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DESIGN OF BUILDINGS AND STRUCTURES UNDER THE VIETNAMESE STANDARDS

1. GENERAL 1.1. Compulsory Vietnamese building standards Design of buildings and structures (hereafter referred as structures) in Vietnam shall be in conformity with the compulsory Vietnamese Standards in the following areas: a) Climatic conditions; b) Hydro-geological and hydro-meteorological conditions; c) Seismic zoning map including the PGA map or table (PGA-Peak Ground Accelerations); d) Fire and explosion protection and prevention; e) Environmental protection; f) Work safety. For items d), e) and f), if there is no Vietnamese Standard available or not are fully covered in the current Vietnamese Standards, then relevant International Standards will be allowed to be used in the frame of the present regulation of the application of foreign building standards in construction activities of Vietnam. 1.2. Basic design standards

Most of TCXD and TCXDVN have been renamed to TCVN

Structural and foundation designs shall be in compliance with the following Standards: (1) TCXD 40:1987 Building structures and foundations – Principles for calculations (2) TCVN 2737:1995 Loads and actions – Norm for Design (3) TCXDVN 375:2006 Seismic resistance design (4) TCXDVN 356:2005 Concrete structures – Design Standard (former one is TCVN 5574:1991) (5) TCXDVN 338:2005 Steel structures – Design Standard (former one is TCVN 5575:1991) (6) TCVN 5573:1991 Masonry and reinforced masonry - Design Standard (7) TCXD 198:1997 High-rise buildings – Design of monolithic concrete structures for multi-story buildings (8) TCXD 45-78 Design Standard for foundations (9) TCXD 205:1998 Pile foundations – Design Standard (10) TCXD 195:1997 High rise buildings – Design of bored piles

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(11) TCXD 198:1996 Small section concrete piles – Design standard When the Vietnamese authority issues decrees to replace, cancel or simultaneously use one or several of the above-mentioned Standards, analysis and design shall be carried out in compliance with those decrees. In certain circumstances, relevant international standards may also be applied for buildings constructed in Vietnam. However, the usage of those standards must be in compliance with the present Regulations for Application of Foreign Standards in Construction Industry. And the present regulation is Regulations for Application of Foreign Standards in Construction issued with the Decision No 09/2005/QD-BXD on the 7th April 2005 by Minister for Construction of Vietnam. 1.3. Limit States for structural design Buildings and structures, designed following the limit states approaches, must be safe (not to be collapsed and cause no harms to occupants) throughout their construction time and their intended design lives under all combinations of the considered loads and actions possibly acting on the structures including the most disadvantageous loading combinations. 1.3.1. There are two limit states to be considered in design analysis (calculations and verifications) (1) Ultimate Limit States group: The ultimate limit states are the limit states that a structure looses its strength and/or stability and consequently collapses, endangering safety to occupants due to: - Damages under loads and actions; - Instability; - Fatigue damages. (2) Serviceability Limit States group: The Serviceability Limit States are the limit states that a structure cannot maintain its normal serviceability due to excessive amounts of the following criteria: - Deformation: displacement, deflection, drift angle, crack widths (mainly for concrete and reinforced concrete structures); - Vibration. 1.3.2. Structural analysis for ultimate limit state shall be performed in the condition of: T ≤ TTD

(1)

where,

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T – critical value caused by an internal force or several internal forces under the most adverse load combination; TTD – minimum limit of a cross-section’s strength corresponding to internal force T (calculated using a specified probability or specified partial safety factors). T shall be determined from design loads (i.e. characteristic (unfactored) loads multiplied with reliable factors or load factors) and shall be chosen from the adverse load combinations considering both magnitude and direction. TTD shall be determined from the geometries of sections, design strengths of materials (chracteristic strengths deviding to safety factors of material strengths and multipled by the working condition factor), and the slenderness of a structure or a structural element (for stability). Condition (1) shall be satisfied for all sections under all load combinations occurring during construction, use, repair and maintenance of the structures. 1.3.3. Structural analysis for serviceability limit state shall be performed in the condition that: f ≤ fgh

