attachments are avoidable with proper design of the mixing system. This flow also exhibits a subsequent buoyant liftoff and an unstable near field (MixZone 2005). Two types of attachment are typically found: wake attachment forced by the crossflow and Coanda attachment forced by the entrainment demand of the influent jet itself. A physical description of these processes is given below (MixZone 2005). In wake attachment, the presence of the discharge structure and the jet influx interrupts the ambient velocity field and causes a recirculation area in the wake downstream from the discharge (MixZone 2005). A Coanda attachment is a rapid dynamic attachment that occurs when a jet discharges close to a nearby parallel boundary. This process is referred to as a Coanda effect. It occurs because of the entrainment demand of the jet flow at its periphery. If a boundary limits the approach flow of ambient water, then low-pressure effects cause the jet to be deflected toward that boundary, thereby forming a wall jet. Thus, the mixing process of Coanda-attached flow is governed by the dynamics of the wall jet (MixZone 2005). Figure 9-22 shows a negatively buoyant turbulent jet flow with wall attachment. This implies that a tank mixing system discharging very close to the bottom or surface may cause flow attachment to the bottom or a buoyant film at the top with reduced mixing.

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FIGURE 9-22 Negatively buoyant turbulent jet flow showing wall attachment. (Source: MixZone.)

Boundary interactions occur when the flow contacts the surface, bottom, or sides or forms a terminal layer in a density-stratified ambient environment (Fig. 9-23). Boundary interactions also determine whether mixing is controlled by stable or unstable conditions at the discharge source (MixZone 2005). Boundary interaction generally

FIGURE 9-23 Turbulent jet flow into density-stratified tank model. (Source: MixZone.)

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Operation

Operation provides the transition from near-field (discharge source–controlled, or micromixing region) to far-field (ambient environment–controlled, or macromixing region) mixing processes. However, boundary interactions in the form of dynamic plume attachments to the bottom are considered near-field mixing processes (MixZone 2005). As the turbulent buoyant jet travels farther away from the source, the source characteristics become less important. Conditions existing in the ambient environment control trajectory and dilution of the flow through the spreading of buoyant density-current motions and passive diffusion due to ambient turbulence. This region is referred to as the far field (MixZone 2005). A counterrotating vortex pair has been noted in the far field of a transverse jet; however, the mean flow state is not necessarily a symmetric vortex pair but can be unsteady and asymmetric under certain conditions (Shan and Dimotakis 2001). The rotational flow created by the vortex is what helps to make the flow regime more homogenous. Hence, a properly designed tank mixing system would attempt to encourage or extend the creation of lateral vortices that do not reach the surface so that it enhances the mixing of the largest volume of water. The assessment of near-field stability (i.e., distinguishing stable from unstable conditions) is a key aspect of analyzing influent dilution and modeling the mixing zone. It is especially important for understanding the behavior of the two-dimensional plumes resulting from multiport diffusers (MixZone 2005). Discharge plumes may be classified as having the following characteristics:

r Stable discharge conditions usually occur for a combination of strong buoyancy and weak momentum (MixZone 2005).

r Unstable discharge conditions occur when a recirculation phenomenon appears in the discharge vicinity. This local recirculation leads to re-entrainment of already mixed water back into the buoyant jet region (MixZone 2005). The previous discussion of stability emphasizes the importance of eliminating recirculation at the discharge vicinity. Recirculation at the inlets can be minimized by (1) reducing influent velocity to minimize the recirculation region size or (2) having a boundary surface very close to the point of discharge to prevent or minimize rotational flow in the direction opposite that of the discharge. In the second case, this means not using discharge nozzles.

Buoyant Discharges Information about the density distribution in the ambient water body is very important for correctly predicting influent discharge plume behavior. Density currents are buoyancy driven far-field flows that are defined by transverse horizontal spreading while being advected

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Chapter Nine downstream by an ambient current. These spreading processes can intrude into the ambient flow, forming a buoyant upstream wedge and stagnation point. These flows are caused by the density difference of the mixed flow relative to the ambient density. Density currents are preceded by turbulent jet mixing in the near field and are followed by passive diffusion in the far field. Density currents may or may not form upstream intrusions, depending on the crossflow magnitude and internal buoyancy at boundary interaction (MixZone 2005). Buoyant jets discharged horizontally along the water surface from a laterally entering channel or pipe bear some similarities to the more classical submerged buoyant jet. For a relatively short initial distance, the effluent behaves like a momentum jet, spreading both laterally and vertically due to turbulent mixing (MixZone 2005). After this stage, vertical entrainment becomes inhibited due to buoyant damping of the turbulent motions, and the jet experiences strong lateral spreading. During stagnant ambient conditions, ultimately a reasonably thin layer may be formed at the surface of the receiving (ambient) water; that layer can undergo transient density-current buoyant spreading motions (MixZone 2005). In the presence of ambient crossflow, buoyant surface jets may exhibit any one of following three types of flow features (MixZone 2005):

r They may form a weakly deflected jet that does not interact with the bottom or surface.

r When the crossflow is strong, they may attach to the downstream boundary, forming a bottom-hugging plume.

r When a high discharge buoyancy flux combines with a weak crossflow, the buoyant spreading effects can be so strong that an upstream intruding plume is formed that also stays close to the surface near the inlet. Density currents are effective transport mechanisms that can quickly spread a mixed effluent laterally over large distances in the transverse direction, particularly in cases of strong ambient stratification. In this case, influent of considerable vertical thickness at the terminal level can collapse into a thin but very wide layer, unless this is prevented by lateral boundaries (MixZone 2005). If the influent water is nonbuoyant or weakly buoyant, there is no buoyant spreading area in the far field—only a passive diffusion area. Depending on the type of near-field flow, ambient density stratification, and boundary interaction process, several types of density current buoyant spreading may occur:

r Spreading at the water surface r Spreading at the bottom r Spreading at a sharp internal interface with a density jump Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Operation

FIGURE 9-24 Buoyant discharge from single port inlet at 45-degree angle into stagnant tank.

r Spreading at the terminal level in a continuously (e.g., linearly) stratified ambient.

Turbulence in the ambient environment becomes the dominating mixing mechanism in the far field at sufficiently large distances from the discharge point. In general, the passively diffusing flow grows wider and thicker until it interacts with the vessel bottom and/or sides (MixZone 2005). The strength of the ambient diffusion mechanism depends on several factors that relate mainly to the geometry of the ambient shear flow and the amount of ambient stratification. From classical diffusion theory, gradient diffusion processes in the bounded flows can be described by constant diffusivities in the vertical and horizontal direction that depend on turbulent intensity and on channel depth or width. In the presence of a stable ambient stratification, the vertical diffusive mixing is generally strongly damped (MixZone 2005). In the surface approach condition, the weakly bent flow impinges on the surface at a near-vertical angle (>45 degrees) (Fig. 9-24). After impingement, the flow spreads more or less radially along the water surface as a density current. In particular, the flow spreads some distance upstream against the ambient flow and laterally across the ambient flow. The strong buoyancy of the discharge dominates this spreading. The lateral spreading of the flow in the surface impingement area is driven by both the flow momentum and the buoyancy force (MixZone 2005). Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Chapter Nine Since influent water is rarely at the same temperature as ambient water in the tank, buoyant flow can be used as a free source of mixing energy. Substantial additional mixing can be created and further optimized if the tank mixing system is designed to use density streams as part of the entrainment scheme. This would further negate the use of inlets at the bottom or near the surface in a tank mixing apparatus. Such inlets prevent the density streams’ interfaces from folding due to stratification and prevent the engulfment or entrainment of large volumes of water needed for mixing efficiency.

Flow Diffusers Influent A multiport diffuser is a linear structure consisting of many more or less closely spaced ports or nozzles that inject a series of turbulent jets into the ambient receiving water body. These ports or nozzles may be connected to vertical risers attached to an underground pipe or tunnel or they may simply be openings in a pipe lying on the bottom (MixZone 2005). Flow diffusers in water tanks’ mixing systems should be designed to use and accommodate all the physical phenomena associated with turbulent jet mixing. Such systems should optimize mixing efficiency using as little energy as possible, as follows:

r Use reasonable inlet velocities. Higher velocities are not only energy consuming, but are also detrimental to proper mixing as shown earlier. This requires a multitude of inlets to sufficiently divide the flow so that velocities are lower.

r Do not use nozzles—not only because of higher velocities and

head loss, but also because more recirculation is associated with nozzles. Orifices are closer to the conduit/pipe surface and minimize such recirculation.

r Entrain larger volumes of ambient water by having flow

streams at different levels and sides, creating three-dimensional mixing effects. A three-dimensional arrangement would be required for the inlets such that the influent creates undisturbed streams or currents, which engulf the majority of ambient volume.

r Enhance folding of the interfaces in the far field (Fig. 9-25).

The inlets must be positioned so that inflow streams impinge on each other and interact with boundary surfaces in such a manner that they create large-scale folding and lateral vortices.

r Use density currents.

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Operation

FIGURE 9-25 Sheet flow from multiport diffuser into tank model showing formation of vortices and folding of interfaces in far field. Note: better mixing distribution in far field.

Effluent The flow diffuser discussed so far is an influent structure. However, it may also be a tank effluent structure. The filling cycle typically uses a fraction of the total time of a tank’s operating cycle. The tank may remain idle for some time or may draft for a long time, feeding back into the system. Reliance on influent mixing alone is not optimum, because throughout the longest part of the operational cycle, no mixing is taking place. To optimize mixing, it is ideal to also mix during the draft cycle. After the fill cycle, ambient water in the tank will stratify, lose disinfectant, or be rendered otherwise nonhomogeneous because of some physical or biochemical activity. As a result, water quality may progressively decrease. More importantly, impurities or disinfection by-products may settle, stratify, or accumulate unevenly because of temperature gradients and removal of fluid solely at one or two locations on the tank bottom. To prevent the possibility of high concentrations of accumulated impurities (such as some disinfection by-products or solids being fed back into the distribution system by excessive drafting), it is prudent to mix or blend the effluent water from various areas and elevations of the tank as it is drafted. At a minimum, effluent mixing will accomplish the following:

r Prevent the sequential removal of stratified or accumulated components

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Chapter Nine r Minimize the number of dead spots due to drafting from the regions close to the outlet

r Decrease reliance on passive diffusion which may be disrupted by many environmental factors

r Decrease the possibility of icing in colder climates due to distributed motion of the fluid and removal of fluids near the top

r Remove water close to the top, where biological activity is higher and disinfectant residual is lower

r Make effluent more uniform and provide a more homogeneous starting point for the next fill cycle, making turbulent jet mixing more effective

r Because of uniformity of mixing, achieve better and more meaningful tracking of water age

r Eliminate mixing cost, since water from the tank or reservoir is mixed by gravity and forced back into the distribution system. When water is combined from different areas and elevations through the diffusers, it creates large interface areas internally which are then stirred by the turbulence in the diffusers and tanks’ piping, valves, and fittings. This further mixes the flow to create a more homogeneous effluent and consistent water quality.

Tank Venting Most water storage tanks are nonpressurized tanks that require adequate venting. By allowing the removal or replenishment of air as water enters or exits the tank, venting prevents both pressurized and vacuum conditions. Atmospheric tanks are not designed to handle pressurization; the absence of sufficient venting to handle the air outflow generated as water enters the tank would cause the air in the tank to compress and exert pressure on the tank walls that may exceed design stress limits. Likewise, tanks are not designed to handle the vacuum conditions created when water is drafted from a tank without adequate venting. Buckling of tank walls takes place even when differential pressure is small. The styles of air vents most commonly found in water tanks are the mushroom, pan, and 180-degree types. AWWA D100, Standard for Welded Steel Tanks for Water Storage, requires that one tank vent, even if more than one is required, always be located near the center of the roof. A reasonable offset is allowed for tanks designed with center dry-access tubes. Vent designs, examples of which are given in Figs. 9-26 to 9-28, should meet the following requirements:

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Operation

24 in. (588 mm)

Screen

Tank roof assembly

FIGURE 9-26 Double 90-degree elbow roof vent detail. (Source: AWWA Manual M42, Steel Water Storage Tanks.)

r Prevent insects and animals from entering the tank (a noncorrodible mesh is recommended)

r Prevent rainwater or surface water from entering the tank r Prevent air drafts from entering the tank (Outside diameter) 3/16

er ov

C A

Ve nt

di am .

A

am

di

Tank ro of (Inside diameter)

er

et (Outside diameter) (Hole in roof) Plan view

Section A-A

FIGURE 9-27 Pan deck vent detail. (Source: AWWA Manual M42, Steel Water Storage Tanks.)

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re

su

res

p Air PTFE gaskets (typical)

1/2- no. 13 × 15 flattened expanded metal bird screen

Pres

sure

Vacuum

palle

t

pallet

Screen (brass material is normal) Support bars Air vacuum Carbon-steel body

Roof

Install vent vertical 5 +

FIGURE 9-28 Typical clog-resistant vent detail. (Note: PTFE = polytetrafluoroethylene. Pallets should be removed during coating to prevent clogging of the screens. Periodic inspection and maintenance are required to keep in proper working condition.) (Source: AWWA Manual M42, Steel Water Storage Tanks.)

r Exclude dust and debris, as much as possible, from entering the tank

r Provide some level of security against accidental or intentional contamination

r Prevent direct sunlight from entering the tank r Be frostproof in cold-weather areas r Be tall enough, or installed high enough, not to be blocked by drifting snow or debris

It is a requirement of the Ten States Standards that overflows not be considered as vents. Obviously, a tank using its overflow as a vent would be left without venting during overflow conditions. It also points out that vents on ground-level tanks should terminate in an inverted U shape with its opening 24 to 36 in. (609.6 to 914 mm) above the tank’s roof or ground. The U-shaped overflow should be covered with 24-mesh noncorrodible screen installed within the pipe at the location least susceptible to vandalism. AWWA manual M42, Steel Water Storage Tanks, recommends clog-resistant vents with pressureand vacuum-releasing pallets. Large tanks should be provided with more than one vent. One should be installed near the center of the roof, and the other(s) closer

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Operation

Operation to the tank’s walls. This facilitates crossflow ventilation through the tank. Vent sizing is of special concern in the case of tanks in systems that have experienced demand growth. Increased tank inflow and outflow rates must be handled by tank venting. Undersized vents must be replaced with properly sized ones to prevent problems related to increased maximum flow rates.

Telemetry Most storage facilities for potable water are located in unmanned sites. Some tanks are located in sites manned by a handful of operators whose main responsibilities are to monitor a water treatment process. In either case, it is desirable to have automated systems that monitor hydraulic and water quality parameters of tanks. These devices can store data in electronic form or on paper. They can also transmit information collected to a central location or manned facility where an operator can keep track of and control multiple facilities throughout a plant and/or a distribution system. Telemetry is the science and technology of automatic measurement and transmission of data by wire, radio, or other means from remote sources, pumping stations, distribution system tanks, or other facilities or processes to receiving stations for recording and analysis. Most telemetry systems used by water utilities are commonly known as SCADA (for supervisory control and data acquisition) systems.

Tank Water Elevation Water utilities that operate a SCADA system have a central monitoring facility where one or more operators are able to remotely control the fill or draft of tanks, the opening and closing of motor-operated valves, and chemical feed processes such as disinfectant boosting. In addition, the SCADA system monitors, records, analyzes, and identifies trends regarding myriad parameters from online sensors, analyzers, and transmitters at each facility in the communication network. Although some of the systems monitored by SCADA may be automatic (e.g., the closing of an altitude valve to prevent tank overflow, the sounding of alarms, and so on), some may be entirely controlled by the SCADA operator (e.g., starting and stopping pumps). Tank elevation information lets SCADA operators know when pumps should be turned on or off as part of normal distribution system operation. In many cases, tank elevations are the only source of information to operators regarding distribution system pressures. Trended elevation data over time paints a picture of a tank’s daily fill and draw cycles. Parameters such as rates of inflow and outflow throughout the fill and draw cycles can be indirectly determined from

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Chapter Nine elevation data if the tank geometry is known, and changes in tank volume can be calculated over time. This trended flow and volume data can help water quality personnel monitor mixing patterns in tanks over the course of the year and provide operators with strategies to enhance tank mixing for each tank’s operating characteristics. A balance must be maintained between the need for optimum water quality and emergency storage. Allowable low levels in tanks should leave sufficient water in storage to satisfy potential emergency demands from fires, power outages, water main breaks, and so on. Excessively high water elevation can trigger an overflow alarm to alert the SCADA operator that an altitude valve may be malfunctioning and that tank overflow is probable. The operator can respond by turning pumps off at upstream pumping stations and/or closing a remotely controlled motor-operated isolation valve, if one is present at the tank. Depending on the particular circumstances, the operator can dispatch a road crew to the tank. Skillful operation and knowledge of the distribution system are required to appropriately address high-pressure conditions. Isolating a tank from the distribution system without taking other measures, such as shutting pumps off, may create abnormally high pressure and leave a system vulnerable to water main breaks and other catastrophic failures. Tanks that are left open to a distribution system provide surge relief should pressure transients be generated. Such systems are deemed soft systems. A system operated at high pressures with its tank(s) offline loses this surge protection and is said to be a hard system. It should be noted that some utilities have chosen to forego altitude valves and rely entirely on telemetry and motor-operated isolation valves to control water level in the tank. Many strategies are available for sensing water level. A few of the most common technologies are listed here, divided into two categories: contact sensors and noncontact sensors.

Contact-Level Sensing Technology Bubbler systems use a source of compressed air to push bubbles out of a conduit at the bottom of the tank. The higher the pressure required to push the bubbles, the higher the water level. Bubblers provide continuous level sensing relatively accurately, but they require an external source of compressed air. The air pressure is transmitted as an analog voltage or current signal. Radio-frequency (RF) capacitance sensors, tuning-fork sensors, and floats are switches that are engaged when submerged in water and disengaged when water levels drop below them. Several of these switches can be installed on a track or some other means of support at several tank depths. An elevation signal is generated for each particular depth where switches are located. A transmitter unit is commonly

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Operation used to monitor each switch in the tank and transmit digital outputs corresponding to tank level. Icing is a concern with this technology.

