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General Guidelines for the Preliminary Design For Segmental Concrete Box Girder Super... Page 1 of 10

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General Guidelines for the Preliminary Design For Segmental Concrete Box Girder Superstructure General Guidelines for the Preliminary Design For Segmental Concrete Box Girder Superstructure [1]

Structural engineering [2] The design of a segmental bridge commences with consideration of the function. The most significant difference from other bridges is the emphasis given to the method of construction, which provides segmental bridges the adaptability to many applications and categorizes this type of bridge. The method of construction must be established before proceeding with the design. The segmental concrete bridge construction forms an inter-relationship of design and construction knowledge. Given the impact of construction methods and construction loads on the basic design of the system, a design for service load alone is inappropriate, the engineer has to evaluate each progression stage of construction and account for imposing loads. An engineer must evaluate the project as a whole before deciding on the method of construction, span arrangement and cross section to be used. A feasible, cost effective and complete construction sequence and methodology should be presented on the design drawings. Maximum leeway for contractor's modifications with regard to construction technology should be provided in the specifications, but the basic system should be clearly and concisely presented to facilitate the receipt of accurate and responsive bids. Given the changing structural system during construction of segmental bridges, the designer is required to determine and check for all service and ultimate loads upon completion of construction and after all long term losses (creep, shrinkage and steel relaxation) has occurred. This is usually assumed at the 10th year of structure being in

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service. The construction method will be a major determinant in the dead load Stress State. While temporary measures and /or details may be required, segmental structures typically do not require additional permanent strengthening. Should a design require additional strengthening, one should review the choice of construction methodology in order to determine a method or sequence with no impact to the permanent structure. Design should reference the AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges.

Construction Method The structural design is dependent upon the construction method because it is necessary to take into account all the intermediate construction stages including changes in statical scheme, sequence of installing tendons, maturity of concrete at loading and load effects from erection equipment. The various types of segmental construction available today are: a. Span by Span Method: In span by span method the form traveler consists of steel superstructure generally a span length and supported on piers, form traveler is moved from the completed structure to the next span to be cast. The formwork is suspended with steel rods during casting. After concrete placed and post tensioned, the forms are released and moved forward. This type of construction usually requires additional clearances due to the supporting truss. From a competitive point of view, the capital investment in the equipment for this type of construction is considerable. b. Incrementally Launched: Segments of the bridge superstructure are cast in place in lengths of 30 to 100 ft in stationary forms located behind the abutments. Each unit is cast directly against the previous unit. After sufficient concrete strength is reached, the new unit is post-tensioned to the previous one. The assembly of units is pushed forward in a stepwise manner to permit casting of structure succeeding segments. Stringent dimensional control, however, is an absolute necessity at the stationary casting site. Bridge alignment in this type of construction may be either straight or curved; however, the curve must have a constant radius. To allow the superstructure to move forward, special low-friction sliding bearing are provided at the various piers with proper lateral guides. c. Progressive Cantilever: Progressive placement is similar to the span-by-span method. In progressive placement the precast segments are placed from one end of the structure to the other in successive cantilevers on the same side of the various piers rather than

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by balanced cantilever on each side of pier. Because of the lenght of cantilever (one span) in relation to construction depth, a movable temporary stay arrangement must be used to limit the cantilever stresses during construction to a reasonable level. d. Free Cantilever: In cast-in-place construction the formwork is supported from a movable form carrier. The form traveler moves forward on rails attached to the deck of the completed structure and is anchored to the deck at rear. With the form traveler in place, a new segment formed, cast, and stressed to the previously constructed segment. Where a long viaduct is to be constructed of cast-in-place segments, an auxiliary steel girder may be used to support the formwork. This equipment may also be used to stabilize the free-standing pier by the anchoring of the auxiliary steel girder to the completed portion of the structure. e. Balanced Cantilever: In the balanced cantilever construction, segments are placed in a symmetrical fashion about a pier. The cantilever tendons are the principal reinforcement of the structural system; they are located in the deck slab and are anchored at the ends of the segment. The midspan tendons, located in the bottom slab near the webs to resist the positive moments in the middle third of the span and the continuity tendons are used as reserve prestressing, designed on the basis of actual deformations measured after closure. f. Cable Stay: When a span is beyond the reach of a conventional girder bridge, a logical step is to suspend the deck by a system of pylons and stays. The cables are in a harp arrangement rendering an aesthetically pleasing. The pier table, the first segment of the deck at the pylon is built on falsework or supported from the pier. Form traveler is installed on each side of the pier table. The superstructure can be cast in place or precast elements and connected to the pylons via cables. The appropriateness of the method of construction to be used depends on several factors that include the horizontal and vertical clearances, alignment, geometry, construction schedule, site constraints, environmental requirements, and aesthetics. The options to use cast-in-place or precast segmental for the various methods of construction will depend primarily on the project size, construction schedule, span length, and access to the site. Following are economical span ranges for various types of segmental construction. For overlapping ranges, the site constraints will dictate the use of the segmental construction technique.