(2)

where f – deformations of structures including deflection, rotation, lateral displacement, difference in lateral displacements between stories in high-rise buildings, vibration amplitudes, crack width etc. caused by characteristic loads. fgh – limit value depends on the characteristics, the usage conditions, and the importance of the structures; working conditions for people and equipments; psychology of people and aesthetic views. 1.4. Limit state analyses for foundation design Foundation design can also be performed based on the following limit states: ultimate limit states and serviceability limit states. Ultimate limit states shall be carried out to ensure the foundation’s stability and integrity under the basic load combinations and the special load combinations. Serviceability limit state analysis (for settlements and deformations) shall be conducted to ensure the deformations of the structures not to exceed the limit values and maintains the

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normal services and aesthetic views of the structures. Foundation design using serviceability limit state analysis is based on the basic load combination without considering the temperature effects. In addition, other adverse effects by rainwater, flooding, tide and ground water shall also be considered in foundation design. Special attention shall be paid to the seasonal variation of the ground water level and soil moisture content during construction and usage of structures. Corrosion protection measures for foundations and piles shall be applied if the ground water, tide, run-off water and process water are corrosive. 1.5. Loads and load combinations Loads and actions used for structural and foundation design shall be determined following the specifications of the current Vietnamese Standards for loads and actions and other relevant standards. (Note: Current Vietnamese Standard for loads and actions used in practise is TCVN 2737:1995 Loads and actions – Design standard). 1.5.1. Load levels Loads are defined into two levels: (i) standard loads (or characteristic loads) and (ii) design loads. The standard loads are those of main characteristics. The design loads are the products of the standard loads and reliability factors (or load factors) γ. The load factors account for adverse variations of loads compared to the standard values and are associated with the limit states considered. In common practice, design loads shall be used for ultimate limit states analysis and standard loads shall be used for serviceability limit state and fatigue analysis (if no specific requirements in relevant standards are available). 1.5.2. Load classifications Loads are classified as: a. Permanent loads (can be either standard loads or design loads): are those remain unchanged during construction of structures and the structures in operation or services. The permanent loads include self-weights of main structural elements and covering elements, weights and soil pressures of soils and fills, static hydraulic pressures, prestressed forces and weights of equipments.

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b. Short-term and long-term temporary loads include weights of temporary partitions; weights of equipments; weight and hydraulic pressure of contained water; dust weight; crane loads; equipment operational loads; pressure of bulk materials; roof and floor live loads; actions from soil deformation (exclusive of changes in soil structure); temperature, shrinkage and creep effects; loads occur during construction and maintenance; loads occur during start-up, commissioning and relocation of equipments and machinery; lifting loads; vehicle loads; wind loads; dynamic hydraulic pressures (by water flow) etc. c. Special loads include seismic load; explosion load; fire load; tornado-like-wind loads; accidental loads; effects of soil deformation due to variation of soil structure or due to soil cracks, mining activities and cavities. The values of the standard loads and the load factors are given in TCVN 2737:1995, TCXDVN 375:2006 and related standards. 1.5.3. Load combinations Load combinations are divided into two categories: basic load combinations and special load combinations. A basic load combination normally consists of permanent loads, short-term and long-term temporary loads. A special load combination includes permanent loads, long-term temporary loads, possible short-term temporary loads and one of the special loads. The load-combination factors shall also be determined from TCVN 2737:1995. 1.6. Building and structure classification Buildings and structures are classified for service and importance level. The importance level of a structure is associated with risks to human lives after collapse, emergency level, public safety, rescue and protection of people after a natural disaster occurred, and the consequences of damaged or collapsed structures on national economy. Classification of buildings and structures are presented in Appendix K. Determination of the importance level is specified in the seismic resistance design section.