Noncontact Level Sensing Technologies Ultrasonic level sensors and radar level sensors are noncontact devices that are suspended above the water surface. They measure the rate of travel of energy through air to measure the distance between the sensor and the surface of the water. Although some sensors are susceptible to condensation, many styles are available with features that prevent condensation from affecting the measurement. Icing may or may not be a concern, depending on how the sensor has been installed. Another concern is the proximity to the sensor’s energy beam of tank walls or other tank appurtenances. Some sensors are able to “calibrate out” such obstacles. Under certain conditions, a stilling well can be used to focus the sensor’s beam through a pipe dipped into the water. This method has the added advantage of reducing water surface turbulence that may affect accuracy, but it increases the chance for icing damage. Pressure transmitters measure the pressure, or head, of the tank at some point along the tank’s piping or wet riser. These devices use the compression of a pressure-sensing element, typically a strain gauge or a capacitor, to generate a continuous-voltage or current-analog signal corresponding to tank elevation. A tap is required in the tank’s piping or riser where small-diameter piping (copper that is 0.25 to 0.75 in. [6.3 to 19.0 mm] in diameter is common) connects to the transmitter. This small-diameter pipe is susceptible to freezing and should be installed in a heated or insulated enclosure in cold-weather regions. Pressure transmitters are the most common type of level sensor used in SCADA systems for distribution system storage tanks. Level-sensing methods require accurate information regarding tank elevation and dimensions. A maintenance and calibration schedule should be followed, and good records should be kept.

Street Pressure Street pressure is measured on the street side of the altitude or tank isolation valve. The most common technology used is the pressure transmitter. A continuous-voltage or current-analog signal corresponding to the pipe pressure at the sensor elevation can be continuously sent to the SCADA operator. (The pressure transmitter need not be at the same elevation as the pipe centerline, but this discrepancy must be accounted for in the determination of street pressure.) If the altitude valve is not locally controlled, a decrease in street pressure signals the operator that the altitude valve or tank isolation valve should be opened and the tank drafted to meet demand. Data on street pressure enable the operator to monitor distribution system pressures even if

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Chapter Nine the tank has been isolated. For instance, a sharp decrease in street pressure may be an indication of system failure such as a water main break.

Disinfectant Residual Water quality managers are installing monitors for disinfectant residuals at representative distribution system locations. Some installations are made because of regulatory requirements, others as a voluntary measure. Tanks are ideal for such systems since they are usually in secured locations, electrical power is available, and utility personnel periodically visit the tank site. Various technologies are commercially available to measure and transmit concentrations of disinfectant residual. The most common secondary disinfection chemical in the United States is chlorine. The following methods are described for measuring concentrations of chlorine residual at water storage facilities.

N,N-Diethyl-p-phenylenediamine (DPD) Colorimetric Method DPD is oxidized by chlorine in solution. This results in two oxidation products. The contrast between the colors of these two compounds, as measured by a colorimeter or a spectrophotometer, reveals the amount of free or total chlorine in the water. A voltage or current-analog signal corresponding to the calibrated concentration of residual is then transmitted to SCADA.

Iodometric Method Potassium iodide reacts with free chlorine in the sample water to produce iodide. The iodide concentration is measured by the instrument to yield total chlorine. Free chlorine is not measurable by this method.

Polarographic Membrane Sensor Technology A pair of electrodes is immersed in a conductive electrolyte and separated from the sample water by a chlorine-permeable membrane. Free chlorine travels through the membrane and is reduced to chloride at the electrode’s surface. The reduction of free chlorine generates an electric current between the electrodes that is proportional to the free chlorine concentration.

Amperometric Electrodes Combinations of probes consisting of a silver anode and a platinum cathode measure free chlorine concentration, pH, and temperature. A current proportional to the free chlorine concentration is produced within the electrodes. The amperometric electrodes require replacement after a manufacturer-specified lifetime (Pollack et al. 1999). Although automated, these systems may require chemical replenishment and periodic maintenance and calibration to sustain accuracy.

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Operation Disinfectant-residual sensors can also act as feedback for booster disinfection at storage facilities. The SCADA operator can remotely control the feeding system. System parameters such as start, stop, feed rate, remaining disinfectant, and set point may be controlled and/or monitored.

Temperature Temperature transmitters and thermocouples can be used to measure and transmit temperatures. Low air temperature inside equipment enclosures can alert the SCADA operator to potential freezing conditions that are detrimental to tank-monitoring devices. A water temperature probe inserted either in the tank or in the pipeline leading to the tank, or in both, can provide the utility with information regarding tank stratification conditions. A complete temperature profile of a tank can be obtained by using a weighted line of thermocouples designed to measure temperature at various water depths. The information may be transmitted to SCADA or stored locally and downloaded manually. Trending of such temperature profiles over time can help water quality managers and operators to determine operational parameters for seasonal or changing conditions.

Flow Many different types of meters can be used to measure water flow. SCADA monitoring of flow into and out of a tank can indicate problems in the distribution system, assist water quality managers in determining optimum tank operation, determine water depletion time during emergencies, and so on. Some of the systems used to measure flow at tanks are differential-type flowmeters such as venturis, insertion meters (V-Cone, Annubar, etc.), and orifice plates and electronic-type meters such as ultrasonic, temperature, and magnetic flowmeters. Detection of flow direction is inherent to the operation of some of the meters, such as magmeters. Other meters, such as venturis, require additional devices to determine flow direction. Each meter named, whether as a primary or a secondary device, makes use of a transmitter to calculate and convert the flow into an analog current or voltage signal.

Security As discussed earlier in this chapter, security at water storage facilities is a concern to utility officials and law enforcement. SCADA systems can also transmit data from security sensors and video from cameras either to the SCADA operator or directly to a separate security SCADA monitoring center or to law enforcement monitoring officials. A variety of sensors are available to detect intrusion to a tank site or

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Chapter Nine tampering with tank appurtenances such as hatches and vents. Each of these sensors is capable of sending a digital signal to SCADA that indicates the appropriate security-related reaction. Pan/tilt/zoom cameras can be operated remotely to track an intruder’s activities. Available camera technology is able to register images even at very low light levels. Video recording devices at each site may be accessed remotely through SCADA to download video images of the intrusion.

Local Monitoring and Control The section of this chapter on water quality monitoring in distribution system tanks outlined several parameters that a utility may choose to monitor. Water corrosivity, pH, conductivity, total organic carbon (TOC), and turbidity are among the most commonly encountered. All and any of these parameters, as well as the ones described in detail previously, can be relayed by telemetry to the utility’s SCADA center. Data about all parameters measured at each tank are routed through a common device for processing and communication. There are essentially two devices found throughout the industry—the remote terminal unit (RTU) and the programmable logic controller (PLC). RTUs are generally associated with remote monitoring of field devices. PLCs have been traditionally used for the automatic and/or remote control of processes. These differences have become cloudy in recent years; each device can now serve both monitoring and control duties. Debate rages on over the reliability of each system, their control and sampling rates, and their capability or lack thereof to handle large numbers of data points, store data during power outages, and so on. RTUs and PLCs require a protective splash-proof/weatherproof enclosure. Each data point is wired to the device’s input/output cards. The data are analyzed according to the terminal or the controller’s programming and are stored or transmitted. It is possible for either device to be connected to a local personal computer. Operators and maintenance personnel can use these local computers to monitor or troubleshoot data and device performance locally without the assistance of, or feedback from, the SCADA center operator. The local computer, which is called a human/machine interface (HMI), can be a laptop computer brought from site to site or a desktop computer stationed at the site. Technology options available for remote telemetry communication can be categorized as follows: telephone, cellular, radio frequency, fiber, and satellite. Telephone communication technology requires a hard line be installed. The information is transmitted and received in analog form by means of modems at the site and at the SCADA center. Several communication rates are available depending on the utility’s budget and the need for fast transmission of large volumes

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Operation of information. Available services range from basic telephone modem communication (2,400 bits per second [bps]) to leased duplex lines with high broadband capability (>2 million bits per second [Mbps]). Because telephone lines are already available near most water storage facilities and the needed equipment is relative inexpensive and commonly available, telephone communication is the most popular technology used for SCADA systems. Telephone lines are susceptible to weather such as lightning or windstorms, which may disable SCADA access to the facility. Careful consideration should be given during design of any SCADA system to default operation of controlled devices if there is a communication failure. Cellular communication technology can be used if the water storage facility and the SCADA center lie within the coverage range of a cellular communication company. A cellular modem is installed at the tank and at the SCADA center, but no hard wire exists. Instead, data are transmitted via a wireless cellular network of communication towers and cellular antennas between the site and the control center. Cellular communication is a good choice for locations where hard phone lines do not reach, as the cost is relatively higher than for a hard telephone line and fewer broadband options are available. Many electronic devices, such as pressure transmitters, small process controllers, security sensors, and so on are now available with integrated cellular modems capable of sending a small number of monitoring and/or control signals to a cellular modem at the SCADA center. Radio-frequency communication systems use a radio modem and a low-powered transceiver at the tank location, and a transceiver is connected to an RF base station at the SCADA center. Several tank sites or other remote facilities can be polled over a single ultrahighfrequency (UHF) or very-high-frequency (VHF) system. Any station can serve as a repeater to extend the line-of-sight transmission of the SCADA center (Pollack et al. 1999). In a typical application, the SCADA base requests data from a remote location, such as a tank, by transmitting a wake-up signal to send data. When the remote begins transmitting, the base reverts to the receive mode and collects the data package. After transmitting the data, the remote goes back to the receive mode and awaits instructions from the base. The output of the sensors at the remote site has usually been converted to digital data by the RTU or PLC. This signal (typically in the range of 300 to 3,000 Hz) is delivered to a modem that converts it to an analog form that can be frequency-modulated to the RF carrier. When the base receives the analog data, the base modem converts it back to digital data. The Federal Communications Commission (FCC) has allocated certain frequencies that can be used for fixed operation. Certain frequencies are available for RF transmission in the low band (25 to 50 MHz), midband (72 to 76 MHz), VHF band (150 to 174 MHz), UHF band (450 to 512 MHz), and 900 MHz (928 to 960 MHz). The low band provides the

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Chapter Nine best communication range because the path loss is less than at higher frequencies. However, it is susceptible to interference from electrical noise. The UHF band is the most frequently used because of the large number of available channels and its relative freedom from electrical noise. Line-of-sight studies are always required to test the feasibility for RF systems. Obtaining a license from the FCC for the exclusive use of a frequency can be expensive and time-consuming. Start-up cost is high, and although maintenance cost is low, the utility becomes the sole owner and operator of the RF communication system and is responsible for its maintenance and upgrades (Pollack et al. 1999). Fiber SCADA communication systems require the installation of expensive fiber-optic cable (often several miles or kilometers) to a fiber utility cable. Fiber provides the best broadband of any communication method, often surpassing 100 Mbps. This expensive option should only be considered when a utility requires the fast transmission of very large volumes of data, including real-time video. The fiber lines can be leased or owned, depending on availability and agreement with the provider. When the water utility owns the line, it becomes responsible for its maintenance and upgrades. When a tank is located where telephone lines, cellular communications, or RF systems are impractical, satellite communications is an option if a satellite covering the distribution system area is in space. In this case, the satellite acts as a relay station between the tank and the SCADA center. Transmitters and receivers are required at both ends to communicate through the satellite. This option is more expensive than hard phone lines or cellular technology, but it may be well worth the cost when no other communication alternative is viable.

SCADA Systems The sophistication of the SCADA system depends on the utility’s budget, the equipment supplier, and the programmer/system integrator. Often a single operator is in charge of remotely controlling and monitoring thousands of data points throughout the distribution system. A master station at the SCADA center is usually a single device (centralized system), a master with submasters (hierarchical system), or a parallel group of processors (distributed system). For the purpose of this discussion, each will be referred to as the master station. The functions of the master station include scanning PLCs and RTUs throughout the distribution system. This is accomplished by monitoring the proper operation of remote control devices, ensuring that messages from these devices are error free, retrying when messages are incorrect, and reporting PLC or RTU failures. A master station also processes data received from RTUs and PLCs. The

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Operation master station does this by checking for alarm conditions, averaging and trending data, storing event changes, and entering data into a database. The transmittal of operator commands is another important function of the master station. The transmittal of commands involves several steps: interrupting the scan and arming the proper remote, encoding and transmitting the command, verifying that the proper command has been received, permitting execution of the command, and verifying command execution. Master stations must also maintain a database for historical data. To accomplish this, data received from the remote location are typically condensed into hourly and daily averages, peak values are delineated, and various data-compression techniques are used to minimize storage. Additionally, the historical data might include status information such as valve positions, water level elevations, and similar items to allow later correlation with flows and pressures. The historical database must also provide very flexible data retrieval capabilities. The master station is also responsible for driving the human/ machine interface. This is done by presenting data on video display screens, map boards, printers, or similar mediums; providing the ability to define screen formats, including graphics; and providing the ability to define report formats. Master stations provide the important function of providing failover to a backup when necessary. This involves maintenance of duplicate data files in a backup processor and monitoring of the primary processor (by the backup) and switch to the backup (i.e., failover) on detection of a stall or error. Master stations may also perform advanced functions such as supply prediction, demand prediction, optimal pumping, and leak detection. The human/machine interface is the point at which the operator interacts with the SCADA system. Current SCADA systems offer interactive HMI modules. These allow building of display screens by those with no programming knowledge. This permits operations personnel who will be using the system to design and build graphic and tabular displays that precisely meet their needs. These displays may be interactive—that is, the symbol for a pump may change color depending on pump status, or a reservoir icon may “fill” as the reservoir level increases. Inputs to a SCADA system occur either as real-time events automatically sensed and reported by the remote control device or as manual inputs through an HMI. Inputs from RTUs or PLCs include status, flow, pressure, and level. Inputs from HMIs include commands for open/close, run/stop, and set point. SCADA outputs are either for driving the HMI or for executing the commands at the remote location. HMI outputs include periodic reports, alarms, alarm summary reports, graphic pictures, displays of

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Chapter Nine real-time data, displays of averaged or trended data, and historical reports. Control outputs include set point and on/off or start/stop. SCADA technology should permit building of reports by persons with no programming skills. An operator or engineer, for instance, can readily define a report format. The resulting report can then be produced for a special study or scheduled to be automated output as a routine operational report (Gotoh et al. 1993). HMI screens should be organized and labeled in a way that they are easily identified and uploaded to the screen. The interface with the operator should be clear and simple (feedback, on-screen help, and option menus are useful tools). A sufficient number of control and monitoring screens should be provided to keep screen cluttering to a minimum. Alarms should be in a scroll window that does not overtake the main window. Symbols, colors, and terms should be consistent and self-descriptive. The overall screen environment should be intuitively simple and allow operation with minimal computer programming skills.

Energy Conservation in the Distribution System In many communities, water utilities are the largest consumers of electric power. Pumping is the largest consumer of electric power for utilities and is their largest operational cost. Pumping is a continuous process that is typically rarely interrupted. It is used to meet high demand in the early morning and late afternoon (and/or evenings) and to fill the tanks between those times. Utilities can take advantage of elevated water storage to reduce these power expenses. Electrical energy is converted to pressure and velocity head through the pumping process. When the water reaches an elevated storage facility, the water rises to an elevation equal to the remaining energy in the pipe. Hence, energy is stored in tanks in the form of potential energy, or head. Tanks can be filled during low-demand hours to take advantage of reduced power rates. The volume of stored water and its specific head reduces the need for pumping (additional electric energy) during peak-demand hours. This process is known as peak shaving. Peak shaving not only reduces power consumption, it reduces the size of pump stations and trunk mains to satisfy the same demand. This capital cost savings is in addition to the operational energy cost reduction. However, this must be weighed against the cost of additional storage to satisfy demand. An optimum design is one that achieves the lowest life-cycle cost including maintenance and cost of demolition and replacement. Tanks help save energy in other ways. Drafting of tanks in conjunction with pumping during hours of peak demand reduces water flow in trunk mains. Lower pipe velocities result in less head loss

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Operation

Operation (energy loss) due to friction. Maintaining water elevation in tanks within a specific range can help set limits on the minimum and maximum system curves. Coordinating that with the pump curves forces pumps to operate at or close to their highest possible efficiencies. The size and number of pumps installed must be optimized in keeping with the demand conditions and storage available. Pumps operating efficiently can reduce electric consumption drastically. In many cases, pumps with best efficiency points (the combination of flow and pressure at which a pump operates most efficiently) that are greater than 90 percent provided wire-to-water efficiencies of less than 25 percent due to inappropriate operation. Pump wire-to-water efficiencies during operation are commonly less than 50 percent. To improve pump efficiency, elevated tanks should be used to satisfy the marginal demand, and pumps should be used only when there is enough demand in the system for them to operate near their optimum efficiency. In addition, allowing tanks to float on the system eliminates the need to start pumps to meet marginal demand increases, which eliminates maximum electric demand surcharges. Most water utility power rates are based on demand or capacity surcharge. That is, the water utility pays based on its peak power consumption during the billing period, for the entire billing period. Rates are also higher for power consumed during peak hours. Therefore, water administrators and operators should look for ways to decrease overall and peak-hour power consumption. The action of starting pumps, in particular, draws large and instantaneous amounts of inrush current from power grids. Electrical distribution systems may experience serious problems if a power company does not have enough standby power to meet this instantaneous demand. In fact, utilities that require large pumps for distribution pumping often must get clearance from the power company before starting a pump. Power companies may also require water utilities to install soft-starter technology to reduce starting motor current. Some small utilities with sufficient storage are able to shift pumping to periods of low electrical demand and pay a reduced rate for power. This strategy, however, is difficult to implement in large systems because of the excessive volume of storage that would be required. Many utilities have resorted to variable-speed pumping to meet variable demand. Variable-speed drives allow pumps to operate below normal speeds to reduce flow and pressure output. Although wire-to-water efficiencies may be low at lower speeds, the amount of energy used is less, reducing energy consumption. Hence, peak power consumption is reduced, because only a fraction of the total potential pump power is used. The efficiencies achieved by variablespeed pumping can be exceeded by properly designed and operated constant-speed pumping systems and tanks.

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Chapter Nine Ideally, the most efficient means of conveying water is to have no more than two pumps operating continuously at constant speed near or at optimum efficiency. The number of pumps may increase or decrease on a seasonal basis if demand patterns change, but it remains constant otherwise. Tanks are filled when daily demand drops and drafted when it increases; the draft and fill limits are set by the boundaries of the pumps’ high-efficiency region. Of course, not all systems have pump stations or tanks that are set up to do that. In addition to the peak-shaving volume for energy considerations, tank size should provide sufficient reserve storage for fire flow and emergencies while maintaining tank turnover for water quality. It is not easy to optimize the tank’s diameter versus height for the required storage volume to satisfy all or most of these considerations.

