bnbc 93 chapter 2

Part 6 6-1 Structural Design

CHAPTER 2 Loads

2.1 INTRODUCTION 2.1.1 Scope This chapter specifies the minimum design forces including dead load, live load, wind and earthquake

loads, miscellaneous loads and their various combinations. These loads shall be applicable for the design of buildings and structures in conformance with the general design requirements provided in Chapter 1.

2.1.2 Limitations Provisions of this chapter shall generally be applied to majority of buildings and other structures subject to

normally expected loading conditions. For those buildings and structures having unusual geometrical shapes, response characteristics or site locations, or for those subject to special loading including tornadoes, special dynamic or hydrodynamic loads etc., site-specific or case-specific data or analysis may be required to determine the design loads on them. In such cases, and all other cases for which loads are not specified in this chapter, loading information may be obtained from reliable references or specialist advice may be sought. However, such loads shall be applied in compliance with the provisions of other sections of this Code.

2.2 DEAD LOADS 2.2.1 General

Part 6 Structural Design

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The minimum design dead load for buildings and portions thereof shall be determined in accordance with the provisions of this section. In addition, design of the overall structure and its primary load-resisting systems shall conform to the general design provisions given in Chapter 1.

2.2.2 Definition Dead Load is the vertical load due to the weight of permanent structural and non-structural components of

a building such as walls, floors, ceilings, permanent partitions and fixed service equipment etc. 2.2.3 Assessment of Dead Load Dead load for a structural member shall be assessed based on the forces due to : i) weight of the member itself, ii) weight of all materials of construction incorporated into the building to be supported permanently

by the member, iii) weight of permanent partitions, iv) weight of fixed service equipment, and v) net effect of prestressing. 2.2.4 Weight of Materials and Constructions In estimating dead loads, the actual weights of materials and constructions shall be used, provided that in

the absence of definite information, the weights given in Tables 6.2.1 and 6.2.2 shall be assumed for the purposes of design.

Table 6.2.1

Unit Weight of Basic Materials

Material

Unit Weight

(kN/m3)

Material

Unit Weight

(kN/m3)

Aluminium Asphalt Brass Bronze Brick Cement Coal, loose Concrete - stone aggregate (unreinforced) - brick aggregate (unreinforced) Copper Cork, normal Cork, compressed Glass, window (soda-lime)

27.0 21.2 83.6 87.7 18.9 14.7 8.8

22.8* 20.4* 86.4 1.7 3.7

25.5

Granite, Basalt Iron - cast - wrought Lead Limestone Marble Sand, dry Sandstone Slate Steel Timber Zinc

26.4 70.7 75.4

111.0 24.5 26.4 15.7 22.6 28.3 77.0

5.9-11.0 70.0

* for reinforced concrete, add 0.63 kN/m3 for each 1% by volume of main reinforcement

2.2.5 Weight of Permanent Partitions When partition walls are indicated on the plans, their weight shall be considered as dead load acting as

concentrated line loads in their actual positions on the floor. The loads due to anticipated partition walls, which are not indicated on the plans, shall be treated as live loads and determined in accordance with Sec 2.3.3.3.

2.2.6 Weight of Fixed Service Equipment Weights of fixed service equipment and other permanent machinery, such as electrical feeders and other

machinery, heating, ventilating and air-conditioning systems, lifts and escalators, plumbing stacks and risers etc. shall be included as dead load whenever such equipment are supported by structural members.

2.2.7 Additional Loads

Chapter 2 Loads

Bangladesh National Building Code 6-3

In evaluating the final dead loads on a structural member for design purposes, allowances shall be made for additional loads resulting from the (i) difference between the prescribed and the actual weights of the members and construction materials; (ii) inclusion of future installations; (iii) changes in occupancy or use of buildings; and (iv) inclusion of structural and non-structural members not covered in Sec 2.2.2 and 2.2.3.

2.3 LIVE LOADS 2.3.1 General The live loads used for the structural design of floors, roof and the supporting members shall be the

greatest applied loads arising from the intended use or occupancy of the building, or from the stacking of materials and the use of equipment and propping during construction, but shall not be less than the minimum design live loads set out by the provisions of this section. For the design of structural members for forces including live loads, requirements of the relevant sections of Chapter 1 shall also be fulfilled.