2. Guidelines for analysis of buildings and structures under the actions of the tropical typhoon actions 2.1. General

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Actions (or loads) caused by tropical typhoons on buildings and structures are not classified as special load case. Analysis and design of buildings and structures under the tropical typhoon loading shall be conducted under the specifications and guidelines of the Vietnamese standards including TCVN 2737:1995, TCXDVN 356:2005 (TCVN 5574:1991), TCXDVN 338:2005 (TCVN 5575:1991) and other related design standards. In cases the international building standards are used for the structural design, the tropical typhoon (or cyclonic) loading shall be calculated based on the parameters such as: the basic wind speed or the basic wind pressure depending on the applied code. Values of these basic parameters will however be determined based on the to the standard wind pressure considering the gust-time and the return period as well as the terrain categories at site stipulated in the Vietnamese loading standard. 2.2. Basic design parameters 2.2.1. Wind pressure zones A wind pressure zone of a construction site shall be determined from the wind pressure zone map of Vietnam (Appendix A) or from the wind pressure zones for Vietnam’s administrative locations (Appendix B). Value of the load induced by the tropical typhoons acting on buildings and structures are dependent on the wind pressure zone and the standard wind pressure (W0) of the construction site based on its geographic location or its administrative name. The territory of Vietnam can be divided into 9 zones according to wind pressures for construction works. Table 7.1 classifies the wind pressure zones and the value of standard wind pressure W0 based on the wind pressure zone map of Vietnam. Table 1: Wind pressure zones in Vietnam for construction work (Rivised version of TCVN 2737:1995 but not officially issued) Storm and tropical cyclonic impacts No Wind Standard pressure wind zone pressure W0 (daN/m2) 1 IA 55 Regions not affected or minorly affected by tropical typhoons 2 I 65 Regions of mountains, hills, valleys and plains belong to zone IA in wind pressure zone map 3 IIA 83 Regions not affected or minorly affected by

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tropical typhoons 4 IIB 95 Regions affected by tropical typhoons 5 IIIA 115 Regions not affected or minorly affected by tropical typhoons 6 IIIB 125 Regions affected by tropical typhoons 7 IVB 155 Regions affected by tropical typhoons 8 VB 185 Regions affected by tropical typhoons 9 VIB 215 Regions affected by tropical typhoons (Source: TCXDVN 2006: Loads and Actions – Design Standard)

If buildings and structures are to be constructed in the mountainous regions or on islands with same altitudes, terrain conditions and nearby the meteorological stations mentioned in Appendix C, then wind pressure values with return period of 50 years shall be taken at these stations (Tables C-1 and C-2, Appendix C). For complicated terrains such as defiles and mountain passes etc., the standard wind pressure W0 shall be determined from the basic wind speed v0 obtained from the data provided by the Department of Hydro-meteorology or from the processed data of the actual site surveys taking into account the historical use of buildings and structures. 2.2.2. Basic wind speed (v0) The basic wind speed v0 (m/s) is the mean wind speed corresponds to 3-second gust wind speed exceeded once in 50 years at 10m above ground in Terrain Category B (see section 2.2.3). Values of the basic wind speed is generally used to calculate the standard wind pressures (W0) on buildings and structures at the areas of the construction sites. These values shall be obtained from the current Standard or from the available data provided by authorised government bodies which are more up-to-date that the data given in the Standard. In general, the standard pressure is computed using the expression below: W0 = 0.0613 x v02

(3)

If international standards are utilised for design of buildings and structures in Vietnam, the basic wind speed v0 (3-second gust wind) of the Vietnamese Loading Standard can be used as a basic parameter to determine the basic wind speed or the basic wind pressure used in these international standards. Appendix C shows the values of 3-second gust speeds for the return periods of 10 years, 20 years, 50 years and 100 years at 10m above ground in Terrain Category B for different zones in Vietnam.

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2.2.3. Terrain categories There are four terrain categories for wind load calculations including: Terrain category A, Terrain category B, Terrain category C and Complicated terrain. Terrain category A addresses the regions of flat and open spaces without or with very small number of obstructing objects of less than 1.5m in heights. Beaches, river surface, large lakes, salt flats, open rice fields, etc. are in this category. Terrain category B is the term describes the regions of fairly open spaces with scattered obstructing objects of less than 10m in heights (low populated suburban areas, towns, villages, young forests, sparse tree growing areas, etc.). Terrain category C defines the regions heavily shielded with close obstructing objects of more than 10m in heights (inner cities, dense forests etc.). Complicated terrain is termed for the regions (which are not included in Categories A, B or C). Mountain ridges, islands, defiles, mountain passes fall into complicated terrain. A building or structure is said to be in one terrain category if the topographical features of that terrain does not change within the distance of 30Z (Z is the height of the building) for Z≤60m and of 2km for Z ≥60m from the windward edge of the building. 2.2.4. Reference level and building height (Z) The reference level and building heights are 2 main parameters essential to determine the wind loads acting on buildings and structures. The current Vietnamese Loading Standard specifies the procedure to obtain the reference level and the building height as follows: (1) If the ground slope i ≤ 0.3, the building height Z shall be measured from the level at the base of the building to the building’s top or to points on the building where the wind load is considered. (2) If the ground slope 0.3 < i < 2, the height Z shall be measured from the conventional level Z0, which is lower than the actual ground level, to the considered point. Determination of the conventional level Z0 is illustrated in Fig. 1.