Bibliography American Water Works Association (AWWA). 2003. Principles and Practices of Water Supply Operations—Water Treatment. 3rd ed., p. 210. Denver, CO: AWWA. AWWA. 1986. Maintaining Distribution-System Water Quality. Denver, CO: AWWA. AWWA. 1990. Water Quality and Treatment, A Handbook of Community Water Supplies. 4th ed., p. 14.4. New York: McGraw-Hill. AWWA. 2010. Water Quality and Treatment, A Handbook on Drinking Water. 6th ed. New York: McGraw-Hill. Clark, R. M., and W. M. Grayman. 1998. Modeling Water Quality in Drinking Water Distribution Systems. Denver, CO: AWWA. Clesceri, L. S. (ed.), A. E. Greenberg, and A. D. Eaton. 1998. Standard Methods for the Examination of Water and Wastewater. 20th ed. Washington, D.C.: American Public Health Association, AWWA, and Water Environment Federation. Code of Federal Regulations. 2004. Title 14—Aeronautics and Space, Chapter 1, Subchapter E Airspace, Part 77. Objects Affecting Navigable Airspace. Washington, D.C.: Federal Aviation Administration, Department of Transportation. Code of Federal Regulations. 2004. Title 40—Protection of Environment, Chapter 1, Part 141. National Primary Drinking Water Regulations. Washington, D.C.: US Environmental Protection Agency. Connell, G. F. 1996. The Chlorination/Chloramination Handbook. Denver, CO: AWWA. Crozes, G. F., et al. 1999. Improving Clearwell Design for CT Compliance. Denver, CO: American Water Works Association Research Foundation (Awwarf). De Zuane, J. 1997. Handbook of Drinking Water Quality. 2nd ed. New York: John Wiley & Sons. Gotoh, K. (ed.), J. K. Jacobs, S. Hosoda, and R. L. Gerstberger. 1993. Instrumentation and Computer Integration of Water Utility Operations, pp. 113–4. Denver, CO: Awwarf and Japan Water Works Association.

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Operation Grayman, W. M., L. A. Rossman, C. Arnold, R. A. Deininger, C. Smith, J. F. Smith, and R. Schnipke. 1999. Water Quality Modeling of Distribution System Storage Facilities. Denver, CO: Awwarf and AWWA. Guidance Manual for Maintaining Distribution System Water Quality. 2002. Denver, CO: Awwarf and AWWA. Hjertager L. K., B. H. Hjertager, N. G. Deen, and T. Solberg. 2008. Experimental and Computational Studies of Turbulent Mass Transfer in a Mixing Channel. International Journal of Chemical Reactor Engineering 6:A105. Jirka, G. H. 1999. Five Asymptotic Regimes of a Round Buoyant Jet in Stratified Crossflow. 28th International Association of Hydraulic Engineering and Research (IAHR) Biennial Congress, Graz (Austria). Kirmeyer, G. J., L. Kirby, B. M. Murphy, P. F. Noran, K. Martel, T. W. Lund, J. L. Anderson, and R. Medhurst. 1999. Maintaining Water Quality in Finished Water Storage Facilities. Denver, CO: Awwarf and AWWA. Knoy, E. C. 1991a. Good Design Eliminates Frozen Storage Tanks. Opflow 17(2):1. Knoy, E. C. 1991b. Solving Cold Weather Problems for Storage Tanks. Opflow 17(1):2. Lee, S. Y., and R. A. Dimenna. 2001. Performance Analysis for Mixing Pumps in Tank 18. Report WSRC-TR-2001-00391 prepared for US Department of Energy, contract DE-AC09-96SR18500. Aiken, SC: Westinghouse Savannah River Co. Lindeburg, M. R. 2001. Mechanical Engineering Reference Manual. Belmont, CA: Professional Publications. Mays, L. W., ed. 1999. Hydraulic Design Handbook. American Water Works Association. New York: McGraw-Hill. Mays, L. W., ed. 2000. Water Distribution Systems Handbook. American Water Works Association. New York: McGraw-Hill. MixZone, Inc. 2005. www.cormix.com. Moegling, S. D. 1992. Modeling the Effects of Reservoir Mixing on Water Quality in Water Distribution Systems. Graduate thesis, University of Akron, Akron, OH. Nathman, J. C., R. C. Aguirre, and H. J. Catrakis. 2004. Far-Field Turbulent Mixing Efficiency and Large-Scale Outer-Fluid-Interface Dynamics. 42nd American Institute of Aeronautics and Astronautics (AIAA) Conference, Sacramento, CA. Pollack, A., A. S. C. Chen, R. C. Haught, and J. A. Goodrich. 1999. Options for Remote Monitoring and Control of Small Drinking Water Facilities, pp. 52–120. Columbus, OH: Battelle Press. Recommended Standards for Water Works [Ten States Standards]. 1992. Great Lakes–Upper Mississippi River Board of State Public Health and Environmental Managers. Albany, NY: Health Research. Roberts, P. J. W., X. Tian, S. Lee, F. Sotirepoulos, and M. Duer. 2004. Physical and Numerical Modeling of Mixing in Water Storage Tanks: Progress Report. Denver, CO: Georgia Institute of Technology and Awwarf. Sanks, R. L., ed. 1989. Pumping Station Design. Stoneham, MA: ButterworthHeinemann. Schlichting, H. 1968. Boundary Layer Theory. 6th ed. New York: McGraw-Hill.

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Chapter Nine Shan, J. W., and P. E. Dimotakis. 2001. Turbulent Mixing in Transverse Jets. Report CaltechGalcitFM:2001.006. Pasadena, CA: Graduate Aeronautical Laboratories, California Institute of Technology. Ten States Standards. 1992. See Recommended Standards for Water Works. Great Lakes–Upper Mississippi River Board of State Public Health and Environmental Managers. Albany, NY: Health Research. US Environmental Protection Agency (USEPA). 1974. Manual of Methods for Chemical Analysis of Water and Wastes. Washington, D.C.: USEPA. US Environmental Protection Agency (USEPA). 1991. Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources [Surface Water Treatment Rule Guidance Manual]. EPA 570391001. Washington, D.C.: USEPA. US Environmental Protection Agency (USEPA). 1999. Disinfection Profiling and Benchmarking Guidance Manual. Appendix D, Determination of Contact Time. EPA-815-R-99-013. Washington, D.C.: USEPA. US Environmental Protection Agency (USEPA). 2006. Stage 2 Disinfectant and Disinfection Byproducts Rule (Stage 2 DBP rule). EPA 815-F-05-003. Washington, D.C.: USEPA. Compliance Help: http://www.epa.gov/safewater/ disinfection/stage2/compliance.html#quickguides. Accessed February 2008. von Huben, H. 1999. Water Distribution Operator Training Handbook. 2nd ed., p. 16. Denver, CO: AWWA. Walski, T. M., D. V. Chase, and D. A. Savic. 2001. Water Distribution Modeling, p. 31. Waterbury, CT: Haestad Press. Walski, T. M., J. Gessler, and J. W. Sjostrom. 1990. Water Distribution Systems: Simulation and Sizing. Chelsea, MI: Lewis Publishers.

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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

CHAPTER

10

Maintenance, Inspection, and Repair Jennifer Coon, C.H.M.M., C.E.T. Tank Industry Consultants

Why have a maintenance program? The answer is simple: Preventive maintenance has been, and always will be, less expensive than crisis maintenance. Preventive maintenance allows owners to identify potential problems and develop solutions before the problems reach crisis proportion. For example, it can be much cheaper to identify and arrest coating failure and corrosion before they turn into metal loss requiring more extensive repair. Additionally, tank painting, if done properly, is typically required at intervals of 15 to 20 years. If the coating adhesion is monitored regularly during inspections, topcoats can be applied to the exterior to restore the aesthetics and extend life of the original or underlying coating system beyond the anticipated 15 to 20 years. Topcoating can cost only a fraction of the cost of full repainting.

Tank Evaluations and Resources Three types of evaluations are recommended during the life of the tank: (1) initial or baseline tank evaluations, (2) update evaluations, and (3) operator evaluations. Several organizations have established standards by which water storage tanks are evaluated and maintained. AWWA publishes standards dealing with specific aspects of tanks. Additionally, all water storage tanks should be compliant with any local building codes.

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Initial Evaluation An initial evaluation is a thorough evaluation performed to determine the tank’s structural, sanitary, safety, and coating condition. The AWWA manual M42, Steel Water Storage Tanks, recommends that a professional engineer familiar with the design and maintenance of water storage tanks perform this type of evaluation. The initial evaluation is the most detailed and intensive evaluation.

Update Evaluation An update evaluation should be performed approximately every 3 to 5 years following the initial evaluation. The update evaluation is performed to monitor changes in the coating condition and rate of corrosion and to verify that tank conditions have not changed significantly since the previous evaluation or rehabilitation. The same person or firm that performed the original initial evaluation should perform the update evaluation. The update evaluation is not as time-consuming and detailed as the initial evaluation. The advantages of having an initial evaluation and subsequent update evaluations performed by a professional are that these evaluations will identify the optimum time for tank repainting and repairs, and the owner can better plan for and budget for proper tank maintenance or rehabilitation. The evaluations will identify all of the work that is required to properly maintain the tank. This eliminates surprises and change orders during a repair or repainting project.

Operator Evaluation Tank owners should perform a cursory evaluation of the tank’s condition at least annually. The purpose of this evaluation is to identify items that can be easily remedied by maintenance personnel and to indicate if any issues exist that require professional evaluation. If any significant deterioration is found, or if the tank has been damaged in some way, a professional should be called in to evaluate the problem. Items requiring basic maintenance can be remedied by operating personnel at this time. If the owner’s personnel are not properly equipped or qualified to climb the tank, the professional person or firm that performs the initial and update evaluations could perform this function. The advantages of operator evaluations are that any significant or serious changes that may require further evaluation by a professional, such as a potential leak or metal loss on the anchor bolts, can be identified by the operator. Also, routine maintenance can be performed by water department personnel, thus saving the costs of replacing items or repairing items at the next rehabilitation.

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Resources for Tank Owners

r The AWWA Standard D100 for steel water storage tanks was originally published in 1935 and has undergone continual upgrading and modification.

r The National Fire Protection Association (NFPA) initially

adopted NFPA 22 Standard for Water Tanks for Private Fire Protection in 1914.

r AWWA M42 Steel Water Storage Tanks Manual. This manual incorporates and updates much of the information contained in AWWA D101 Standard for Inspecting and Repairing Steel Water Tanks, Standpipes, Reservoirs, and Elevated Tanks for Water Storage. The publication of D101 has been discontinued.

Additional Steel Tank References

r AWWA D100-05—AWWA Standard for Welded Carbon Steel Tanks for Water Storage

r AWWA D102-06—AWWA Standard for Coating Steel WaterStorage Tanks

r AWWA D103-97—AWWA Standard for Factory-Coated Bolted Steel Tanks for Water Storage

r AWWA D104-04—AWWA Standard for Automatically Controlled, Impressed-Current Cathodic Protection for the Interior of Steel Water Tanks

r API Standard, ANSI/API 65-1992—Welded Steel Tanks for Oil Storage

r API Standard, ANSI/API 653-1995—Tank Inspection, Repair, Alteration, and Reconstruction

r API Standard, ANSI/API 620-1992—Design and Construction of Large, Welded, Low-Pressure Storage Tanks

r NFPA 22—Standard for Water Tanks for Private Fire Protection

Composite-Tank References

r ACI 371R-98—Guide for the Analysis, Design, and Construction of Concrete-Pedestal Water Towers

r AWWA D107—AWWA Standard for Composite Elevated Tanks for Water Storage

Inspection and Repair by Operator The first step of any preventive maintenance program is inspection to identify the items requiring maintenance. Following are items that

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C h a p t e r Te n should be inspected periodically by the operator and instructions regarding repair.

Site Access Inspection The tank and the site should be monitored for signs of unauthorized access and vandalism, which are a potential liability for the tank owner. Signs of unauthorized access include damage to the tank or site, graffiti on the tank or site appurtenances, paint chipping caused by rocks being thrown at the tank, and bullet holes or indentions in the steel caused by from guns being shot at the tank. Personnel should look for damage or loose wiring in the site fence and barbed-wire strands as well as gaps between the fence and the ground. The proper operation of the gate locking mechanism, site motion detectors, site lighting, and surveillance cameras should also be verified. If the tank is equipped with an exterior ladder, the proper operation of its vandal deterrent and locking mechanism should also be confirmed.

Repair If the site is not already enclosed by a fence, a fence at least 6 ft (1.8 m) tall and topped with barbed wire should be installed around the entire tank site. Barbed-wire strands should be added to the top of the fence if they are not already present. Personnel should then regularly maintain the fence and barbed wire so that they are in good condition. Any holes, broken wire, or bent sections should be repaired. The fence should be close enough to grade to prevent intruder passage under it. All barbed-wire strands should be taut. The fence should be equipped with a gate or gates that can be locked whenever the site is unattended. Vegetation should be regularly trimmed back from the fence so that it does not damage or restrict view of the fence. All light fixtures, motion detectors, and surveillance cameras should be regularly maintained. If the tank has an exterior ladder, it should be equipped with a locked vandal deterrent.

Site Maintenance Inspection The operator’s personnel should evaluate the condition of the tank site not just for appearance purposes, but also to help protect the tank from damage and corrosion. The presence of any trees, bushes, or other vegetation touching the foundation, bottom plate, or tank should be noted. Foliage traps moisture against the steel and creates a damp atmosphere that can accelerate corrosion. These areas should also be checked to see if grass clippings or other debris have accumulated there. If saturated or eroded soil not caused by precipitation or

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Maintenance, Inspection, and Repair

Maintenance, Inspection, and Repair overflow effluent is noted around the base of the tank, a professional engineer familiar with water storage tank issues should be contacted, as this may signify a structural problem.

Repair Trees and bushes should be trimmed back to prevent the limbs and foliage from touching the tank. Vegetation should be trimmed so it does not grow up on the foundation, base plate(s), and tank. Personnel should remove any debris found on the foundation and base plate(s). When the tank site is mowed or other maintenance operations are performed with similar equipment, the discharge should be directed away from the base of the tank to prevent any rocks discharged from hitting the tank and damaging its coating. This will also prevent grass clippings from accumulating on the foundations and base plates and accelerating corrosion. Care should be taken that maintenance equipment, trucks, and so on do not come into contact with the tank or foundation. If necessary, personnel should regrade the site so that the foundation projects 6 to 12 in. (152 to 304 mm) above grade and adequate drainage away from the foundation occurs. Rainwater runoff and overflow discharge should be directed away from the foundation.

Foundation(s) Inspection The foundation should be checked to verify that it projects between 6 and 12 in. (152 and 304 mm) above grade and that there is proper drainage away from the foundation. The foundation(s) should be examined for signs of settlement and the concrete inspected for evidence of cracking, spalling, or exposed reinforcing steel. Deep cracks or extensive crumbling of the foundation signal, a potentially serious issue, and a professional evaluation should be conducted. Also, if the foundation tops are not approximately level with each other, this may be evidence of differential settlement of the tank foundation, and a professional evaluation should be conducted.

Repair Personnel can apply a bonding agent and vinyl emollient concretepatching mortar to any deteriorated areas or voids found in the concrete foundation to build up the surface to its original contour. The condition of this repair should then be monitored.

Grout, Fiberboard, and Sealant Inspection The condition of any grout, fiberboard, or sealant located at the interface of the foundation and the bottom plate should be evaluated

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C h a p t e r Te n for any cracks, voids, or deterioration. These can allow moisture to build up between the tank foundation and bottom plate and cause underbottom corrosion.

Repair A flexible sealant can be applied to any deteriorated areas or voids found in these materials to restrict the ingress of moisture through the voids and under the tank bottom plate. The condition of these repairs should then be monitored.

Anchor Bolts and Chairs Inspection The anchor bolts and chairs should be inspected regularly by the operator’s personnel for any signs of corrosion and metal loss. Metal loss is most critical at or below the nut. If metal loss is observed, the area of metal loss should be measured and compared with the diameter of the bolt where no corrosion has occurred. Typically, if the anchor bolt has deteriorated by more than the thread depth (to the root diameter or less), if any of the anchor bolts are bent or otherwise damaged, or if any of the nuts are not completely threaded, a structural engineer familiar with the design and maintenance of water storage tanks should be contacted to evaluate the anchor bolts and chairs further.

Repair Personnel should keep the chairs free of debris, vegetation, and grass clippings.

Manholes and Access Doors Inspection Manholes and doors should be checked periodically to confirm that they are secured against unauthorized entrance. Unlocked manholes and doors are a potential liability for the tank owner.

Repair Personnel should install locks on manholes and access doors. They should also replace any manhole gaskets that do not create a positive seal. If any of the bolts have corroded, they should be replaced with stainless-steel bolts to prevent rust staining from streaking onto the tank surfaces.

Exterior Overflow Pipes Inspection The overflow pipe should be checked to verify that no potential exists for cross connection between the potable water stored in the tank and

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Maintenance, Inspection, and Repair

Maintenance, Inspection, and Repair the water in the storm or sanitary sewer. The proper operation of a flap gate or elastomeric check valve should be confirmed, and the inspector should verify that no gaps exist between the flap gate and the pipe. The condition of the screen should be assessed to verify that it is adequate to prevent the ingress of insects and small animals. The brackets and associated attachments should be evaluated for corrosion and metal loss.

Repair If gaps are noted, personnel should reposition the flap gate or elastomeric check valve on the overflow pipe to eliminate them. Any damaged screening on the overflow pipe discharge should be replaced to prevent the ingress of insects and small animals.

Venting Inspection The proper operation of the clog-resistant vent and its pallets needs to be checked before and after freezing weather. The condition of the vent screening needs to be assessed to verify it is adequate to prevent the ingress of insects into the tank. Shielding over any vertically oriented screening also needs to be assessed so that wind-driven dust and debris do not enter the tank.

Repair Personnel should replace any damaged vent screens to prevent the ingress of insects into the tank. Any damaged shielding over the screening should also be replaced so wind-driven dust and debris do not enter the tank.

Ladders Inspection All ladders should be carefully checked for deteriorated members that might pose a danger to climbers. The ladder brackets and their attachments to the tank and the ladder itself should be checked for missing or deteriorated bolts and/or cracked welds. The rungs should be inspected for metal loss, especially where they join the side rails.

Repair If deteriorated members are noted on a ladder or its associated brackets, repairs should be made. If the ladder is equipped with a safeclimbing device, the device should be shielded from any paint or solvent being used to ensure its continued proper operation.

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C h a p t e r Te n

Balcony and/or Platform Inspection Any balcony and/or platform should be evaluated to verify that the surface does not retain water. If the surfaces allow water to pond, a professional engineer should be contacted to discuss options for drainage. The floor or safety railing access openings should be assessed. Closable covers should be located over all floor openings, and protective chains or bars should be located at all railing access openings.