2.3.2 Definition Live load is the load superimposed by the use or occupancy of the building not including the environmental

loads such as wind load, rain load, earthquake load or dead load. 2.3.3 Minimum Floor Live Loads The minimum floor live loads shall be the greatest actual imposed loads resulting from the intended use or

occupancy of the floor, and shall not be less than the uniformly distributed load patterns specified in Sec 2.3.3.1 or the concentrated loads specified in Sec 2.3.3.2 whichever produces the most critical effect. The live loads shall be assumed to act vertically upon the area projected on a horizontal plane.

Table 6.2.2

Weight of Construction Materials

Material

Weight per

Unit Area

(kN/m2)

Material

Weight per

Unit Area

(kN/m2)


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Floor Asphalt, 25 mm thick Clay tiling, 13 mm thick Concrete slab (stone aggregate)* --- solid, 100 mm thick solid, 150 mm thick Galvanized steel floor deck (excl. topping) Magnesium oxychloride- normal (sawdust filler), 25 mm thick heavy duty (mineral filler), 25 mm thick Terrazzo paving 16 mm thick

Roof Acrylic resin sheet, corrugated ---- 3 mm thick, standard corrugations 3 mm thick, deep corrugations

Asbestos cement, corrugated sheeting --- (incl. lap and fastenings) 6 mm thick (standard corrugations) 6 mm thick(deep corrugations) Aluminium, corrugated sheeting --- (incl. lap and fastenings) 1.2 mm thick 0.8 mm thick 0.6 mm thick Aluminium sheet(plain) --- 1.2 mm thick 1.0 mm thick 0.8 mm thick Bituminous felt(5 ply) and gravel Slates --- 4.7 mm thick 9.5 mm thick Steel sheet, flat galvanized --- 1.00 mm thick 0.80 mm thick 0.60 mm thick Steel, galvanized std. corrugated sheeting --- (incl. lap and fastenings) 1.0 mm thick 0.8 mm thick 0.6 mm thick

0.526 0.268

2.360 3.540

0.147-0.383

0.345 0.527 0.431

0.043 0.062

0.134 0.158

0.048 0.028 0.024

0.033 0.024 0.019 0.431

0.335 0.671

0.082 0.067 0.053

0.120 0.096 0.077

Roof (contd.) Tiles --- terra-cotta (French pattern) concrete , 25 mm thick clay tiles

Walls and Partitions Acrylic resin sheet, flat, per mm thickness Asbestos cement sheeting ---- 4.5 mm thick 6.0 mm thick Brick masonry work, excl. plaster --- burnt clay, per 100 mm thickness sand-lime, per 100 mm thickness Concrete (stone aggregate)* --- 100 mm thick

150 mm thick 250 mm thick Fibre insulation board, per 10 mm thickness Fibrous plaster board, per 10 mm thickness Glass, per 10 mm thickness Hardboard, per 10 mm thickness Particle or flake board, per 10 mm thickness Plaster board, per 10 mm thickness Plywood, per 10 mm thickness

Ceiling Fibrous plaster, 10 mm thick Cement plaster, 13 mm thick Suspended metal lath and plaster (two faced incl. studding)

Miscellaneous Felt (insulating), per 10 mm thickness Plaster --- cement, per 10 mm thickness lime, per 10 mm thickness PVC sheet, per 10 mm thickness Rubber paving, per 10 mm thickness Terra-cotta Hollow Block Masonry --- 75 mm thick 100 mm thick 150 mm thick

0.575 0.527

0.6-0.9

0.012

0.072 0.106

1.910 1.980

2.360

3.540 5.900 0.034 0.092 0.269 0.961 0.075 0.092 0.061

0.081 0.287 0.480

0.019

0.230 0.191 0.153 0.151

0.671 0.995 1.388

* for brick aggregate, 90% of the listed values may be used.

2.3.3.1 Uniformly Distributed Loads : The uniformly distributed load shall not be less than the values

listed in Table 6.2.3, reduced as may be specified in Sec 2.3.9, applied uniformly over the entire area of the floor, or any portion thereof to produce the most adverse effects in the member concerned.