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Construction Method

Superstructure Depth (ft)

Economical Span Range (ft)

Span-by-span

Precast

Constant 6

up to 110

Precast

Constant 6 to 8

110- 150

Precast/ Cast-in-place Constant 7 to 12

120- 160

Incremental Launch

Cast-in-place

Constant 8 to 12

up to 240

Progressive Cantilever

Precast

Constant 8 to 10

up to 200

Balanced Cantilever

Precast

Constant 6 to 12

160 - 260

Precast

Variable 6 to 20

200 - 450*

Cast-in-place

Variable 6 to 40

260 - 750

Cable Stay

Precast or Cast-in- Constant 6 to 15 place by cantilever erection

500-1500

* The weight of the haunched segments near the pier diminishes the feasibility of using precast segments in balanced cantilever much beyond the 400 feet range unless the segment weight can be decreased by splitting the segment, using a combination of precast and cast-in-place construction, or other means.

Cross Section: The optimum selection of the proportions of the box section is generally a matter of experience. The AASHTO, ASBI-PCI segmental box girder standards should be referenced for most bridges. The various parameters that should be considered at the start of a design are:








Constant vs. variable depth Span to Depth ratio Number of parallel box girders Shape and dimensions of each box girder, including number of webs, vertical or inclined webs, thickness of webs, top and bottom flanges. Accessibility/ inspection of superstructure

All these factors are closely related to each other, and they also depend largely upon construction requirements. Bays with clear height of less than 7 feet is uncomfortable for the inspectors to inspect inside of the bridge.

Constant vs. variable depth: Constant girder depth is the easiest choice and affords the best solution for short and

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moderate spans approximately 260 ft. However, constant depths have been used for aesthetic reasons for spans up to 450 ft. such as Pine Valley Bridge in California. When the span increases, the magnitude of dead load moments near the piers normally requires a variation of structural depth and parabolic soffit; it also would be more economical to vary the section.

Span-to-Depth Ratio: Girder depth determined in accordance with the following criteria will generally provide satisfactory for service, deflection and ultimate behavior: A. Constant depth girder 1/15>h/L>1/30

Optimum 1/18 to 1/20

In the case of incrementally launched girders the girder depth should preferably be within the following limits: L=100' L=200' L=300'

1/151/20 optimum 1/18 At center of span: 1/30>ho/L>1/50

Superstructure shape: A single cell box should preferably be used when the top flange width is less than or equal to 6 times the box depth. A 50 to 60 feet top slab width is the economical upper limit for single cell box. Web spacing is usually selected between 15 and 25 feet to reduce the number of webs to a minimum, simplifying construction problems while keeping the transverse bending moment in the top and bottom flanges within reasonable limits. Large overhang cantilevers and a large span lengths between webs are accepted with special provisions to carry the deck live load transversely. Transverse prestressing, edge beams at the tip of the overhang and ribs to carry the load can accommodate this. Alternatively several boxes may be used side by side to make up a superstructure with Joining deck between the two box section.

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The greatest ease of construction is achieved through the use of cross sections with two webs. Two webs may nevertheless be insufficient in certain cases. Wide bridges can have either one wide cell or parallel single box girders connected with longitudinal in-situ pour. The main disadvantage of single cell cross section is that the entire bridge must be closed during major repairs or demolition.

Cross section dimensions: L1 / L2>0.45 where L1 is measured from center of girder to the edge of deck. L haunch / L slab~0.2 to 0.3 t1/ t2; 1:1.5 to 1:2 (typical min. depth t2 to anchor longitudinal Cantilever tendons = 18 inches) Top slab thickness = t3>L3/30 t5>L5/30 t1 > 9" if transverse P/S, otherwise 8 inches minimum. t3 > 8" If L3 < 15' (Reinforced deck) > 10" If L3 < 15' (Transverse prestressed deck) Bottom Slab

= t5 = 7" If L5 < 15' = 8" If L5 < 15'

t4 > 14" to facilitate concrete placement Transverse prestressing tendons shall be used for decks where the clear span between webs or haunches L2 is 15 feet or greater. Dimension of t6 to be determined through stress analysis to insure the allowable stresses are not exceeded.