Repair Personnel should replace any missing covers from floor openings and/or protective chains from safety railing.

Interior Lighting Inspection Personnel should check to make sure all interior lighting fixtures operate properly. The condition and presence of the protective cages and globes on the fixtures should be verified. The conduits associated with the lighting should be assessed to confirm that they enclose all wiring and are adequately supported. If they do not appear to be adequately supported, a professional engineer should be contacted.

Repair Personnel should maintain any interior light fixtures so that they operate properly. Burned-out bulbs should be replaced, as should damaged protective cages and globes. If the fixtures or associated conduits expose wiring, it should be covered in accordance with National Electric Code (NEC) guidelines.

Obstruction Lighting Inspection If obstruction lighting is required on a tank, personnel should make sure it is operational and lit. The condition of any globes and bulbs should be verified. The lighting should be evaluated to confirm that it is adequately braced and that it and the associated conduits do not have exposed wiring. The condition of the photoelectric cell should be checked. The proper operation of the lighting should be verified both at night and during the day to ensure that the lights are on when required and off during daylight hours (unless otherwise required by the Federal Aviation Administration) to reduce electricity use and the frequency of replacing bulbs. If the fixture and its conduits do not appear to be adequately supported, a professional engineer should be contacted.

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Maintenance, Inspection, and Repair

Repair Personnel should replace damaged bulbs or globes. If wiring is exposed, it should be covered in accordance with NEC guidelines.

Inspection of the Tank Exterior Exterior Steel Welded Tanks and Leaks The general condition of the exterior coating and any evidence of corrosion should be monitored. If the exterior of the tank is in poor condition, the condition of the interior coating could be as poor or worse. The tank should be observed for signs of leakage or rust streaking that a leak could have caused. Leaks can develop in flat steel plates, but the most common sites are at seams and joints. Dark rust stains are usually evidence of leakage. Small leaks in the seams may rust closed over time, so water may not actually be running down the tank at the time of the inspection. Also, if the tank is empty, a leak may not be visible. If a leak or excessive corrosion is noted, a professional engineer should be consulted regarding repair, as the leak may indicate a more serious issue.

Exterior Steel Riveted Tanks and Leaks The general condition of the exterior coating and any evidence of corrosion should be monitored. If the exterior of the tank is in poor condition, the condition of the interior coating could be as poor or worse. The tank should be observed for signs of leakage or rust streaking that a leak may have caused. Leaks can develop in flat steel plates, but the most common sites for leaks are seams and joints. Dark rust stains are usually evidence of leakage. Small leaks in the seams may rust closed over time, so no water will actually be running down the tank at the time of the inspection. Also, if the tank is empty, a leak may not be visible. Rivet heads should be closely evaluated as extreme metal loss on these items may indicate a structural issue. If a leak or excessive corrosion is noted or severe corrosion observed on rivet heads, a professional engineer should be consulted regarding repair, as the leak may indicate a more serious issue.

Exterior Steel Bolted Tanks and Leaks The general condition of the exterior coating and any evidence of corrosion should be monitored. If the exterior of the tank is in poor condition, the condition of the interior coating could be as poor or worse. The tank should be observed for signs of leakage or rust streaking that a leak may have caused. Leaks can develop in flat steel plates, but the most common sites for leaks are seams and joints. Dark rust stains are usually evidence of leakage. Small leaks in the seams may rust closed over time, so water may not actually be running down the tank at the time of the inspection. Also, if the tank is empty, a leak

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C h a p t e r Te n may not be visible. Additionally, the gaskets or sealants between the bolted joints should be evaluated to confirm they create a watertight seal. Glass-lined coatings should also be checked for damage around the bolts from over-tightening. If a leak or excessive corrosion is noted, a professional engineer should be consulted regarding repair, as the leak may indicate a more serious issue.

Professional Evaluation AWWA recommends that water storage tanks be professionally evaluated at least every 5 years and otherwise whenever conditions warrant evaluation. A thorough professional evaluation will enable the tank owner to accurately schedule required maintenance, prolonging the structure’s useful life. A professional evaluation should consist of a careful study of the tank’s interior, exterior, foundation, and accessories. All necessary surfaces on the tank should be accessed by rigging and rappelling the interior and exterior as required by the condition and design of each tank.

Selecting a Professional Inspection Company When retaining a firm or person to perform a professional tank evaluation, the owner should refer to the AWWA manual M42 Steel WaterStorage Tanks, which states: “The tank maintenance engineer should have knowledge of the traditional engineering disciplines and have specialized training and practical experience in the design, fabrication, erection, inspection, sanitary integrity, coating, and maintenance of steel water-storage tanks.” A tank owner who invests in a professional tank evaluation should expect the evaluation to be thorough, professional, and complete. In addition to supplying the usual components of a professional evaluation, the evaluation team should identify any peculiarities associated with the tank.

The Evaluation Report A certified engineering report should be issued concerning the condition of the tank. The evaluation report should describe the observations of the technicians and engineers and their recommendations for optimum rehabilitation. Color photographs of the tank interior and exterior provide aid to the tank owner in analyzing the data presented. The report should address the condition of the tank— structural, coating (including heavy-metal content analysis), corrosion control (including cathodic protection), safety (OSHA compliance), operational and sanitary conditions, and compliance with other applicable standards—and environmental considerations such as

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Maintenance, Inspection, and Repair

Maintenance, Inspection, and Repair containment and proper disposal of abrasive blast residue. Those items not in compliance with current OSHA regulations concerning safety, sanitation, and operation should be identified so that the tank owner can make informed decisions regarding compliance with these important issues. A registered professional engineer familiar with the design, construction, and maintenance of water storage tanks should certify the report, which should serve as a decision-making document for the tank owner. The report should also include budget estimates for the recommended work, anticipated life of the coatings and the structure, and estimated replacement cost of the tank. Recommendations should address what rehabilitation work needs to be performed to meet the short-term and long-term needs of the water system.

Inspection of the Tank Interior Underwater Evaluation Using a Diver Although this method does not require the tank to be drained, it should be taken off line and isolated from the system the entire time the diver is in the tank. When performing an underwater evaluation, the diver must wear a full dry suit and full-face diving mask to prevent contact between the diver’s body and the potable water. Before entering the tank, the diver and all related equipment must be thoroughly disinfected in accordance with the latest revision of AWWA Standard for Disinfection of Water-Storage Facilities C652.

Underwater Evaluations Using a Remotely Operated Vehicle (ROV) Remotely operated vehicles can be used to evaluate the interiors of water tanks without interrupting service and isolating the tank from the system by operating valves. These ROVs provide closed-circuit video to an on-site technician who operates the unit. As with divers, the ROV must always be disinfected before use in potable water tanks in accordance with AWWA C652. The vehicles are typically made of nonporous materials, and the bearing seals must be filled with a foodgrade glycerin.

Float-Down Evaluation The interior of the tank is full of water as a float-down evaluation begins. A field technician in a small raft evaluates the interior surfaces as the owner drains the tank. The interior wet riser of elevated tanks is typically evaluated by rigging after the float-down evaluation has concluded. The duration of this evaluation is determined by the rate at which the tank is drained.

Drained (Dry) Evaluation During a dry evaluation, the tank is drained before the evaluation and dewatered. The remaining water and sediment are removed from

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C h a p t e r Te n the tank to access the bottom plates. Technicians get to the necessary surfaces of the tank by rigging and rappelling the interior (including the interior of riser pipes 36 in. [0.9 mm] in diameter and larger) as required by the condition and design of each tank. Both dry and wet evaluations have limitations. First, with a diving evaluation, the diver is able to access all of the steel surfaces. During a dry evaluation, the tank evaluation crew can only access, via simple rigging from roof manholes and vents, surfaces that are adjacent to ladders. However, the diver must evaluate the steel surfaces up close, as the limited light does not allow him or her to move away from the tank shell and evaluate the overall corrosion patterns inside the tank. Also, because there is usually silt in the bottom of the tank, the tank bottom cannot be as thoroughly evaluated by diving. When a diver stirs up this sediment, visibility is impaired, diminishing the quality of the evaluation of the tank bottom and the lower portion of the tank shell. In addition, the diver is working with a limited supply of air, which typically causes him or her to accelerate the evaluation. Some of the physical tests that are normally performed, including adhesion tests and dry film thickness readings, cannot be accomplished on the submerged surfaces. Perhaps most importantly, only one diver sees the tank interior, whereas it is ideal for several members of the tank evaluation crew to visually assess the tank interior and evaluate the problems found. This provides for greater accuracy in the evaluation.

Structural Evaluation Structural evaluations are normally only performed if the owner or the engineer believes a tank does not meet current structural standards or if the structural integrity of the tank is suspect. Structural evaluations should evaluate metal loss compared to the apparent or observed original metal loss obtained by ultrasonic testing. A structural analysis is not commonly required for properly maintained existing tanks unless the tank has been modified (if, for example, equipment or antennas have been added to the structure) or has experienced an extreme event such as high winds or an earthquake. The first step of a structural analysis is an engineering evaluation of the tank to determine its condition. A structural engineer should review deterioration of the foundation to determine its effects on the tank’s structural integrity. A level should be used to determine if differential settlement has occurred since construction of the tank. The original design drawings should be reviewed for compliance. Measurements should be taken in the field to analyze the tank and anchorage for compliance with current structural codes and requirements that may have changed or come into effect since the tank was originally designed and constructed. The latest AWWA standards and local building codes should be used. Careful attention should be given

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Maintenance, Inspection, and Repair

Maintenance, Inspection, and Repair to the tank’s compliance with current design requirements for wind, snow, and seismic loadings. The original weld quality on welded steel tanks should be verified by ultrasonic or radiographic testing. The testing is normally performed at locations as required by current AWWA standards and evaluated in accordance with American Welding Society (AWS) standards for weld quality. If the owner has the original radiographs used to verify the original weld quality and has confidence in their accuracy, this additional weld testing may be redundant.

Specialized Inspections Ultrasonic Thickness Measurements Ultrasonic thickness measurements of the steel should be taken, and areas of metal loss and deterioration should be analyzed for structural deficiencies.

Coating Evaluation The coating survey should include laboratory analysis of coating samples to determine the total lead content by weight. Similar tests should be conducted for other regulated heavy metals such as arsenic, barium, cadmium, chromium, mercury, selenium, and silver. Additionally, the coating type, thickness, condition, and adhesion should be tested to assess the ease of applying a topcoat to the existing coating. If such tests indicate that topcoating is an option, recoating costs could be significantly reduced.

Cathodic Protection Evaluation Annual inspection of the cathodic protection system by the manufacturer or other qualified person is recommended. At a minimum, this should include overall inspection of the entire cathodic protection system (including removal of expended or damaged anodes, if required), replacement of all defective parts, complete potential profile survey, a physical check of the anode placement and wiring continuity, observation for corrosion at areas of exposed steel, and a written report.

Inspections Following Extreme Events In the 1970s, national design standards first began to include procedures for designing liquid storage tanks that resist earthquakes. The basic design standards and codes for tanks, which focused on providing better details and structural resistance, were based on observed behavior and problems. Design standards have evolved since that time with regard not just to seismic design, but to wind-load design. Older tanks may not meet these current standards. Therefore, inspection and upgrading, and then maintaining retrofits, may reduce the effect of a seismic or other natural event on the tank. Additionally,

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C h a p t e r Te n inspections can identify problems that potentially could arise during freezing weather. Many of these problems could be easily repaired and maintained before they cause hazardous conditions or before the tank fails during freezing weather.

Owner-Performed Inspection Immediately following a tornado, hurricane, major windstorm, or earthquake, or during freezing weather, tanks should be evaluated for any possible damage. If damage is evident, a professional structural engineer familiar with water tank design and maintenance should be contacted as quickly as possible to evaluate the structural condition of the tank.

Professional Seismic Evaluations In-depth seismic evaluations (as with structural evaluations) are typically above and beyond the standard initial evaluation. Seismic evaluations are typically performed only if the owner or the engineer believes a tank does not meet current seismic requirements. Because of the ever-changing seismic regulations and prescribed design loads, some owners are compelled to do a seismic evaluation of their structures, especially those built before the advent of seismic design. In addition to a professional engineer–performed initial field evaluation of the tank, the engineer should obtain and research all available original tank erection drawings, design calculations, specifications, as-built drawings, and other historical data. Based on the field evaluation and the historical documentation review, a certified engineering report should be submitted outlining the observations and recommendations for replacement or retrofit and maintenance to meet the objectives of the owner. The analysis should determine not only whether the original design complies with the current seismic standards, but also whether the tank in its current condition complies with these standards. Because there have been significant changes in the AWWA design and construction standards (especially in the approaches to design for seismic loadings) and design philosophies, the owner may desire a more complete structural evaluation. Accordingly, the entire tank and anchorage system should be analyzed for compliance with present structural codes and requirements, which may have changed or come into effect since this tank was originally designed and constructed. Careful attention should be given to each tank’s compliance with the present design requirements for wind and seismic loadings. Additionally, the original foundation design drawings and soil report should be reviewed for compliance. This design review of the foundations and the amount of reinforcing steel actually inside the concrete will be based solely on the drawings, as there is no economical method of verifying the amount and the location of reinforcing steel and/or

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Maintenance, Inspection, and Repair

Maintenance, Inspection, and Repair concrete construction practices. Without the original foundation design drawings, this review could not be reasonably performed, and several assumptions would have to be made. The engineer should always provide a recommendation after reviewing the foundation design regarding the necessity of additional physical foundation evaluation. If original drawings of the foundation(s) are not available, then portions of the existing foundation may have to be excavated. During the seismic evaluation, the original weld quality on welded steel tanks should be verified by ultrasonic or radiographic testing. The testing is typically performed at locations as required by present AWWA standards and evaluated in accordance with AWS standards for weld quality. If the owner has the original radiographs used to verify the original weld quality and has confidence in the accuracy of their radiographs, this additional weld testing may be redundant. If one has not been recently performed, a soil investigation may be part of the seismic evaluation of the tank. The additional information gained from a site-specific soil investigation is important in assessing the overall tank behavior. The soil information may also be useful in determining the design load to comply with the building code requirements and to identify potential soil abnormalities that may affect the performance of the tank. However, a soil investigation is not always a necessary expense that adds value. It is typically in the owner’s best interest to require soil investigations only when the information gathered may have a significant influence on the outcome of the seismic evaluation. It is recommended that a soil investigation be conducted only for the following conditions:

r Tanks where the foundation is extremely sensitive to the integrity of the tank

r Sites believed to have a potential or known soil problems, or r Sites where the potential properties suggest that the default building code soil factors used in determining the seismic load are not credible. For example, when tanks are located in areas subject to soil liquefaction or gross slip failures, additional site investigation and remediation may be required. The size, location, and type of tank influence the relative value of a soil investigation. A tank of low height with an unanchored flat bottom typically imposes less load on the soil and may not be susceptible to soil and foundation problems during an earthquake. Conversely, a large standpipe or elevated tank with substantial anchorage requirements may be significantly affected by the soil behavior. The existing building codes and national tank design standards all specify factors to adjust the seismic design load for the site soil classification. All of these documents also have a default value when

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C h a p t e r Te n sufficient detail is not available. For many sites, the default site classifications are conservative for determining the design load. Another important factor in assessing the need for a site-specific soil investigation is the history of tank failures during seismic events. The types of tank failures most often encountered are related to piping flexibility, damage to the shell anchorage, shell buckling, or sloshing damage to the roof and roof support structure. Few foundation problems resulting from earthquakes are reported. When foundation problems are reported, they are often related to gross soil failures (e.g., a tank sliding down the hill) that may not be addressed by the typical soil report or may be a consequence of inadequate anchorage design.

Tank Inspection Issues Confined Space and Other Safety Issues Personnel accessing the interior of a tank should be trained in proper procedures for entering confined spaces. This includes training in measuring atmospheric conditions for oxygen levels and lower explosive limits, emergency response procedures, and roles of each of the workers on the site crew. Worker training programs are available from a variety of sources including OSHA, which offers outreach training courses through its Outreach Training Program. Before climbing a tank, the operator’s personnel should be trained to work at heights and should be comfortable doing so. The worker should use all appropriate safety equipment and follow all safety procedures. Whenever someone enters the tank, at least one additional person should act as a ground person who is available to get help, if needed. Emergency response procedures should be established and reviewed with all crew members at the start of the tank access. Training may be available through an OSHA Outreach Training Program, through your local fire department, or from recommendations by distributors of fall-protection equipment.

Tank Cleaning/Washouts As water is held in the tank, suspended solids begin to settle out of the water and onto the tank bottom. Without regular washouts, large amounts of sediment may accumulate in the tanks. In addition, proper evaluation of the interior surfaces of the tank cannot be conducted with sediment covering the bottom of the tank.

Draining the Tank Before scheduling work crews to wash out a tank, it is a good idea to determine if the tank is equipped with a drain. If so, its location should be noted.

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Maintenance, Inspection, and Repair

Maintenance, Inspection, and Repair

Washing Out the Tank The tank can be washed out with low-volume, moderate-pressure (2,500 psi [17,237 kPa]) pumps, firefighting equipment, or other means. Water should be sprayed on all interior surfaces to remove as much residue as possible. In areas where sedimentation is a problem or where strict local environmental regulations apply, it may be necessary to separate the sediment from the washout water and properly dispose of it by some means other than allowing it to enter storm sewers or nearby streams. Also, care should be taken so that large amounts of sediment are not allowed to enter the tank piping; this could clog pipes or damage valves. If the tank has been equipped with aluminum cathodic protection anodes, many of them may have fallen since the previous washout. Because these anodes may damage the interior coating, they should be removed from the tank during the washout.

Operating Without the Tank Operating without a tank may require notification of local businesses and residents so that temporary large uses of water such as lawn watering or equipment washing can be scheduled for other days, thereby leaving the operator with adequate fire protection capacity. It may be necessary to provide pressure relief valves for the one-tank pressure planes when the single tank is out of service.

Refilling the Tank and Disinfection The disinfection of water storage facilities should be done in accordance with the latest revision of AWWA C652. This standard offers three chlorination methods by which disinfection can be accomplished.

r Method 1. This method requires that the tank be filled with chlorinated water (10 mg/L chlorine) for the sole purpose of disinfecting the tank. After the required retention period, the disinfection water is drained and the tank is filled with potable water. After the potable water has satisfied bacteriological tests and is determined to be of acceptable aesthetic quality, the water may be delivered to the distribution system.

r Method 2. This method requires that a chlorine solution

(200 mg/L) be applied with brush or spray equipment to all parts of the tank that would be in contact with water when the tank is full to the overflow elevation. After rinsing, the tank is then filled with potable water. After the potable water has satisfied bacteriological tests and is determined to be of acceptable aesthetic quality, the water may be delivered to the distribution system.