2.3.3.2 Concentrated Loads : The concentrated load to be applied non-concurrently with the uniformly

distributed load given in Sec 2.3.3.1, shall not be less than that listed in Table 6.2.3. Unless otherwise specified in Table 6.2.3 or in the following paragraph, the concentrated load shall be applied over an area of 300 mm x 300 mm and shall be located so as to produce the maximum stress conditions in the structural members.

In areas where vehicles are used or stored, such as car parking garages, ramps, repair shops etc., provision

shall be made for concentrated loads consisting of two or more loads spaced nominally 1.5 m on centres in absence of the uniform live loads. Each load shall be 40 per cent of the gross weight of the maximum size

Chapter 2 Loads



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vehicle to be accommodated and applied over an area of 750 mm x 750 mm. For the storage of private or pleasure-type vehicles without repair or fuelling, floors shall be investigated in the absence of the uniform live load, for a minimum concentrated wheel load of 9 kN spaced 1.5 m on centres, applied over an area of 750 mm x 750 mm. The uniform live loads for these cases are provided in Table 6.2.3. The condition of concentrated or uniform live load producing the greater stresses shall govern.

2.3.3.3 Provision for Partition Walls : When partitions, not indicated on the plans, are anticipated to be

placed on the floors, their weight shall be included as an additional live load acting as concentrated line loads in an arrangement producing the most severe effect on the floor, unless it can be shown that a more favourable arrangement of the partitions shall prevail during the future use of the floor.

In the case of light partitions, wherein the total weight per metre run is not greater than 5.5 kN, a

uniformly distributed live load may be applied on the floor in lieu of the concentrated line loads specified above. Such uniform live load per square metre shall be at least 33% of the weight per metre run of the

partitions, subject to a minimum of 1.2 kN/m2. 2.3.3.4 More than One Occupancy : Where an area of a floor is intended for two or more occupancies at

different times, the value to be used from Table 6.2.3 shall be the greatest value for any of the occupancies concerned.

2.3.4 Minimum Roof Live Loads Roof live loads shall be assumed to act vertically over the area projected by the roof or any portion of it

upon a horizontal plane, and shall be determined as specified in the following sections : 2.3.4.1 Regular Purpose - Flat, Pitched and Curved Roofs : Live loads on regular purpose roofs shall be

the greatest applied loads produced during use by movable objects such as planters and people, and those induced during maintenance by workers, equipment and materials but shall not be less than those given in Table 6.2.4.

Table 6.2.4

Minimum Roof Live Loads(1)

Type and Slope of Roof Distributed

Load, kN/m2

Concentrated Load, kN

I Flat roof (slope = 0)

1.5

1.8

II 1. Pitched or sloped roof (0 < slope < 1/3) 2 Arched roof or dome (rise < 1/8 span)

1.0 0.9

III 1. Pitched or sloped roof (1/3 slope < 1.0) 2. Arched roof or dome (1/8 rise < 3/8 span)

0.8 0.9

IV 1. Pitched or sloped roof (slope 1.0) 2. Arched roof or dome (rise 3/8 span)

0.6 0.9

V Greenhouse, and agriculture buildings

0.5 0.9

VI Canopies and awnings, except those with cloth covers

same as given in I through IV above based on the type and slope.

Note : (1) Greater of this load and rain load as specified in Sec 2.6.3 shall be taken as the design live load for roof. The distributed load shall be applied over the area of the roof projected upon a horizontal plane and shall not be applied simultaneously with the concentrated load. The concentrated load shall be assumed to act upon a 300 mm x 300 mm area and need not be considered for roofs capable of laterally distributing the load, e.g. reinforced concrete slabs.