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Additional deck thickness (½ to 1 inch) is usually provided for profile grinding of the deck unless a separate overlay is to be provided. An integral overlay may be provided by increasing the cover over the top reinforcing by the depth of the integral overlay. (Additional overlay protection such as high performance concrete or polyester concrete is recommended for corrosion protection of the superstructure).

Longitudinal Post -Tensioning: Two types of longitudinal tendons are used in segmental construction. External tendons (external to the concrete section) and internal tendons (internal to the concrete section). Generally for span-by-span construction, external tendons are used and for cantilever construction internal tendons are used. Cantilever construction generally uses cantilever tendons located in top slab and positive moment tendons located in the bottom slab. Continuity tendons when required are draped and usually are external. In seismic zones all tendons has to be internal.

Physical and weight limitations Precast segmental: Trucking: Generally typical length between 8 to 10 feet and weight of 40-60 TONS, assuming 155 pcf concrete and rebar can be transported. Lane width and bridge clearances on trucking route can limit the size of the segments to be transported. Shipping waterways: No limit Erection: typical equipment 80 TON max. With gantry 80-100 TON. Typical length 8-12 feet. Handling stresses: Lifting induces stresses, must be investigated.

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No structural limits due to length / weight.

Cast in place balance cantilever segmental:








No. of segments150: it is justified to fabricate the traveler for the job. Therefore length and weight of the segments can be tailored to the job. Typical form traveler weight: Contact manufacturers and refer to AASHTO 25.4

Additional considerations for balance cantilever construction: Small portions of the superstructure at piers are constructed on falsework and are usually designated as a "Pier table". On cast in place segmental bridges, the pier table must be long enough to place two form travelers back to back (generally 30 to 40 feet long). The pier table usually constructed ½ segment length out of balance to minimize the unbalanced effects during segment construction. Designer should perform an initial hand (or spreadsheet) calculation of cantilever construction with placement of last segment to determine approximate cantilever P/S area required and check loads on pier section. On larger structures twin wall piers may be advantageous to reduce lateral stiffness for thermal and seismic forces while being very efficient to resist large segmental construction moments. On narrow structures with short overhangs deck drainage system may be difficult to impossible to install due to conflict with cantilever tendons and drainage box and or piping. Minimize variations (special segment lengths). Standardization is the key to cost effective segment design. Limit the size of cantilever tendons to one size for the entire project. For reduction in future maintenance, maximize the length of superstructure continuity to minimize expansion joints and minimize use of bearings. If bearing used, plan for future bearing replacement. In balanced cantilever construction the end spans are usually 0.6L to 0.8L of the adjacent span and often 0.5L to 0.6L is used. When 0.5L end span is considered the bridge may require ballast to prevent uplift and

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where end span is over 0.5L, the end span usually constructed on cast in place falsework and connected to the cantilever portion by cast in place closures. It usually takes about 3-6 days to fabricate and cast a segment with post tensioning followed the day after casting completed.

References: 1. AASHTO, Guide Specification for Design and Construction of Segmental Concrete Bridges, second edition 1999. 2. Walter Podolny, Jr. and Jean M. Muller, " Construction and Design of Prestressed Concrete Segmental Bridges. 3. Rafael Manzanarez, " Design and Construction of Segmental Concrete Bridges" Caltrans Seminar Notes, July 1999. 4. AASHTO LRFD Bridge Design Specifications. 5. AASHTO LRFD Bridge Construction Specification. 6. ASBI Recommended Practice for Design and construction of Concrete Segmental Bridges, 2001. by Majid Madani Related Content: Dynamics of Structure and Foundation [3] Scaling of structural strength [4] Engineering Structural Welding [5] Structural engineering Home | Privacy Policy | Contact Copyright © 2008 2doworld Group, platform by Drupal // // Source URL: http://www.2doworld.com/structural-engineering/general-guidelines-preliminary-designsegmental-concrete-box-girder-superstructure.html Links: [1] http://www.2doworld.com/structural-engineering/general-guidelines-preliminary-design-segmentalconcrete-box-girder-superstructure.html [2] http://www.2doworld.com/elearning/structural-engineering.html [3] http://www.2doworld.com/school-engineering/dynamics-structure-and-foundation.html [4] http://www.2doworld.com/content/scaling_structural_strength.html [5] http://www.2doworld.com/content/engineering_structural_welding.html

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