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C h a p t e r Te n r Method 3. This method requires that the tank be filled to approximately 5 percent of the total storage volume with a mixture of potable water and chlorine containing 50 mg/L of available chlorine. After a retention time of not less than 6 hours, the tank is filled to the overflow level with potable water. After a 24-hour retention period, the water should be tested. Once the water has been tested for bacteria and aesthetic quality, the water may be delivered to the distribution system. Of the three disinfection methods listed in AWWA C652-02, Method 1 is the least popular because an entire tank full of water must be wasted to accomplish disinfection. In addition to wasting the water, discharging large volumes of highly chlorinated water is not environmentally acceptable. The primary drawback to Method 2 is that personnel disinfecting a tank must be equipped with proper respirators and protective clothing to help protect them from the vapors released into the air when chlorine is applied.

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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

CHAPTER

11

Potable Water Security John McLaughlin, P.E. Jordan, Jones and Goulding

The use of water as a leveraging tool in conflict is not new, or is the concept of water security. Besides the air we breathe, water is the single most critical element to human survival. In Water Conflict Chronology (Gleick 2008), more than 100 incidents are documented in which water was the cause of, or was integral to, a major conflict or event. These events or types of conflicts are grouped into one or more of the following categories: control of water resources, water as a political or military tool, terrorism, water as a military target, and disputes related to development of water resources. As early as 2,500 bc, water was used as a military tool to help defeat an enemy. With humans’ reliance on safe and sustainable potable water, its use as a tool of war and conflict should be no surprise.

Threats to Water Systems Any lack of attention to or understanding of the critical importance of potable water changed dramatically after September 11, 2001. Suddenly, the concepts of unconventional threats and how they could use critical infrastructures against a population became real. An immediate reaction to the events of September 11 was the introduction of federal legislation to require US water systems to complete vulnerability assessments (VAs) and emergency response plans (ERPs). This federal legislation became Public Law 107-188, and it required every public water system in the United States serving more than 3,300 people to complete a VA and an ERP, on a staggered schedule, before

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Chapter Eleven December 31, 2004. The actual schedule for completion of the VAs was the following:

r March 31, 2003, for systems serving a population of 100,000 or more

r December 31, 2003, for systems serving a population of 50,000 or more but less than 100,000

r June 30, 2004, for systems serving a population greater than 3,300 but less than 50,000 In each case, the system was to complete an ERP as soon as possible, but no later than 6 months after completion of the VA. The ERP was to incorporate the results of the VA. Water systems were not fundamentally insecure before September 11; most had fences, locks, and other systems to detect and delay intruders. Larger water systems sometimes had guards and more intricate electronic security systems. What fundamentally shifted was the focus—away from protecting water systems against natural disruption and contamination and toward reducing the risk from an intentional malevolent human attack (and not necessarily from an international terrorist organization). Some of the most prevalent, best documented, and least appreciated threats to water systems come from disgruntled current or former employees, a lone vandal or a group of vandals, and common criminals. Almost monthly since September 11, news stories have documented break-ins at water facilities. These types of events almost certainly occurred as often before 9/11, but they received little publicity or attention. The main difference between the pre- and post9/11 incidents is that the Federal Bureau of Investigation (FBI) and other law enforcement agencies paid the former—usually unorganized attempts at vandalism—little attention. Nevertheless, the incidents directly pointed to the need for better risk reduction at water systems, which quickly began to improve formerly minimal security practices.

Definitions People tend to think of “providing security” at water systems, and this chapter uses that terminology, but the real goal is to reduce risk by eliminating vulnerabilities. This process of risk reduction ultimately leads to the security that water system operators and the public seek. In that connection, the following definitions are provided (Sandia 2002).

r Risk—Measure of the potential damage to or loss of an asset based on the probability of an undesirable occurrence.

r Risk assessment—Process of analyzing threats to and vulnerability of a facility, determining the potential for losses,

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Potable Water Security and identifying cost-effective corrective measures and residual risk.

r Vulnerability—An exploitable security weakness or deficiency at a facility.

r Physical protection system—Integration of people, procedures, and equipment for the protection of assets or facilities against theft, sabotage, or other malevolent human attacks. The goal of any security system is to prevent an attack if possible. It is generally not cost-effective, though, to stop any and all attacks. Can a water system afford the level of physical protection used at a nuclear facility or strategic military facility (armed and trained guards; “kill zones,” clear areas outside the perimeter where deadly force is authorized; and so on)? The answer to this question is almost always no, and so water system management must be willing to develop strategies to mitigate the consequences of an attack. This ensures that even though a water system may not be able to stop the attack from happening, it can still cost-effectively reduce the overall level of risk. Consequence mitigation, in addition to reducing the risk from an intentional malevolent human act, also helps reduce the risk to a system from a natural disaster. By providing a double benefit, consequence mitigation measures may be the most cost-effective risk reduction measures of all. Certain aspects of risk reduction at a potable water storage system also benefit the system during natural disasters. The focus of this chapter, however, is still on reducing risk from an intentional, malevolent human act.

Types of Threats A water-storage facility can be intentionally attacked in three basic ways: physical disruption, contamination (radiological, chemical, or biological), and interference with supervisory control and data acquisition (SCADA), computer, and information technology (IT) systems.

Physical Disruption Much has been written about contamination being the worst-case scenario for a water-storage facility. This is valid and worthy of discussion, but perhaps the simplest and the most effective way of having an impact on potable water storage is through physical disruption. The amount of water that humans actually consume is only a fraction of a percentage of the total potable water produced. In Milwaukee, Wisconsin, and Albuquerque, New Mexico, for example, the percentage of potable water actually consumed is one-half to one-quarter of 1 percent of the total produced (Danneels 2001). Having storage, and

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Chapter Eleven therefore supply, of nonpotable water still allows fires to be fought, industry to operate, contamination to be contained, and basic sanitation to continue. In cases where potable water storage is compromised, potable water can be temporarily provided using bottled water, mobile treatment systems, and bulk water that has been hauled in. The purpose of this chapter is not to identify specific vulnerabilities of a water-storage facility or to provide direction for adversaries; therefore, the discussion will remain general. Physical disruption of storage facilities generally requires some knowledge of the specific water system to be truly effective. However, almost every water system relies on critical storage facilities that, if eliminated, would critically disrupt its ability to supply water to the distribution system or to critically important customers. It is easy to disable or eliminate a storage facility without sophisticated chemical or biological knowledge and equipment. Imagine the damage that can be done to electrical systems with basic tools. Sugar in the fuel tanks of emergency generators can create substantial damage. Valves can be broken and extensively damaged without explosives. Any simple Internet search reveals recipes for various homemade explosives capable of doing substantial damage.

Contamination Three types of contaminants are of concern in water systems. These are, in order of concern, biological, chemical, and radiological. Traditional water treatment has focused on removal or inactivation of naturally occurring contaminants and contaminants unintentionally introduced by humans. Each case of intentional or malevolent contamination can cause unique problems. Besides the obvious—customers getting sick or dying—one of the most likely overall problems is the widespread public perception and panic that water is not safe to drink (Burrows, Valcik, and Seitzinger 1997). Additionally, there is the problem of timely determination of what agent (or agents) has been introduced. Charlotte-Mecklenburg Utilities (in North Carolina) has dealt with this issue twice. The first event was unintentional and involved Foamgate (Krouse 2001); the other occurred after 9/11 and was intentional. In each case, even with rapid detection of the contamination, the testing necessary to determine its exact nature and potential harmful effects was one of the most difficult parts of the entire response effort. A chemical agent might be easily detected through the taste, odor, or appearance of the water, especially if enough of the agent is present to do physical harm to a person. The problem with radiological or biological agents is that they are much more difficult to detect and deal with. The first means of detecting these agents in water, even large quantities of agents, might be through symptoms that do not appear in an affected population until days or weeks later. Moreover,

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Potable Water Security symptoms could still be difficult to trace back to the water system without good coordination, cooperation, and relationships between the water system and public health personnel. Add to these difficulties the fact that most potable water storage facilities are still not well protected and thus contamination is relatively easy to accomplish. If the threat has knowledge of the system and chooses a storage facility that serves a critical part of the distribution system, the situation can easily be made worse. It would be difficult for a terrorist or other threat to have a broad, long-term impact on a water system through use of a contaminant. At a minimum, an adversary would need each of the following to create widespread consequences: (1) specific knowledge of which storage facilities are in the most critical parts of the distribution system, (2) access to agents and knowledge of which agent(s) might be most effective and difficult to detect or inactivate, and (3) access to the equipment to distribute the agent. Many agents can be introduced into a water supply system. Any one of them can cause panic among the public (Deininger 2000). This means that the contamination threat, though difficult to carry out, cannot be minimized.

SCADA/IT Interference A third method of disabling a water-storage facility is through cyber attacks against a SCADA system. Fortunately, many water systems still practice manual operation and allow their SCADA systems to perform very little, if any, control. Those that do not practice manual operation or that allow maximum control by their SCADA systems run the very real risk of losing control through hackers entering their system. These hackers can be current insiders or employees, disgruntled former employees, lone thrill-seeking hackers, or a group of organized and highly capable hackers bent on significant and coordinated destruction.

General Site Considerations Location Possible locations for existing storage facilities are as varied as each facility’s vulnerabilities. It is difficult to conclude what would be an ideal location from a security standpoint. A facility in a heavily populated area might be less vulnerable because it would be harder to attack with so many people potentially watching; it is more critical, though, because it serves more customers. In a remote setting, there are fewer people to observe and possibly detect an intrusion, but the criticality of the service area is probably lower. This section will only review the more common security issues for remote and urban locations.

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Remote Location When a storage facility is remotely located, its primary vulnerability is that few people are around to detect an intrusion. Unless it is a manned facility, such as a clearwell at a water treatment plant, the only reliable means of detecting an intrusion would be through an accurate, automated detection system, which many remote facilities do not have. Even when a system has the capability of accurately detecting an intrusion attempt at a remote site, response would normally take too long because of the distance from a regular patrol area.

Urban Location An urban or heavily populated location does not have the same vulnerabilities as a remote location, but several inherent vulnerabilities still exist. Location in a congested area means that many more people have close access to the site and are potentially aware of the facility’s importance. In general, in many urban areas, a lot of criminal activity goes unnoticed and unreported. One thing common to virtually all water system facilities is the presence of graffiti, especially on tanks. Most water systems have not worried about this in the past, but the presence of graffiti points to the ease of access by and the poor detection of intruders. In addition, because these storage facilities are so close to large population centers and because they tend to serve more critical customers, they are usually much more vital assets. In both remote and urban settings, the key is good detection. Obviously, until a system accurately detects an intrusion attempt in the first place, delay of the intruder will not be possible. A response force, no matter how close or aware, will have not have any impact, and no facility location will be safer than any other.

Accessibility Accessibility, as discussed here, has to do with the number of people allowed to access the facility. Almost all potable water storage systems allow nonutility personnel to have unmonitored access to storage tanks. These are most often employees of telecommunication companies, electrical utilities, and other city departments. An unscientific survey of results of many vulnerability assessments shows that almost all facilities allow this access without maintaining any direct control over who accessed the facility or when. An equally critical vulnerability is the common practice by many water systems of allowing too many of their own personnel to have keys to facilities. Maintaining access control over the water department’s own personnel is a more difficult problem to solve than

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Potable Water Security controlling access by employees of other agencies. Many tank personnel legitimately need access to the site, but many others have no real need for keys or key cards. Both cases require policies that call for monitoring of all personnel, utility or nonutility, who might access a site. Access should be limited to those who legitimately require it. Background checks should be conducted on anyone who has access privileges.

Visibility, Perimeter, and Size The visibility, perimeter, and size of any site are difficult to control. Most sites are selected on the basis of hydraulic considerations and the ability to acquire suitable property without any forethought to vulnerabilities and security, a practice that must change. Even then, cost will be a primary concern, and creative means of eliminating vulnerabilities will be required. A tank’s visibility is a given. Large ground storage tanks and elevated tanks of any size are obvious. What often works in the tank owner’s favor is that most people take water tanks for granted and forget they are there. As long as police, fire, and the tank owner’s personnel do not do this, the visibility issue can be minimized. What should always be avoided is taking a potentially bad visibility issue and making it worse. Neighborhood aesthetics may dictate some screening, but hiding a tank too well makes it more difficult to detect intrusions. Site perimeters should not be camouflaged or screened unnecessarily. A tank owner should enlist the public relations staff to help explain this to the community. Whether the site is large or small, the tank and related critical facilities should not be placed near the perimeter. A small site may be dictated by economics or location (tight, congested area), but as long as good detection of potential intruders is maintained, additional layers of delay can be added without huge cost, especially at a new site.

General Tank Considerations Water storage tanks tend to be fairly standard in how they are designed and accessed. The biggest differences are elevated versus ground storage tanks and, in the realm of elevated tanks, leg supports versus enclosed pedestal supports. There are differences in construction material (steel, concrete, or a steel/concrete composite) and variations within each category of tank (standpipes, clearwells). These specific differences tend to have less impact on tank security. For this chapter, only design elements that are pertinent to security of storage facilities will be discussed.

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Elevated Storage Tanks Elevated tanks generally offer a security advantage over ground storage tanks in that they do not usually require integral, on-site booster pumping. That is not to say that pumping is not part of the design of an elevated tank system, but maintaining pumps on site is usually unnecessary. Where pumping is not integral to the tank site, the number of vulnerabilities is reduced accordingly.

Enclosed-Base Elevated Tanks The two main types of enclosed-base elevated tanks are the flutedcolumn tank and the pedestal/spheroid tank. Figure 11-1 shows a typical fluted-column type of enclosed-base elevated tank; Fig. 11-2 shows a typical pedestal/spheroid type of elevated tank. Both types usually contain a single pedestrian access door with an integral lock. The fluted-column tank, with its (usually) larger-diameter base, can often accommodate a vehicle protected by a lockable door similar to an automatic garage door. Both styles of tanks almost always contain in their bases tank-specific piping (Fig. 11-3), including the supply and

FIGURE 11-1 Typical fluted-column type of enclosed-base elevated tank.

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FIGURE 11-2 Typical pedestal spheroid type of elevated tank.

FIGURE 11-3 Tank-specific piping for enclosed-base and pedestal spheroid elevated tanks.

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FIGURE 11-4 SCADA components stored in enclosed tank base.

discharge piping, sampling ports, overflow piping, shutoff valve(s), and an altitude control valve (if used). An integral pumping system is commonly provided in the base of a fluted-column tank, but usually not in a pedestal/spheroid tank. In one sense, the large size of the enclosed base is one of the pluses of a fluted-column tank, because more assets can be stored in the space. The other side of the coin is that storing all critical assets in one place creates potential vulnerabilities, because only a single door serves to delay an intruder. Within the base of each style are usually found SCADA components such as tank pressure gauges, residual chlorine analyzers, remote terminal units (RTUs), and radio/dialer equipment (Fig. 11-4). Internal ladders providing access to the top of the tank bowl are almost always located in the base of both styles of tanks. These ladders allow direct access to the water storage portion of the tank by way of direct hatch access or through the water-storage vent.

Multicolumn Tanks Multicolumn tanks have many of the same features as an enclosedbase tank, but without the same level of protection. Figure 11-5 shows the base of a typical multicolumn elevated tank with a ladder guard. Usually, multicolumn tanks have detached underground vaults to

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FIGURE 11-5 Typical base of multicolumn style tank with ladder guard.

house critical piping and shutoff and altitude control valves (see Fig. 11-6 for an example of this arrangement). SCADA components and other related instrumentation are sometimes housed in the same vault, but more often they are located in the open on the tank leg or possibly in an unprotected shed detached from the tank.

FIGURE 11-6 Detached underground vault for piping and valves, multicolumn tank.

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Chapter Eleven In the past, ladder access was available at ground level in many systems, but that practice began to change even before September 11. The practice of cutting off ladders 20 ft (6 m) or so above ground level and adding locked access gates began as a way of controlling vandalism, and it has now become an even more accepted means of limiting access.

Ground Storage Tanks/Standpipes In both ground storage tanks and multicolumn tanks, the piping, valves, SCADA, and so on. are usually located in underground vaults or separate sheds, or they are mounted outside on the tank itself. Ladders are now being cut off above ground level, and lockable access gates are being installed. Figures 11-5 and 11-7 show examples of how this is accomplished on both multicolumn and ground storage facilities. As noted previously, ground tanks often differ from elevated tanks because a booster pump station is often integral to ground tanks’ operation. Usually, both the tank and the pump station are located on the perimeter of the same site. Often the pump station is a more critical and easily accessed asset and becomes more of an issue to

FIGURE 11-7 Ladder cutoff and guard on ground storage tank.

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Potable Water Security secure. The principles of detection, delay, and response (Sandia 2002), along with consequence mitigation, apply to the pump station if it is located on site.

Construction Materials Materials of construction play only a minor part in the security of a tank. Almost all tanks are constructed of concrete, steel, or a steel/concrete composite. A study of explosives, tank characteristics, and materials of construction would be needed to determine which of the three would be most susceptible to destruction. Suffice it to say that a steel, concrete, or composite tank of proper structural design will withstand about the same level of explosive force, all other factors being equal.

Water-Storage Vulnerabilities This section is general and avoids describing specific methods and means of contaminating or disrupting a water system through intentional acts at a potable water–storage facility. Most key elements of water system vulnerability have been covered previously. Specific locations exist on most storage facilities that are the most vulnerable points. These include vents, sampling ports, fiberglass hatches, and local chemical feed stations. Many utilities have hatches that are lightly screened or not screened at all because of wear and tear. Fiberglass hatches are common on ground storage tanks and present a minimal barrier to a determined adversary. The locks usually provided for metal hatch covers are of the type found at the hardware store and are easily cut with large bolt cutters. Readily accessible sampling ports, fire-hose connections, or local chemical feed systems (for maintaining residual chlorine levels, for example) are simple points of access for possible contamination. Disruption of a water system through physical destruction at a water-storage facility is a bit more difficult, but it is possible just the same. It would take a large amount of explosive placed strategically close to a storage facility to ensure complete destruction. Because of this, we tend to focus on the possibility that an adversary would attempt the same level of disruption through focused destruction of critical piping, valves, booster pumping, or other on-site components. As with a tank’s access hatches and vents, most enclosed tank base doors or exterior vaults are only secured with a minimal hasp-andlock system. SCADA/IT vulnerabilities are not currently severe or common, because not many water systems rely on SCADA/IT to control functions. Many utilities use SCADA only to monitor a few key parameters and are alerted either when the signal is lost or when values are out

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Chapter Eleven of range. This does not mean, though, that these vulnerabilities can be ignored. Reliance on SCADA signals without verification can be dangerous, and often SCADA systems alarm over many minor occurrences, leaving operators to filter these alarms and potentially miss something of real importance. SCADA-related vulnerabilities will probably increase as security systems (closed-circuit television [CCTV], perimeter alarms, and so on) begin running signals through the same SCADA system used for operational data. This also opens up a new avenue to be concerned about: A disgruntled employee who controls not only the operation of a system but the security system as well is known as a super insider.