Chapter 2 Loads


2.3.4.2 Special Purpose Roofs : For special purpose roofs, live loads shall be estimated based on the

actual weight depending on the type of use, but shall not be less than the following values :

a) roofs used for promenade purposes - 3.0 kN/m2

b) roofs used for assembly purposes - 5.0 kN/m2

c) roofs used for gardens - 5.0 kN/m2 d) roofs used for other special purposes - to be determined as per Sec 2.3.5 2.3.4.3 Accessible Roof Supporting Members : Roof trusses or any other primary roof supporting

member beneath which a full ceiling is not provided, shall be capable of supporting safely, in addition to other roof loads, a concentrated load at the locations as specified below :

a) Industrial, Storage and Garage Buildings - Any single panel point of the lower chord of a roof truss, or any point of other primary roof supporting member - 9.0 kN b) Building with Other Occupancies - Any single panel point of the lower chord of a roof truss, or any point of other primary roof supporting member - 1.3 kN 2.3.5 Loads Not Specified Live loads, not specified for uses or occupancies in Sec 2.3.3.1 and 2.3.3.2, shall be determined from loads

resulting from : a) weight of the probable assembly of persons; b) weight of the probable accumulation of equipment and furniture, and c) weight of the probable storage of materials. 2.3.6 Partial Loading and Other Loading Arrangements The full intensity of the appropriately reduced live load applied only to a portion of the length or area of a

structure or member shall be considered, if it produces a more unfavourable effect than the same intensity applied over the full length or area of the structure or member.

Where uniformly distributed live loads are used in the design of continuous members and their supports,

consideration shall be given to full dead load on all spans in combination with full live loads on adjacent spans and on alternate spans whichever produces a more unfavourable effect.

2.3.7 Other Live Loads Live loads on miscellaneous structures and components, such as handrails and supporting members,

parapets and balustrades, ceilings, skylights and supports, and the like, shall be determined from the analysis of the actual loads on them, but shall not be less than those given in Table 6.2.5.

Table 6.2.5

Miscellaneous Live Loads

Structural Member or Component Live Load(1) (kN/m)


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1. Handrails, parapets and supports : a) Light access stairs, gangways etc. i) width 0.6 m ii) width > 0.6 m b) Staircases other than in (a) above, ramps, balconies : i) Single dwelling and private ii) Staircases in residential buildings iii) Balconies or portion thereof, stands etc. having fixed seats within 0.55 m of the barrier vi) Public assembly buildings including theatres, cinemas, assembly halls, stadiums, mosques, churches, schools etc. vi) Buildings and occupancies other than (i) through (iv) above

0.25 0.35

0.35 0.35 1.5

3.0

0.75

2. Vehicle barriers for car parks and ramps : a) For vehicles having gross mass 2500 kg

b) For vehicles having gross mass > 2500 kg

c) For ramps of car parks etc.

100(2)

165(2) see note (3)

Note : (1) These loads shall be applied non-concurrently along horizontal and vertical directions, except as specified in note (2) below.

(2) These loads shall be applied only in the horizontal direction, uniformly distributed over any length of 1.5 m of a barrier and shall be considered to act at bumper height. For case 2(a) bumper height may be taken as 375 mm above floor level.

(3) Barriers to access ramps of car parks shall be designed for horizontal forces equal to 50% of those given in 2(a) and 2(b) applied at a level of 610 mm above the ramp. Barriers to straight exit ramps exceeding 20 m in length shall be designed for horizontal forces equal to twice the values given in 2(a) and 2(b).

2.3.8 Impact and Dynamic Loads The live loads specified in Sec 2.3.3 shall be assumed to include allowances for impacts arising from normal

uses only. However, forces imposed by unusual vibrations and impacts resulting from the operation of installed machinery and equipment shall be determined separately and treated as additional live loads. Live loads due to vibration or impact shall be determined by dynamic analysis of the supporting member or structure including foundations, or from the recommended values supplied by the manufacture of the particular equipment or machinery. In absence of a definite information, values listed in Table 6.2.6 for some common equipment, shall be used for design purposes.

Table 6.2.6

Minimum Live Loads on Supports and Connections of Equipment due to Impact (1)

Equipment or Machinery Additional load due to impact as percentage of static load including self weight

Vertical Horizontal

1.