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Wind direction

On the left-side of point A: Z 0 = Z 1 On segment BC: Z 0 = H x (2 - i) / 1.7 On the right-side of point D: Z 0 = Z 2 On segments AB and CD: Z0 shall be linearly interpolated. Figure 1: Determination of reference level and building heights if i < 2 (TCVN 2737:1995)

(3) If the ground slope i ≥ 2, the conventional level Z 0 shall be determined as shown in Fig. 2.

Wind direction

On the left-side of point C: Z0 = Z1 On the right-side of point D: Z0 = Z2 On segment CD: Z0 shall be linearly interpolated. Figure 7.2: Determination of reference level and building heights if i ≥2 (TCVN 2737:1995)

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2.2.5. Determination of the wind load reliability factor (load factor) γ The revised version of the Vietnamese Loading Standard TCVN 2737:1995 sets out the value of 1.2 for the wind load factor γ , which corresponds to the building’s intended life of 50 years. An adjustment factor is introduced as given in Table 2 if other intended life is chosen. Table 7.2: Adjustment factors for determining the wind load for different intended lives of buildings (TCVN 2737:1995 – revised version) Intended life, years 10 20 30 40 60 70 80 90 100 50 Wind load 0,664 0,805 0,888 0,948 1,000 1,035 1,069 1,099 1,125 1,151 adjustment factor

2.3. Determination of wind load in accordance to Vietnamese loading standard 2.3.1. Components of force in the wind direction The wind load acting on a structure consists of the following components: normal pressure We, frictional force Wf and normal pressure Wi. The normal pressure We is acting on the external surfaces of a building and its elements. In opposite, the normal pressure Wi is acting on the internal surfaces of a building not fully enclosed or with openings (either temporary or permanent open) on the building envelope. Frictional force Wf is tangential to the external surface and is proportional to the plan projected area (for saw-tooth roofs, wavy roofs and roofs with windows) or the side projected area (for walls with openings and similar structures). The wind load can also be analysed into two components of normal pressures Wx and Wy corresponding to the obstructing surfaces of a building on x-axis and y-axis respectively on the plan view. The obstructing surface of a building is the projection area of the building on the plane perpendicular to the corresponding axis. 2.3.2. Dynamic and static components of the wind load Considering the dynamic characteristics and the sensitivity to wind load of a structure, the wind load consists of static component (W) and dynamic component (Wp). It is not compulsory to calculate the dynamic component of the wind load for determination of the internal pressure Wi and for design of domestic buildings with heights of less than 40m or one-storey industrial buildings with the height-to-span ratio of less than 1.5 in the terrain categories A, B and C.

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The dynamic component of the wind load should be considered for design calculations of vertical hollow structures, tower structures, chimneys, column like structures, handrails for belt conveyors, open space frame structures, high-rise building with heights of greater than 40m, portal frames for on-storey industrial buildings with heights of greater than 36m and the height-to-span ratio of greater than 1.5. The total wind load shall be calculated as: Wtotal = W + WP

(4)

2.3.3. Determination of the static component of the wind load (W) (1) Standard value of the wind load’s static component W The standard value of the wind load’s static component W at level Z above the reference ground level shall be computed as: W = W0 x k x c

(5)

where, k – a factor to account for the variation of the wind pressure along a building’s height and in different terrain categories; c – aerodynamic factor. (2) Determination of factor k The factor to account for the variation of the wind pressure along a building’s height and in different terrain categories (k) are given in Table 3. Table 3: Factor k Terrain category Height Z, m (1) 3 5 10 15 20 30 40 50 60