Effective Security/Risk-Reduction Practices All security or risk-reduction measures can be placed in one of several categories. The major categories are physical protection systems (PPS), operational security (OS), and consequence mitigation (CM). Within the PPS are three subcategories: detection, delay, and response (Sandia 2002). The basic concept is to try to prevent an attack from occurring through PPS and OS (and, as a result, through effective detection, delay, and response). The CM piece of risk reduction automatically presumes that the attack has occurred and was successful. Through good CM, a system can effectively respond to an event and minimize the damage. Water systems have an inherent ability to mitigate consequences, because they face similar issues every day when lines break, power goes out, spills occur, and storms move in. In some instances, it is probably more cost-effective for the same risk reduction to focus energy not on preventing the attack but on mitigating its consequences. (The cost and physical difficulties of protecting every part of a water distribution system, or even the most critical parts, would be extreme. However, most systems incorporate beneficial elements such as redundant facilities, system loops, and interconnects. A rapid response by personnel trained in these matters will almost certainly reduce the attack’s effectiveness.) This does not mean to ignore the effort to prevent an attack; it just acknowledges that no water system can truly afford to prevent every attack from all possible threats.

Physical Security Physical protection systems are security measures such as CCTV (camera) systems, motion sensors, alarms, fences, locks, and guards. The basic concept of PPS is to detect an adversary as early as possible. Detection means not just having a camera system record an intruder, but having a person assess the alarm or image and react quickly and effectively to alert whatever response mechanism is planned. Delay is the combination of measures that will slow an adversary who is on the

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Potable Water Security path to the water-storage facility. As noted, early detection followed by effective delay is the ideal sequence for PPS. Response comprises the time and process involved to intervene with the adversary. If the delay is inadequate, the adversary will succeed in carrying out the malevolent act before the response arrives, and therefore the response is ineffective. A response that arrives on scene and reaches the adversary in time, but fails to intercept the adversary, is equally ineffective. An example of this would be having an unarmed guard trying to stop a group of heavily armed adversaries. Although the unarmed guard may arrive in time, he or she can do little to stop the adversary. Detection, delay, and response comprise a three-legged stool. Without all three legs in place and of equal strength, the stool will not stand. In addition to the information provided here, the reader should review information provided in “Guidelines for the Physical Security of Water Utilities,” a Water Infrastructure Security Enhancement guidance document produced by the US Environmental Protection Agency (USEPA) and funded by the American Society of Civil Engineers (ASCE), American Water Works Association (AWWA), and the Water Environment Federation (WEF). Also see the USEPA’s Water and Wastewater Security Product Guide at http://cfpub.epa.gov/ safewater/watersecurity/guide/tableofcontents.cfm.

Detection Practices Digital CCTV Many utilities installed CCTV capability before September 11. Some of these provided digital image storage. The majority used tape and relied on an operator to see an event in real time or to forensically view what happened. After 9/11, digital CCTV systems became more prevalent. These systems store images in digital format and provide an alarm if the viewed image deviates from a stored baseline image. In such a case, in addition to providing the alarm, they pull up the correct segment of video image, including the moments immediately preceding and following the event. With any camera system, lighting conditions and clear lines of sight are critical. An uninterrupted fence line and clear areas at least 15 ft (4.5 m) outside the fence line are essential to successful early detection. Adequate lighting, properly designed with the camera system to provide optimum contrast, is also essential. Lights should be the quick-strike type so that after a power outage has been resolved, it does not take several minutes for the lights to warm up. (Quick-strike lights come up to full candlepower almost instantly after power is restored. They do not operate without power. The best means of powering lights and other critical functions during a power outage is to provide a generator.)

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Chapter Eleven Hand in hand with these systems is the ongoing maintenance of each part of the tank and the tank site. Areas inside and outside of fences must be kept clear, lights that burn out must be immediately replaced, and any camera system must be designed to work with the light level available. These practices fall somewhat into the operation systems category because they are policy-level practices that humans take care of operationally. They are listed here, though, because they are also integral to physical security, and they clearly demonstrate the need for all protection systems to be just that—systems. Table 11-1 shows some basic comparisons of CCTV technologies, with pros and cons for each.

Perimeter Detection Systems Numerous types of perimeter detection systems are available. The highest levels of these involve multiple integrated systems including combinations of microwave, infrared, capacitance, taut wire, and fiber optic. Fiber-optic technology can be cost-effective and can be adjusted or tuned to minimize nuisance alarms. All physical alarm systems must still rely on a human to assess the alarm and react properly. Table 11-2 shows some basic comparisons of detection technologies, with pros and cons for each. As with the CCTV systems, the training and policies necessary for this are discussed in the section “Operational Security.”

Guard Dogs or Geese Depending on the criticality of the facility and whether it is operator attended or not, trained guard dogs may be an option. This option obviously carries certain maintenance and liability issues, but it may be a valid option where human monitoring is difficult, requires augmenting, or is impractical. Similar to guard dogs, but less of a liability concern, are geese. The mess and maintenance for geese may be a problem, but they are very good at sounding an alarm. Once the alarm is sounded, a human must intervene effectively, or the alarm has not been fully assessed. An alarm without human assessment is not an alarm at all.

Access Control Controlling access is another key component of both detection and delay. Access control can be as simple as basic door and window locks or it can comprise state-of-the-art biometrics. Basic lock-and-key systems can be effective against many adversaries, but they require strict key-control policies that are practiced and enforced. If everyone has a key to all facilities and assets, locks cease to be effective. Good key control can detect and delay both insider and outsider adversaries. If padlocks are used at remote storage facilities to which other

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Potable Water Security Photographic Technology Night-vision camera

Black and white (B&W) camera (recommended)

Color camera (not recommended)

Day/night (color/B&W)

Comments r Good for day and night viewing r Will not have to redo or add lights r Expensive r r r r r

Good for day and lower-light vision Inexpensive Not good for dark conditions Not as easy to distinguish during the day Will have to redo site lighting to have effective monitoring

r Good for day viewing r Not good for low-light or dark conditions r Will have to redo site lighting to have effective monitoring r Expensive r Color is good for day viewing r B&W is better for night viewing r More expensive than B&W or color r Will have to redo site lighting to have effective monitoring

Recording Technology No recording

Tape recording

Digital recording

r Must monitor at all times to be functional r Nothing is available that can be used for prosecution r Used for backup validation of alarms r Hard to find previously recorded moments r Cannot record while viewing a previously recorded moment r Used for backup validation of alarms r Begins recording based on motion in the field of view r All recordings are date/time stamped for ease in finding a particular moment when viewing r Accessible from a remote location r Images are in PC-friendly format and can be stored electronically indefinitely

TABLE 11-1 CCTV Summary

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Fence sensors

Types of Detection

r Not overly sensitive to wind r Very reliable r Low false-alarm rate and low nuisance-alarm rate r Immune to electrical or electromagnetic interference (EMI) disruption r Intrinsically safe and uses very stable equipment, resulting in high reliability r Adjustable sensitivity.

r Taut wire

r Fiber optic

Pros r Most economical and easiest to install of the fence sensors r High probability of detection

Technology r Vibration

r The more activity there is at fence, the lower the sensitivity setting r Sensitive to extreme temperature changes and blowing debris r Could be sensitive to large-animal activity r The fence must be stable, free of vibration, and in good condition

with enhancements: r Weather sensor station—feeds weather information to field processor, which then adjusts its vibration alarm sensitivity r Pulse count accumulator—sensitivity is determined by choosing number of pulses needed to create an alarm r Nuisance alarms can be caused by shrubbery, trees, animals, and severe weather that causes fence to vibrate r Regular tensioning maintenance is required r One of the most expensive fence sensor systems because of laborious installation and maintenance time

Cons r Must have properly installed and maintained fence lines r Prone to all types of vibrations, which can be minimized

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TABLE 11-2

In-ground sensors

r Mostly immune to weather and environmental noise

r Ported coaxial buried line

Perimeter Detection Technologies (Continued)

r Mostly immune to weather and environmental noise

r Balanced pressure line

r Strain-sensitive r Capable of “hearing” what may be causing an alarm (similar to cables pressing ear against the wall) r E-field r Self-adjusting circuit rejects wind and ambient noise r Extremely low nuisance-alarm rate r Capacitance r Weather and EMI/radio frequency interference (RFI) have no effect on sensors’ ability.

working with large expanses of concrete r Tree roots may cause problems when tree blows in wind r Sensitive when in close proximity to roads/rails due to machinery. r Avoid installing under chain-link fences; install at least 3 ft (0.9 m) above buried metallic pipes r Susceptible to buried metal r Affected by high-EMI sources such as large electrical equipment or substations (should not be used in close proximity to these areas)

r Very sensitive to high-EMI sources (for example, substations) and radio frequency interference r Sensitive to poor fence construction or maintenance r Adverse weather such as rain, snow, and lightning can create problems r Vegetation and animal movement can cause sensors to react r Generally mounted on top of fence, so use in conjunction with another type of sensor on lower part of fence fabric r Anything making physical contact that changes fence characteristics may cause an alarm r Should use additional surveillance/detection when

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Volumetric sensors

Types of Detection

r Mostly immune to weather and environmental noise r Send multiple-beam pattern, increasing coverage r Good probability of detection r Available in portable versions r Narrow detection zone good for monitoring perimeter sectors r Can be used to monitor an area or a definitive perimeter line r Use monostatic sensors where well-defined area of coverage is needed (400 ft [122 m] coverage) r Bistatic sensors can be used up to 1,500 ft (457 m)

r Buried geophone r Active infrared

r Microwave

optic

Pros r Mostly immune to weather and environmental noise r Immune to electrical or EMI disruption r Adjustable sensitivity

Technology r Buried fiber

r Sensitive to high-frequency spectrum r Sensitive to areas that contain strong emitters of electric fields (radio transmitters) or magnetic fields (large electric motors or generators) r Can interpret ionization cycle created by fluorescent bulbs as motion r Potential health hazards

Cons r Must be installed away from poles and trees at a distance equal to at least the height of the pole or tree) r Should not be installed in or under concrete or asphalt r Susceptible to erosion where either more exposure or deeper burial affects the sensitivities r Sensitive to tree roots as the tree blows in wind r Sensitive to medium in which geophones are buried r Sensitive to trees, fences, light poles, and telephone poles, which can trigger the alarms when blowing in wind r Precise alignment of sensors is critical r Not good with hilly terrain r Sensitive to snow and grass around the sensors r Sensitive to fog, heavy rain, and dust r Sensitive to vegetation overgrowth

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r Can help to limit false alarms r Provides record of events during an intrusion r Monitoring field can be manipulated r Typically used in conjunction with other monitoring technologies

Video sensors r Motion detection

TABLE 11-2

r Greatly reduces false-alarm rate if used in predictable and/or controlled environment r Cost effective (cheaper than purchasing two individual sensors) r Good for detecting helicopter or plane intrusions

r Passive infrared/ microwave

Perimeter Detection Technologies (Continued)

r Radar

r Send multiple-beam pattern, increasing coverage r Good probability of detection

r Passive infrared

r Susceptible to uneven terrain r High maintenance r Potential radio-frequency health hazards r Needs lighting r Needs unobstructed viewing

r As ambient temperature approaches temperature of intruder, sensor is less likely to respond r Sensitive to all heat sources (heaters, animals, and so on) r Precise alignment of sensors is critical r Not good with hilly terrain r Sensitive to snow and grass around the sensors r Sensitive to fog, heavy rain, and dust r Sensitive to vegetation overgrowth r Reduces probability of detection since both sensors must positively detect before sending an alarm r Has all the cons of each technology r Potential microwave health hazards

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Chapter Eleven utilities and/or agencies may need access, the tank owner should avoid daisy-chain systems (several interlocking padlocks); all that is necessary for an intruder to do is to break the weakest lock. Swipe cards and/or personal identification number (PIN) access control can be more secure and allow easier “key” control. The person must remember his or her code and remember to carry his or her key. An advantage of these systems is that they allow logging of who enters the facility—or at least of whose card and PIN were used to enter. This may not stop the adversary (the wrong person with the right key or access code can enter), but it will dissuade those who want to escape undetected. Biometric systems control access by using characteristics and traits that are unique to an individual. Among the most common are fingerprint and retina/eye scanners. These systems are virtually impossible to trick, and they do not involve having to carry a key. Their cost may prevent widespread use, but they can be especially effective against an insider or as a second layer to a perimeter detection system for an especially critical facility. See Table 11-3 for a further breakdown of various access control systems.

Glass-Break Sensors Delay and response are most effective when there is early detection. If an adversary gets through a fence or other outer perimeter undetected, the time available to a response force for intervention is greatly diminished. However, using glass-break sensors on building windows may be necessary if perimeter detection at a fence line is not available or practical. Certainly, it is preferable to have the extra distance and delay, but short of moving entire facilities, that may not be possible. This type of sensor may also be considered a layer in a detection system for a highly critical storage facility or where threat by an insider is the main concern.

Door Alarms Door alarms, too, are more appropriate when the adversary is an insider or as an extra layer in a detection system. The use of alarms for storage facilities within the property’s perimeter can detect an insider who, although legitimately within the perimeter of the facility as a whole, may need to be restricted from entering key buildings that house specific assets.

Contaminant Detection Technology Contamination is less likely to occur than physical disruption and may not have the same impact. Contamination may be more difficult for an adversary to accomplish, and detecting such an attack is also much more difficult. Current technologies generally detect contamination by looking at the effect the contaminant has on certain key

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TABLE 11-3

r Verification of a personal characteristic to authorize access to a restricted area

r Verification by matching PIN to badge number for entry

with predetermined criteria for access to restricted area

r Correct combination of numbers entered on keypad for entry into restricted area r Automated verification of card

stored picture of same person to actual person wearing badge

Entry Method r Verification of a personal picture on a badge to actual person wearing badge r Verification of picture on badge to

Entry Control Summary

Biometrics

Key card/PIN entry

Key card entry

Personal identification number (PIN)

Stored-image badge

Photo ID

Technology

recognition, or face scanning r Each characteristic is unique to the individual r Access can be denied to some individuals and allowed to others, or access can be only during certain times of day r Access privileges can be modified r Automated process

others, or access can be only during certain times of day r Access privileges can be modified r Automated process r Characteristics include fingerprints, retina or voice

r Key cards can be coded per entry point and per card r Access can be denied to some cardholders and allowed to others, or access can be only during certain times of day r Access privileges can be modified r Automated process r Must have both the card and PIN for access r Access can be denied to some cardholders and allowed to

r PIN entry can be coded per entry point, but not per person r Automated process

r Procedural—relies on identification of person by guard r Relies on guard for all access control

r Procedural—relies on identification of person by guard r Relies on guard for all access control

Comments

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Chapter Eleven indicators—among the most common residual chlorine and oxidation–reduction potential (ORP). The theory behind residual chlorine analysis is that a biological contaminant exerts a chlorine demand and therefore creates a drop in the residual. This unusual or unexpected drop would raise an alarm, but there would be no specific information about what caused the drop and whether it was intentional or natural. The same is true for ORP detection. This indicator may react to more contaminants, including chemical and biological contaminants, but there is no way of identifying a specific agent or of determining whether contamination occurred naturally or intentionally. Several criteria should be considered when deciding whether to implement an early warning system for water system contamination. In no particular order, they are:

r Provides warning sufficiently ahead of time to allow for r r r r r r r r r r

proper action Is economically affordable Requires little skill or training Is flexible enough to cover all possible threats Is able to identify the source Is sensitive to changes at regulatory levels Provides minimal false-positive and/or negative results Is durable and robust Provides results that are reproducible and verifiable Can be operated remotely Has year-round all-climate functionality

Any decision to choose an early warning system must be made locally, and the relative costs (monetary, physical, social, and organizational) must be weighed against the relative benefits. It is also important to keep in mind the relative infancy of this technology. Little is known about which contaminants the technology might most accurately detect, and any early warning system currently considered would not likely be able to score high on all the criteria just listed. The following sections detail types of systems and tools that will likely be used when early warning systems become more effective and prevalent. For much more specific information on planning, designing, implementing, and operating an early warning system, please refer to “Early Warning Monitoring to Detect Hazardous Events in Water Supplies,” from which much of this contaminant monitoring information is taken (Brosnan 1999).

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Contaminant Analyzers Current technologies for detecting a contaminant look at its effect on certain key indicators. Among the most common are residual chlorine and ORP, described previously. The very nature of these types of detection means the contaminant is already present in the system and its consequences must be mitigated. Another technique is to use biological analyzers—organisms that react in certain ways to any of several toxic agents. Their reaction is tied to an electronic signal that creates the alarm. The problems here are the lack of any specificity as to the cause of the alarm and the potential for false positives or negatives. A few examples follow:

r In the dynamic fish test, golden ides are exposed to an artificial water flow/current, which they normally swim against. If they detect an upset condition, they turn to avoid it, and this action would be detected and registered. Similar techniques have been used in Europe since the 1970s.

r In the dynamic daphnia test, water fleas are placed in a con-

trolled column of raw water and exposed to several infrared light beams, which they regularly interrupt and which indicates a regular level of activity. If a contaminant is introduced into the water, the activity level initially increases and then sharply declines because of the death or incapacitation of the daphnia.

r Recently, some locations have used the mussel as a monitoring

indicator. The theory is that when mussels are subjected to a contaminant, their shells close at low contaminant levels and then open wide at severe levels. The monitor takes several mussels and glues one half of the shell to a wall. The other half of the shell has a magnet attached that contacts a reed switch to indicate an open or closed position. Electromagnetic sensing between the two shell halves can indicate their interim positions between fully opened and fully closed.

r Delayed algal fluorescence and luminescent bacteria monitors

use the principle that the presence of a contaminant diminishes the luminescent/fluorescent level of either the algae or the bacteria. While these methods may not be desired for use at this stage, they give an indication of the body of knowledge available to enhance security at all your facilities. Technology to monitor and analyze contaminants is constantly being developed and perfected with a goal of providing accurate, real-time capability. Already the Sandia National Laboratories staff

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Chapter Eleven have developed the ChemLab (microChemLab), a palm-sized analytical laboratory that can virtually instantly detect any number of chemical or biological contaminants (Sandia 2002). This type of technology, which can accurately detect and identify an agent in real time, appears to represent the future of contaminant monitoring.