A

B

C

(2) 1,00 1,07 1,18 1,24 1,29 1,37 1,43 1,47 1,51

(3) 0,80 0,88 1,00 1,08 1,13 1,22 1,28 1,34 1,38

(4) 0,47 0,54 0,66 0,74 0,80 0,89 0,97 1,03 1,08

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80 100 150 200 250 300 350 ≥ 400

1,57 1,62 1,72 1,79 1,84 1,84 1,84 1,84

1,45 1,51 1,63 1,71 1,78 1,84 1,84 1,84

1,18 1,25 1,40 1,52 1,62 1,70 1,78 1,84

Notes: 1) The value of factor k shall be linearly interpolated for intermediate heights. 2) For different wind directions, the terrain categories might also be different. 3) For terrain categories other than categories A, B or C, additional research is required.

(3) Determination of aerodynamic factor c Wind load distribution diagram on a building and a structure and the factor c shall be determined as guided in TCVN 2737:1995 (or Appendix D attached to this paper). Linear interpolation shall be allowed for the intermediate values. For special structures not included in Appendix D, the factor c shall be obtained by wind tunnel test or from relevant documents in Vietnam or overseas. 2.3.4. Determination of the dynamic component of wind load (WP) 2.3.4.1. Dynamic characteristics and dynamic correlation of structures (1) Characteristic natural period and frequency of structures The characteristic natural period at mode i of the building (Ti, the index i addresses the ith natural mode of vibration), the i-th natural frequency (fi) and the mode shape of natural vibration of a structure are determined by structural dynamic method (e.g. Rayleigh’s method) or by finite element model using the computer programs that can conduct the dynamic analysis and determine the natural frequencies and vibration modes of the structure. For buildings and structures with the height of less than 40m, the fundamental (or 1st) natural period of structure T1 (in sec) will possibly be approximated by using the following formulae: T1 = Ct x H 3/4

(6)

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where: Ct = 0.085 for structural steel frames; Ct = 0.075 for reinforced concrete frames; Ct = 0.050 for other structures; H = building height, in m, measured from the foundation surface or the top of the rigid base. The ith natural frequency fi (in Hz) of a structure will be calculated as: fi = 1.0 / Ti

(7)

(2) Logarithm reduction factor of vibration δ To determine the dynamic component of the wind load, the logarithm reduction factor of vibration δ (or in other words, the counter vibration effects) shall be taken as: a) δ = 0.3 – for reinforced concrete and masonry buildings and cladding steel frame buildings. b) δ = 0.15 – for tower structures, steel chimneys, and other column-like steel structures supported by reinforced concrete base. •

Limit of the natural fundamental frequency of vibration fL

The limit of the natural fundamental frequency of vibration fL (in Hz) of a building are the value of frequency at which it is not necessary to consider the inertial forces occurring while the building is vibrating at its corresponding natural frequency. This limit is given and Table 4 and dependent on the logarithm reduction factor of vibration δ. Table 7.4: Limits of fL the characteristic frequency of vibration fL fL (Hz) Wind pressure zone δ = 0,3 δ = 0,15 (1) (2) (3) I, IA 1,1 3,4 IIA, IIB 1,3 4,1 IIIA, IIIB 1,6 5,0 IVB 1,7 5,6 VB 1,9 5,9 VIB -

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Space correlation factor of the dynamic component of wind load (ν)

•

The space correlation factor of the dynamic component of wind load (ν) shall be determined from surfaces of a building on which the dynamic correlations are calculated. The surfaces used for calculations include windward surfaces, leeward surfaces, sidewalls, roofs and other structural elements transferring the wind load. If the surfaces are of rectangular shape and parallel to the basic axes x, y, z (see Fig. 3, plane x-y addresses the plan-view of the building, and z is for the building’s height), then the factor ν shall be obtained from Table 5 in correlation with the parameters ρ and χ which in turn are given in Table 6.

x

z

Wind direction

h

0

a

b

y

Figure 7.3: Coordinate system for determination of ν

Table 5: Space correlation factor ν Factor ν

χ (m) ρ (m) 0,1 5 10 20 40 80 160

5

10

20

40

80

160

350

0,95 0,89 0,85 0,80 0,72 0,63 0,53

0,92 0,87 0,84 0,78 0,72 0,63 0,53

0,88 0,84 0,81 0,76 0,70 0,61 0,52

0,83 0,80 0,77 0,73 0,67 0,59 0,50

0,76 0,73 0,71 0,68 0,63 0,56 0,47

0,67 0,65 0,64 0,61 0,57 0,51 0,44

0,56 0,54 0,53 0,51 0,48 0,44 0,38

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Table 6: Parameters ρ and χ Basic plane parallel to the considered surface zoy

ρ

χ

b

h

zox

0,4a

h

xoy

b

a

(2) Determining the standard value of the dynamic component WP The standard value of the dynamic component of the wind load WP at height Z shall be determined as follows: a) For buildings and structural elements with the fundamental natural frequency of vibration fI is greater then fL, WP shall be computed using the below expression: WP = W x ζ x ν

(8)

where, W – standard value of the wind load static component at height Z ζ - dynamic pressure factor at height Z (given in Table 7) Table 7: Dynamic pressure factor ζ Height Z, m ≤5 10 20 40 60 80 100 150 200 250 300 350 ≥ 480

Dynamic pressure factor ζ for different terrain categories A B C 0.318 0.517 0.754 0.303 0.486 0.684 0.289 0.457 0.621 0.275 0.429 0.563 0.267 0.414 0.532 0.262 0.403 0.511 0.258 0.395 0.496 0.251 0.381 0.468 0.246 0.371 0.450 0.242 0.364 0.436 0.239 0.358 0.425 0.236 0.353 0.416 0.231 0.343 0.398

Note: for special terrains, additional research should be conducted to determine ζ .

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b) For buildings and structural elements of single-degree-of-freedom (e.g. steel portal frames of one-storey industrial buildings, water tower etc.), if f1 < fL then WP shall be computed as: WP = W x ζ x ξ x ν

(9)

where,

ξ - dynamic factor can be obtained from graphs in Fig. 4 when parameter ε and logarithm reduction factor δ are known. The parameter ε shall be calculated as:

ε=

1.2 × W0 2972 × f 1

(10)

where, W0 – the standard wind pressure at construction site (in N/m2)

Figure 7.4: Dynamic factor ξ Notes: Curve 1 for δ = 0.3; curve 2 for δ = 0.15.

c) For buildings having symmetrical plan with f1 < fL and for buildings with f1 < fL < f2 (f2 is the second characteristic frequency of vibration), WP shall be calculated as follows: WP = m x ξ x ψ x y

(11)

where,

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m – mass of a portion of the building with the point of gravity is at height Z y – lateral drift of the building at height Z corresponding to the first mode of vibration (for buildings having symmetrical plan, y shall be taken as the drift caused by uniformly distributed static load in horizontal direction. ψ - factor determined by dividing the building into r parts on which the wind pressures are the same, ψ shall be computed as: r

ψ =

∑y k =1 r

k

× Wpk (12)

∑ yk2 × M k k =1

in formulae (12): Mk – mass of kth part yk – lateral drift of the kth part’s gravitational point for the first mode of vibration Wpk – uniformly distributed dynamic component of wind load acting on the kth part (determined from formulae (8)). d) For multi-storey buildings with the rigidity, weight and windward surface relatively uniform with height, WP shall be determined as follows: WP = 1.4 x (Z/h) x ξ x WPh

(13)

where WPh is standard value of the dynamic component of wind load at height h of building’s top (determined from formulae (8)). 2.4. Aerodynamic stability of tall buildings and wind-sensitive structures

It is required to check the aerodynamic stability of tall buildings and wind-sensitive structures (such as: chimneys, canopies etc.) even when f1 < fL. The checking procedure shall be in conformity with the relevant documents as shown in the reference section. The input parameter used for checking the aerodynamic stability is the mean wind speed taken in 10 minutes (v1) at 10m height above the reference level in Terrain category B. Values of v1 shall be taken from Appendix D or be computed as: v1 = 0.714

W0 0.0613

(7.14)

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2.5. Complicated, irregular and wind-sensitive structures

The wind load acting on complicated, irregular and wind-sensitive structures will possibly be assessed by wind tunnel tests. The data obtained from the wind tunnel tests will be used to check the aerodynamic stability of the structures and to find solutions for vibration control.

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