Placement of Detection Devices Accurately detecting a contaminant is only the first step. Where do you put these analytical devices to do the most good? How do you know where the contaminant originated and where it may be headed in the distribution system? Several hydraulic models on the market provide a level of water quality prediction. The most widely used for contaminant transport are Haestad’s WaterCad, MWSoft, and MIKENET from the Danish Hydraulic Institute. MIKENET is already being used in Europe. The model takes analytical measurements from a series of parameters to detect a contaminant and then applies its algorithm to predict the fate and transport of the contaminant from start to finish. Critical to this or any other model is its calibration to real-world conditions, the number and locations of analytical devices, and the operator’s knowledge of the system. As is the case for any detection effort, a contamination event cannot be considered truly detected until the alarm has been accurately assessed. For water-storage facilities identified as critical through pairwise comparisons, fault tree analysis, or accurate hydraulic modeling, realtime contaminant analyzers should probably be located on site.

Delay Practices Delay measures generally are the most cost-effective part of a risk reduction system that comprises detection, delay, response, and consequence mitigation. There is a multitude of number and types of measures; the only limits are the constraints of the particular site. Whether it is an operator-staffed facility such as a water treatment plant clearwell or a remote, unmanned facility such as an elevated water storage tank, the most common delay features are fencing and gates. As with any protection system, fencing and gates are useless without proper maintenance and training of the staff on how to maximize their effectiveness. A simple way of making a regular chain-link fence more secure is to use razor wire at the top of the fence instead of three strands of barbed wire. Traditional fences consist of 6-ft to 8-ft (1.8-m to 2.4-m) chain link with three strands of barbed wire on outward-facing outriggers. Where necessary and practical, the fence can be made more secure by replacing the three-strand barbed wire with at least one coil of concertina or razor wire (Fig. 11-8). Even more delay can be built in by using two layers of fencing. This system is prevalent at critical

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FIGURE 11-8 Chain link fence with razor wire at top.

military or nuclear sites and may be appropriate for certain waterstorage facilities. If the facility is highly visible and is located in a neighborhood, ornamental or architectural-type security fences may be necessary. Figure 11-9 shows an example of an ornamental fence that can also provide security benefits. There are numerous varieties of this type of fence (the example in Fig. 11-9 is from Delgard) that can help with both security and public acceptance. Regardless of the type of fencing used, both fence and clear areas must be properly maintained. This can be assured by establishing and complying with a policy to regularly check the entire fence line—that is, to perform a touch test on the entire perimeter. Finally, tamperproof nuts and bolts for gates and fences should always be used. When reviewing the effectiveness of a perimeter fence, one of the first checks is to see if the nuts and bolts can be loosened by finger pressure only. This is frequently the case, and it negates the effectiveness of hardened locks, razor wire, and the like. At perimeter entrance points for personnel, gates with effective locks, swipe card, or biometric access control are effective. Because vehicle access is commonly needed at water-storage facilities, the same locking systems as used for personnel access should be used. Figures 11-10 and 11-11 show examples of vehicle gate entrances at remote sites. Usually, the gates are only of the vehicle-access type; because

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Chapter Eleven

FIGURE 11-9 Architectural security fence.

the sites are unmanned, they are almost always accessed by vehicle. If gates exist but vehicle access is no longer allowed, Jersey-type barriers are very effective. These come in various forms, including plastic barriers that can be filled with liquid to add weight. If they are to be effective, they must remain filled with liquid.

FIGURE 11-10 Typical chain link entrance gate.

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FIGURE 11-11 Typical architectural gate and multilock system.

A special caution needs to be sounded against using daisy-chain locks if at all possible. These systems are allowed to exist because of the normal practice of permitting those other than the tank owner and personnel (e.g., people from the phone company, emergency services, police, and so on) to have unrestricted access to a site because they have equipment there. As noted elsewhere in this chapter, this practice should be stopped or at least severely restricted. In addition to preventing unknown personnel from having unrestricted access to a site, it is important to have an effective lock. Figure 11-12 illustrates the concept of the weak link in the chain, in which a simple lock of the type available at a hardware store is all that stands in the way of an adversary.

r An array of delay features can be placed between the site perimeter and the storage tank itself. Vehicle barriers in zigzag patterns are very effective. Additional layers of fencing will delay an adversary who is on foot. Use reinforced glass for all exterior windows.

r Depending on the nature of the operations at the site, certain assets may be contained within a building on site. The building presents several opportunities to delay an adversary. Heavy exterior metal doors should be installed. Locks and hinges on all exterior doors should be covered with steel plates using tamperproof screws. Reinforced glass should be

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Chapter Eleven

FIGURE 11-12 Daisy-chained locks with weakest link.

used for all windows (if applicable). Figure 11-13 shows a typical entrance configuration for a fluted-column enclosed-base elevated tank. If this configuration is not practical, bars, cages, fence, or mesh can be installed on the inside of the window frame. Of course, these measures are worth nothing if doors are not kept locked and if there is no key control. Presuming the adversary gets to the asset before a response arrives, further delay measures can still be used. The most common and simplest is to build a steel cage around the actual asset. The design of this barrier must allow for adequate normal maintenance, but it can be very cost-effective.

Response Practices The response component of physical security should focus on providing the water tank owner and staff with backup communications systems for all possibilities. This should include landline phones, cellularphone backup, and even radio systems tied into emergency frequencies. Local law enforcement should be made fully aware of all facility locations and should train on these sites. Water system staff should also be familiar with local law enforcement agencies and should have all of their emergency phone numbers up-to-date and readily available at all times.

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FIGURE 11-13 Pedestrian and vehicle access to fluted-column base.

In conjunction with good detection system practice, nuisance alarms must be minimized to prevent the “cry wolf” problem, in which alarms are ignored because there are too many false alarms. If private guards are to be used, many issues must be addressed. Will they be employed 24/ 7/365? Will they be armed? What level of authority will they have? Will they be used at all facilities? Do they regularly train with local law enforcement? Generally, the cost of hiring private guards is prohibitive. An adequate response can be made by local law enforcement if you have worked to improve relationships with those agencies.

Operational Security Operational security (OS) can also provide security, detect and delay an adversary, and enhance response capabilities.

Detection, Delay, and Response Practices for Operational Security The categories of detection, delay, and response still apply in OS, but they are different from the physical security or PPS functions. With OS, policies, procedures, and training—not physical features—have a much greater role:

r From an operational perspective, one of the best ways to improve detection is to have a well-trained and aware staff. Preparing and fully implementing a set of security policies

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Chapter Eleven and procedures along with emergency operations and response plans is absolutely the most important element of any risk reduction program.

r If guards are to be provided, they should be on site 24/7/365 and should be well trained.

r Complete background checks should be conducted for all employees. Focus more thorough and more frequent checks on employees who have critical access.

r Limit access and key availability to only those employees who need them.

r Plan in advance what deliveries are expected, and record what company and driver are expected when. Allow no deviations from this schedule. Require that all delivery personnel be escorted at all times on site. Institute a policy to perform basic assay tests for all chemicals arriving on site.

r Establish Water Watch neighborhoods throughout your sys-

tem, but begin by focusing on areas near your critical facilities. Train people in these groups in the basics of water system operation and, especially, what security problems to look for and whom to call. Whoever is tasked with receiving the calls must be prepared to handle the situation and initiate a response.

r Have local law enforcement stop and check identification on any person working in or around water system facilities (such as hydrants, valve boxes, tanks, booster pumps, and so on). Do not assume that the worker is an authorized employee of the water company.

r Consider splitting SCADA system monitoring into two categories—normal monitoring and monitoring for intrusion detection. Establish two-person control over SCADA and security access to critical assets.

r Strengthen existing backflow prevention policy or establish a new policy. Begin requiring backflow prevention on all connections and change-outs.

r As already stated, probably the most effective way to delay an adversary is by establishing and implementing securityand emergency-related policies and procedures. These should specifically address such areas as key control for all facility locks. If biometrics or other types of access control are used, proper policies and procedures still must be followed. The same type of key control policy should apply to vehicles and at all other points where access needs to be restricted. All employees should be subject to strict sign-in/sign-out procedures around all critical facilities and when using any water system vehicle. Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Potable Water Security If an employee is alone at a facility, a regular passive and active call-in procedure should be strictly observed. The passive program means that a call is regularly placed to the lone operator, either from a base facility or from local law enforcement. The calls can be placed randomly or at regular intervals. The active method requires the lone operator to place the regular or random calls. (Having at least two operators on duty at all times is the ideal situation, thus eliminating or minimizing the need for active or passive call-in procedures. However, if having two people available at all times is impractical, the regular call-in procedure is the next best thing.) These types of policies are especially effective at reducing the risk from an insider adversary, but they are also applicable in defending against the outsider. They are generally very cost-effective to implement. The biggest obstacle is to change the ways in which a tank owner’s staff thinks and functions. Even now, not all operators and staff members consider security to be an important part of utility operations. Thus, it can be difficult to achieve full acceptance of policies such as these.

Consequence Mitigation If detection, delay, and response have failed and a successful attack has occurred, you are left to mitigate the consequences of that attack. For water systems, conducting mitigation may be one of the most costeffective means of reducing the risk of future attacks and ultimately improving the level of security for the system. Consequence mitigation provides benefits after an intentional human act and after natural disasters. As with some high-level adversaries (e.g., international or domestic terrorists, organized criminal enterprises, and saboteurs), a natural disaster cannot be prevented from “attacking” a water system; in either case you must be able to mitigate the consequences. These general mitigation techniques are applicable to all sites:

r Provide and maintain an inventory of replacement equipment, focused on the most critical assets as determined from a Sandia-based RAM-WTM (Sandia 2002) or other vulnerability assessment.

r Do not store replacement or redundant components in the same location or structure as the primary item.

r Provide generators or other backup power at all critical facilities. They should be capable of powering the critical assets, at a minimum.

r For utilities that use gaseous chlorine, store less total chlorine on site, assuming delivery is on time and reliable.

r If you must store large quantities of gaseous chlorine on site, store it in two or more geographically distant locations to lessen the amount available at any single place. Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

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Chapter Eleven r Provide additional gas venting and storage locations for all gaseous chlorine, and regularly provide emergency training.

r In lieu of gaseous chlorine, switch to safer means of disinfection such as hypochlorite delivered in bulk form or created through on-site generation.

r Provide tamperproof, lockable fire hydrants. Schedule regular training and communication with fire department personnel.

r Work toward establishing system interconnects with neigh-

boring utilities whenever possible. Modeling and testing of the feasibility of an interconnect must happen before any physical connection is made.

Bibliography Brosnan, T. M., ed. 1999. Early Warning Monitoring to Detect Hazardous Events in Water Supplies. International Life Sciences Institute (ILSI) Risk Science Institute Workshop Report. Washington, D.C.: ILSI. Burrows, W. D., J. A. Valcik, and A. Seitzinger. 1997. Natural and Terrorist Threats to Drinking Water Supplies. US Army Center for Health Promotion and Preventive Medicine. In Proc. 23rd Environmental Symposium and Exhibition, American Defense Preparedness Association, Arlington, VA. Danneels, J. J. 2001. Department Manager, Sandia National Laboratories. Statement to US House of Representatives Committee on Science, hearing on H.R. 3178 and the Development of Anti-Terrorism Tools for Water Infrastructure, Nov. 14, 2001. Deininger, R. 2000. The Threat of Chemical and Biological Agents to Public Water Supply Systems. Water Pipeline Database, Science Application International Corporation (SAIC), Hazard Assessment and Simulation Division. McLean, VA.: SAIC. Gleick, P. H. 2008. Water Conflict Chronology (revised). Oakland, Calif.: Pacific Institute for Studies in Development, Environment, and Security. Krouse, M. 2001. Backflow Incident Sparks Improvements. Opflow 27:2. Public Health Security and Bioterrorism Preparedness and Response Act of 2002. Public Law 107-188, 42 U.S.C. Washington, D.C.: 2002. Security Systems and Technology Center, Systems Analysis and Development Department, Sandia National Laboratories. May 2002. Risk Assessment Methodology for Water (RAM-WSM ). Notebook Volume I. Copyright 2002 Sandia Corporation. Contract DE-AC04-94AL85000. Export Control Classification Number (ECCN) EAR99. US Environmental Protection Agency. 2006. Guidelines for the Physical Security of Water Utilities. ASCE/AWWA Draft American National Standard for Trial Use. American Society of Civil Engineers (ASCE), American Water Works Association (AWWA), and Water Environment Federation (WEF). Washington, D.C.: USEPA.

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Source: Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair

CHAPTER

12

Tank Rehabilitation Gregory R. “Chip” Stein, P.E. Tank Industry Consultants

Maintaining water-storage facilities is becoming increasingly important because of rising replacement costs and the difficulty of obtaining rate increases and funding for large-scale construction operations. Although the cost of maintenance is also increasing, these smaller outlays can substantially delay or even eliminate the need to replace a utility’s large capital investment in tanks. This chapter is a guide to the proactive rehabilitation of existing water tanks as well as a guide to planning short- and long-range maintenance operations on a new tank. When renovation of an existing tank is being considered, an evaluation must be made to determine the scope of work to be included. The costs of renovation versus replacement must be compared and amortized over the life of a new tank to determine if repair is economically justifiable.

Developing Specifications If an evaluation of the tank’s condition, components, and appurtenances has determined that repair is required—and if repair is economically feasible—it is necessary to generate a set of detailed technical specifications and bonding requirements. The scope of work must be determined by evaluating the recommendations and cost estimates from the inspection report and comparing these to the availability of funds and to the tank owner’s long- and short-term plans for the tank. Often, there are multiple potential solutions to an observed deficiency. To determine the repair that best fits the utility’s needs, these solutions and their associated costs should be evaluated in terms of the level of risk the utility is willing to accept.

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C h a p t e r Tw e l v e

Standards Referenced For the specification writer to be effective, he or she must have a working knowledge of and have access to the following material:

r Applicable American Water Works Association (AWWA) standards

r National Sanitation Foundation (NSF) standards r SSPC Painting Manual Volume 2: Systems and Specifications. Society for Protective Coatings

r Local regulations regarding volatile organic compounds (VOCs)

r National Fire Protection Association regulations r All pertinent regulations from the Occupational Safety and Health Administration (OSHA) and American National Standards Institute (ANSI) In addition to these, the specification writer must have a working knowledge of any state or local regulations that apply to water tank rehabilitation. The writer should also be familiar with the capabilities and availability of qualified contractors to perform work of the nature and magnitude required.

Seismic Design Standards AWWA D100-05 has changed the way tanks in seismic zones are designed. This latest AWWA D100 revision eliminates seismic “zones” altogether—instead, the coordinates of the tank are entered into a computer program, and site-specific seismic design criteria are determined. The change in seismic design standards resulted from a dramatic change in the way engineers view the risk of a seismic occurrence, its potential magnitude, and its effect on a structure. Existing tanks in high-risk areas should be evaluated to determine whether they meet the current seismic criteria. It may be prudent to reevaluate the seismic criteria and the original tank design criteria when planning future structural upgrades or modifications.

Owner’s Standard Requirements Also included in the project specifications should be any special requirements the tank owner or local regulatory agencies might have. Potential contractors must be alerted to special bonding, wage rate scales, taxes, and licenses that may be required. Local ordinances may have stipulations concerning hours of work, acceptable noise levels, requirements for air monitoring, and other construction activities.

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Tank Rehabilitation

Ta n k R e h a b i l i t a t i o n Open communication among the specification writer, the tank owner, and local officials is imperative when preparing specifications.

Environmental/Worker Safety Lead Regulations Regulations regarding removal of paint that contains lead and other regulated heavy metals were changed in the early 1990s. Methods of compliance and the interpretation and enforcement of these regulations to protect the environment and workers have changed dramatically. Many areas of the United States now enforce a policy of no emissions into the atmosphere or past the property line. Add to this the concern for the safety of workers while they are removing the coatings, and it is obvious why the cost of water tank rehabilitation has doubled. The largest problem has been collection of the dust and debris generated by the removal of the tank’s coating while keeping workers’ exposure levels to heavy metals within the permissible range prescribed by OSHA in its regulation 1926.62. One solution is to shroud the entire structure with impervious tarps and conduct open blasting within this containment system. Dust collectors are then used to negate the pressurization effect of the compressed-air abrasive blasting, producing a negative air pressure in the containment enclosure. Workers must be adequately equipped with respiratory protection while they are in this hazardous environment. This relatively expensive method of containment has been very successful and widely used. Other methods of surface preparation include the use of vacuum shrouding around blast nozzles and power tools and the use of ultra-high-pressure (35,000+ psi [241.32 MPa]) water jetting. The most promising technology currently in use and undergoing further development is the robotic blasting system. This system includes a self-contained centrifugal blasting apparatus that seals against the tank surface. The unit is raised and lowered by a winch and cable. There is no compressed air, so there is no pressure to disperse the debris that is generated. The abrasive media is typically recyclable, so the amount of debris is minimized. Additionally, because workers are outside the blasting assembly, they are not exposed to the concentrated dust.

VOC Regulations Volatile organic compounds, the solvents that traditionally have given coatings their liquidity and workability, are being heavily regulated nationwide. To complicate matters, different areas of the country are

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C h a p t e r Tw e l v e adapting different acceptable levels of VOCs in industrial coatings. As the solvents in the coatings are released, the coating dries. To reduce VOCs in their products, coating manufacturers are producing more high-solids coatings and more water-based coatings. These new coatings will greatly affect coating selection for topcoating operations, recoating, and construction of new tanks.

Water Circulation Short-circuiting and stagnation of water in tanks is a concern for tank owners, who have installed baffle walls and piping systems to force circulation and water turnover. Baffle walls should be carefully designed to account for their effect on the tank structure. Additionally, these walls present challenges to future tank maintenance. Piping systems should be evaluated for use of dissimilar metals, increased cost of interior repainting, and degree of head range loss required to run the system.

Description of Repair Work The accurate and thorough description of needed repair work is one of the most important roles of the specifying engineer. The engineer’s goal should be to adequately describe the work so that change orders are minimized or eliminated and bidders are all on the same page with respect to what is required. Sometimes it is easy to accurately estimate and describe the work (e.g, installation of a safe-climbing device on a ladder). However, in other situations, the repair work is not as easily estimated (e.g., the amount of pit welding required or the length of a crack in the concrete). For these situations, the specification should stipulate the method of repair while allowing bidders to submit a unit price for it (e.g., the price per foot to repair the concrete crack). This allows the specifier to minimize the possibility of a change order and solicit prices for the unknown quantity of work in a competitive bidding atmosphere.

Surface Preparation A successful coating application depends largely on the quality of surface preparation. Regardless of the substrate (be it steel, concrete, or a coated surface), the area to be coated must be clean, relatively free of contaminants, and properly abraded to receive a coating. Surface preparation should be specified to conform to the applicable SSPC standards for cleanliness and the coating manufacturer’s surface profile requirements. Depending on the location of the water tank (in coastal or heavy-industry areas, for example), specific requirements regarding the degree of cleanliness and additional testing requirements may be required for surface contaminants.

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Ta n k R e h a b i l i t a t i o n

Coating Systems There is no longer any such thing as a standard coating system. Technology in the painting industry, especially in the water storage tank industry, is in a period of rapid change. There is no longer a “standard spec”—not if you want a coating system that will truly protect your tank. Gone are the days when conventional paints were applied over minimally cleaned surfaces by everyday laborers. Now, in a period of increasingly stringent environmental regulations, highly skilled technicians apply sophisticated coatings onto surfaces cleaned by everevolving surface preparation methods. We must now “design” a coating system for each tank, taking into consideration all the specific conditions that may affect the system’s performance.

Coating System Selection First, we need to realize that in the past, common industry shortcomings caused specifiers to use improper or inadequate coating systems for water tanks. Engineering education was lacking with regard to controlling corrosion by using coatings, and so specifiers relied mainly on coating suppliers for guidance. Thus, a trend developed among specifying engineers of using suppliers’ “canned” specifications rather than developing a specification and system that fit the exact needs of the tank owner. Additionally, the welded-steel tank specifiers, designers, and fabricators frequently failed to recognize the need to incorporate proper design details that extend the coating life. Just because “it’s by the specification” does not mean it is the best coating design for long-term corrosion protection. The first step in designing a proper coating system is to determine the owner’s needs and research specific operating conditions by asking questions about the tank itself. r In what environment is this tank located?

r r r r r r r r

What are the constraints of the tank site? What is the design of this tank? What is the current condition of its coating? What are the types of coating failures observed on this tank? Why did these coating failures occur? What can be done to correct these coating failures? Where are the existing corrosion problems on this tank? What time of year and for how long can the tank be taken out of service for painting?

r What is the level of community acceptance of this tank? r What are the owner’s short- and long-term plans for this tank?

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C h a p t e r Tw e l v e After these questions have been answered, review possible coating alternatives. Weigh the advantages and disadvantages of each system so that the best system can be provided for a specific tank and owner. Research of coating alternatives should begin with a thorough review of applicable standards. This review should include AWWA D102 Standard for Painting Steel Water-Storage Tanks and the applicable standards of the SSPC. The coatings must comply with and be tested in accordance with the requirements of the NSF for coatings in contact with drinking water. Finally, the coating manufacturer’s performance test data and real-life case histories should be investigated, as well as any independent laboratory testing or documented service history.

Coating System Objectives The specifier should set objectives that will be compatible with the needs of the tank painter, engineers, and, most importantly, the tank owner, as follows:

r Reduce initial cost. r Provide the optimum coating life for the tank environment. r Minimize release of VOCs or other harmful materials into the atmosphere.

r Provide a coating system that will be easily maintained by touch-up and maintenance topcoating, thus minimizing the need for abrasive blasting to bare steel until the tank has been topcoated several times.

r Eliminate unsealed or uncoated interfaces of steel surfaces. r Provide excellent resistance to abrasion and be self-healing when subjected to minor abrasions and scratching.

r Provide excellent resistance to ultraviolet (UV) light, moisture, oil, soil, and chemicals.

r Provide a recoat window varying from a few days to as long as years for new tank projects.

r Meet all NSF standards and US Environmental Protection Agency (USEPA) regulations.

r Describe the system in generic or performance terms that do not rule out qualified coating manufacturers yet that uphold the standards of quality and performance necessary to provide the tank owner with the best possible system.

Interior Coating Systems Interior coating systems should offer long life; ease of application; abrasion resistance; and (in the case of open-top tanks) resistance to UV light, oil, dirt, chemicals (chlorine), and other contaminants.

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Ta n k R e h a b i l i t a t i o n Prior to the effective date of NSF Standard 61, Drinking Water System Components–Health Effects addressing direct and indirect water additives, there were many types of coatings: r Vinyls

r r r r r r r

Zinc dust/zinc oxide Chlorinated rubber Bituminous Coal tar Red lead Wax grease

Phenolic aluminum In the past, these coatings have worked with varying degrees of success. However, because strict environmental guidelines for toxins, heavy metals, VOC emissions, and other health threats have since been established, significantly fewer of these coatings are likely to meet new criteria set forth for use on tank interiors. The success of the two-component catalyzed epoxy appears to make it the frontrunner at this time. Epoxies can be and have been formulated with very high solids (low VOCs) and with many chemical varieties available. Two-component catalyzed epoxy is a highly versatile tank lining and coating. Another product for consideration is the solventless 100 percent solids polyurethanes. These products are not mixed, as conventional epoxies and urethanes are; they are sprayed with a dual pump arrangement that mixes the polyurethane at the spray gun tip. The advantage is less waste and a coating that cures for immersion within 48 hours. Manufacturers claim that these coatings offer from 20 percent to 38 percent longer life than epoxies, but in this author’s opinion, extensive field testing and evaluation are required to substantiate this claim. One last coating for consideration for water immersion is not really a coating at all. Spray metalizing using zinc, aluminum, or a combination of both has been used successfully for many years. Only recently has technology made this a viable option when considering costs. Zinc coatings can also be used for direct application to the water tank. While there is a lack of extensive service history in our industry, inorganic zinc coatings could be used on surfaces intended for immersion in potable water if NSF certified. However, these coatings should not be topcoated unless they are fully cured and hydrolyzed.

Exterior Coating Systems Like the interior coating systems, the exterior systems should also offer long life; abrasion resistance; ease of recoating; ease of application; and resistance to ultraviolet light, oil, dirt, salts, chemicals, and other

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C h a p t e r Tw e l v e contaminants. Because of potential community resistance, coatings requiring minimal to no abrasive blasting are attractive alternatives for future repainting needs. With ever-tightening regulations to control VOC emissions, the alternatives for exterior systems have been reduced significantly. Solvent-based aluminums, acrylics, vinyls, and chlorinated-rubber paints do not meet most VOC restrictions and have lately been used very little. Although we are never quite sure what direction the regulatory bodies will take when it comes to environmental issues, the trend appears to be to reduce emissions even further, resulting in lowering the VOCs of all coatings. Generic coating systems that currently meet most areas’ restrictions are high-solids alkyds, water-based acrylics, epoxies, polyurethanes, and inorganic and organic zinc-rich coatings. More stringent regulations will probably eliminate alkyds and all but the high-solids epoxies and polyurethanes in the future. The most widely used exterior system today is the epoxy–urethane system, sometimes with a zinc-rich primer and sometimes with an additional clear urethane topcoat. Water-based acrylics are becoming more popular—especially for overcoating, due to the minimal stress they have on existing coatings during cure and for congested sites where their “dry fall” characteristics are important. (Dry fall coating overspray releases all of its solvents as it falls through the air. The overspray is dry when it contacts the surface below.) A more recent technology is the use of solvent or new water-based fluorourethanes. Previously, these coatings were only available as a baked-coil coating material from which more than 25 years of color and gloss retention was normally expected. Time will tell if the newer air-dried versions will perform as well. Another category of exterior coatings comprises inorganic-based siloxane hybrids that claim to weather as well as, if not better than, conventional polyurethanes, but apply like high-build epoxies. Finally, there are varieties of coatings of several generic types that are formulated so that they can be applied over very minimally cleaned surfaces. They can be applied over rust, rust stain, old chalked and cracked paint, and other existing coating defects. Environmental issues, especially for lead paint removal and abrasive dust generation, have caused specifiers to strongly consider alternatives to conventional cleaning methods. To avoid open-air blasting, the coating industry has developed various methods to achieve the same degree of cleaning. Among these methods are containment of the structure with tarp material and the use of dust collectors to produce negative air pressure inside this containment.

Risks and Benefits of Repainting After considering coating systems and environmental issues, the specifier and the owner must decide the risks and benefits for the various options of repainting. Table 12-1 is a way of reviewing this Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com) Copyright © 2010 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.

Very low Low Moderate High High

Spot repair/spot repaint

Zone painting

Spot repair/overcoat

Complete removal/recoat

Replace tank

TABLE 12-1

High

High

Moderate

Low

Low

Cost 0

Comparison of Relative Risks, Benefits, and Costs

Note: N/A = not applicable.

Benefit None/negative

Alternative Do nothing

Low

Low

High

Moderate

High

Early Coating Failure N/A

Low

High

Low

Low

Low

Environmental Release N/A

Risks

Low

High

Low

Low

Low

Worker Contamination N/A

Low

High

Low

Moderate

High

Adverse Publicity High

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C h a p t e r Tw e l v e information. Costs may be inserted using an engineering estimate or even from the actual bids. Another consideration is when the tank can be drained and available for painting. If this cannot be done in normal warm-weather conditions, the coating system design must specify either coatings that can be applied and cured at colder temperatures or coatings that are applied while using heaters and dehumidification to control the environment. If the schedule is tight and downtime must be limited, multiple shifts using environmental control might overcome schedule constraints. Also, the coating system must meet the owner’s aesthetic requirements. Color availability and color and gloss retention are important aspects of aesthetic appeal. To the public, aesthetic appeal is often the most important aspect of the coating system. One final consideration when designing coating systems—the specifier must keep in mind the knowledge and abilities of the potential low bidder. The specifier must realize that if no independent field inspection of the work will occur, there is greater risk in specifying a coating system that is very sensitive to the quality of workmanship— as are nearly all of the new long-life systems. So, if little or no inspection is to be performed or if prequalification of the bidders is impossible, it is not recommended to specify coatings that are difficult to properly apply. Likewise, the specifier must be prepared to address problems that will undoubtedly occur in the field. This may even include demonstrating to the contractor how the specified coatings are to be mixed, applied, and cured. As new systems are developed, adequate product knowledge and practical field experience with these new coatings are lacking. Beware of being the guinea pig for new coating systems. Know what you are specifying! If you lack previous experience with the new product, observe application procedures and gain information from knowledgeable colleagues. It is important that specifiers continually learn about new coatings, equipment, procedures, regulations, and other important aspects of the coating industry through pertinent professional organizations and societies so they can provide the best service possible.

Overseeing Painting and Maintenance Many water tank owners seek autonomous verification that recoating and repairs are being performed in accordance with project specifications and generally accepted industry practice. By having a qualified and experienced professional be the on-site project representative while the work is being performed, the owner has independent assurance that the coatings will remain in good condition for their intended service life.

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Ta n k R e h a b i l i t a t i o n

Role and Qualifications of the Project Representative Long before any abrasive blasting and painting are performed on the water tank, the tank owner needs to determine what the role of the project representative will be and what qualifications that person must possess. Many water utilities require registration and licensing from one or more of the various industry associations (e.g., SSPC, NACE). However, possession of a specific industry’s license should not take the place of numerous years of experience in administering water tank rehabilitation projects. The role of the project representative must be determined and agreed on before the project begins. It is critical, then, that the description of the project representative’s duties, including the limitation of authority and responsibility, be communicated and made clear to all parties, including the painting contractor. For example, when an independent project representative is on-site, his or her responsibility (or lack thereof) for the safe work practices of the contractor’s personnel should be understood. Generally this is the case, since the project representative does not have direct control or supervision of the means, methods, techniques, sequences, or procedures of the contractor’s personnel, nor would the project representative generally be asked to issue direction regarding or assume control over the contractor’s compliance with environmental regulations. In most cases, the role of the project representative is to conduct on-site observation of the work in process and help the owner determine whether the work is in compliance with the specifications and with generally accepted industry practice. The project representative should also be expected to document and report to the tank owner any work that appears unsatisfactory or defective and advise the owner when additional testing appears necessary. The project representative should document his or her observations daily on an observation form. Topics might include number of contractor’s personnel on-site, surface profile measurements, paint batch numbers, area(s) of tank worked on, and ambient weather conditions. The written narrative should be supplemented with photographic documentation as determined necessary by the project representative. It is critical that this documentation be distributed to the owner and to the contractor’s foreman on a regular (daily) basis so that all parties are on the same page.

Role of the Water Tank Owner As previously discussed, the water tank owner is responsible for selecting the project representative and communicating the authority and limitations of that person’s duties to all parties. Next, the owner should designate someone else to act as the utility’s representative, a person who has the authority to transmit instructions, receive

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C h a p t e r Tw e l v e information, and interpret and define the owner’s decisions. The utility’s representative should also be responsible for arranging for access onto both public and private properties as necessary and for reviewing and providing input on all documentation submitted by the contractor and the engineer.

Observation of the Work in Process As mentioned previously, many utility owners view independent evaluation of the coating application and repairs as an essential part of a successful tank rehabilitation project. The capability of a coating to achieve its anticipated service life is directly related to the quality of workmanship during application. Verification of the workmanship assures the owner that the money spent on tank maintenance will be maximized. As applicable, the following should be verified daily by the on-site project representative:

r Temperature of steel r Weather conditions (temperature, wind velocity and direction, relative humidity, and dew point)

r Paint batch numbers used on the day of the observation r Location of work performed r Quality of work being performed and compliance with the r r r r r r r r r

project documents Wet and dry film thickness readings Calibration record of dry mil thickness gauge Measure of the paint cure Number of workers on the job Equipment on the job Recommendations made Estimated completion date Photographs of significant details Other pertinent data as required or requested

It is necessary that the on-site project representative, in addition to having the previously discussed qualifications, be trained and qualified to competently use the equipment necessary to verify the quality of the work. The project representative generally uses the following equipment and resources to observe the work when required to do so:

r SSPC-Vis 1 visual blasting standards r NACE Visual Standard TM-01-70/75 (available)

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Ta n k R e h a b i l i t a t i o n r r r r r r r r r r r

AWWA standards D100-96 and D102-97 Testex Press-O-Film Profile Measurement System Surface contamination detection device (soluble salts) Wet film thickness gauge Dry film thickness gauge Certified thickness calibration standards Steel temperature gauges Sling psychrometer and psychrometric tables Wet sponge holiday detector (low voltage) Tooke Gage (if required) Adhesive force measurement device (if required)

Contract Document and Specification Options It is important that the project specifications include specific tank upgrades as well as the tank repainting design. Repair specifications can be worded according to the precise methods to be used or the desired end result. The specification writer should review the strengths and weaknesses of each approach.

Precise Methods of Repair For a specification that outlines precise methods of repair, the specification writer (and therefore, ultimately, the tank owner) exercises a good deal of control over the contractor’s activities. This type of specification can result in fewer bidders who are willing to modify their standard procedures to comply with the specifications, and bid prices may be higher. Strictly adhered to, this method of specification preparation is a bit of overkill and places more liability on the specifier.

End Result Only This type of specification defines the repairs to be made but leaves it to the contractor to determine how to accomplish the repairs. The result is more bidders and possibly lower costs. It allows the contractor to use his or her standard methods of repair and can lead to the development of innovative procedures. However, the specifier and the owner have little or no control over methods used. A prudent specifying engineer uses the best of each method and writes a repair specification that results in the best bidders offering the most competitive bids that result in a long-lasting, high-quality repair. Quantity does not necessarily mean quality. The contract documents used in a tank rehabilitation project generally spell out such necessary requirements as insurance limits,

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C h a p t e r Tw e l v e bonding, length of contract time, and bidding information. Sometimes a utility has a complete set of contract documents; sometimes it just provides certain documents and/or input on minimum requirement levels. If the utility lacks a complete set of contract documents, there are organizations that provide boilerplate documents (e.g., the Engineering Joint Contract Documents Committee) that can easily be modified for tank rehabilitation. Regardless of how the contract documents are assembled, the utility must provide precisely exact input in numerous areas, including minimum insurance level requirements, liquidated damages amounts, on-site availability of water and electricity, and bid opening dates.

Contract Administration The specifier and the tank owner should collaborate to administer the project to make sure that the owner’s needs are being satisfied. The owner may prefer that some activities be performed on-site during a rehabilitation project; some activities are best overseen by either the specifying engineer or the on-site project representative. The project engineer should verify compliance with the project specifications and contract documents to ensure that both the letter and the intent of the documents are being followed. The go-ahead for work to proceed should not be given until all submittals have been reviewed and accepted. After all of the submittals have been reviewed and accepted, numerous other administration activities need to be done, including these:

r Pre-job conference attendance r Consultation on adequacy of and compliance with the project specifications

r Specification interpretation r Attendance at the preconstruction meeting and all subsequent r r r r

meetings, and provision of meeting notes Review of all contractors’ submittals and shop drawings Review of construction schedule Review and approval of materials Preparation for negotiations of change orders and assistance with supplemental agreements

r Review and approval of payment requests r Dispute settlement r Public relations

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First Anniversary Evaluation A first anniversary evaluation, as recommended in AWWA D102, should be called for in the project specifications and should be scheduled prior to the end of the one-year bonded guarantee. The purpose of this evaluation is to identify and repair defective work before the bonding period ends. The water tank owner should be responsible for making the tank available and coordinating the date of the evaluation with the contractor and the project representative. The contractor should be required to complete the tank washout the day before the evaluation and should also be responsible for meeting the following requirements:

r Have an experienced foreman present. r Be prepared to perform minor touch-up work. r Bring all rigging necessary to performance of the touch-up work.

r Bring at least 1 gal (3.79 L) each of the exterior primer, intermediate coating, and finish coating.

r Bring at least 1 gal (3.79 L) each of an interior coating that can be placed in immersion service immediately for minor spot repairs.

r Bring Scotch-BriteTM abrasive disks with power tools and sandpaper to clean the steel surface.

r Supply equipment with which to apply coating repairs. r Supply equipment with which to wash out the tank and chlorine to disinfect it following the evaluation and any required touch-up work. The project representative should prepare and submit to the water tank owner a brief report with color photographs of the conditions found during the first anniversary evaluation and of the touch-up work.

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