Lifts, hoists and related operating machinery

100%

2. Light machinery (shaft or motor driven) 20% 3. Reciprocating machinery, or power driven

units. 50%

4. Hangers supporting floors and balconies 33%

Chapter 2 Loads


5. Cranes : a) Electric overhead cranes

25% of

maximum wheel load

i) Transverse to the rail : 20% of the weight of trolley and lifted load only, applied one-half at the top of each rail ii) Along the rail : 10% of maximum wheel load applied at the top of each rail

b) Manually operated cranes

50% of the values in (a) above

50% of the values in (a) above

c) Cab-operated travelling cranes

25%

Not applicable

Note : (1)

All these loads shall be increased if so recommended by the manufacturer. For machinery and equipment not listed, impact loads shall be those recommended by the manufacturers, or determined by dynamic analysis.

2.3.9 Reduction of Live Loads Reduction of live load is permitted for primary structural members supporting floor or roof, including

beam, girder, truss, flat slab, flat plate, column, pier, footing and the like. Where applicable, the reduced live load on a primary structural member shall be obtained by multiplying the corresponding unreduced uniformly distributed live load with an appropriate live load reduction factor, R as listed in Table 6.2.7 and set forth in Sec 2.3.9.1.

2.3.9.1 Load Groups : All possible live loads applied on floors and roof of a building due to various

occupancies and uses, shall be divided into three load groups as described below for determining the appropriate live load reduction factors.

a) Load Group 1 : Uniformly distributed live loads arising from the occupancies and uses of

(i) assembly occupancies or areas with uniformly distributed live load of 5.0 kN/m2 or less, (ii) machinery and equipment for which specific live load allowances have been made, (iii) special roof live load as described in Sec 2.3.4.2, and (iv) printing plants, vaults, strong rooms and armouries, shall be classified under Load Group 1. Reduction of live load shall not be allowed for members or portions thereof under this load group and a reduction factor, R =1.0 shall be applied for such cases.

b) Load Group 2 : Uniformly distributed live loads resulting from occupancies or uses of

(i) assembly areas with uniformly distributed live load greater than 5.0 kN/m2, and (ii) storage, mercantile, industrial and retail stores, shall be classified under Load Group 2. Live load reduction factor, 1.0 < R < 0.7 shall be applied to this load group depending on the tributary area of the floors or roof supported by the member as specified in Sec 2.3.9.3.

c) Load Group 3 : Uniformly distributed live loads arising due to all other occupancies and uses except

those of Load Group 1 and Load Group 2, shall be grouped into Load Group 3. Live load reduction factor, 1.0 R 0.5 as specified in the Sec 2.3.9.3, shall be applied to tributary areas under this load group.


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2.3.9.2 Tributary Area : The tributary area of a structural member supporting floors or roof shall be determined as follows :

a) Tributary Area for Wall, Column, Pier, Footing and the like : Tributary areas of these members shall

consist of portions of the areas of all floors, roof or combination thereof that contribute live loads to the member concerned.

b) Tributary Area for Beam, Girder, Flat plate and Flat slab : Tributary area for such a member shall

consist of the portion of the roof or a floor at any single level that contributes loads to the member concerned.

Table 6.2.7 Live Load Reduction Factors for Various Occupancies and Uses

Load

Group

Occupancy or Use

Tributary (1) Area (floor, or

roof, or combination)

At (m2)

Live Load (2,3) Reduction

Factor, R

1

a) b) c) d)

Assembly areas with uniformly distributed live

load of 5.0 kN/m2 or less. Live loads from machinery and equipment for which specific load allowance has been made Special roof live loads as specified in Sec 2.3.4.2 Printing plants, vaults, strong room and armouries

all

1.0

2

a) b)

Assembly areas with uniformly distributed

live load greater than 5.0 kN/m2 . Storage, mercantile, industrial, parking garage, retail stores

50

60 80 100 120 140 280 220 300 400

800

1.00 0.97 0.92 0.88 0.86 0.84 0.81 0.79 0.76 0.74 0.70

3

a)

Uniformly distributed live loads from all occupancies and uses except those listed in load groups 1 and 2 above.

< 25 25-30

40 50 60 80

100 120 140 180

220

1.00 0.90 0.84 0.78 0.73 0.67 0.62 0.59 0.57 0.53 0.50

Chapter 2 Loads


Note : (1) At = sum of all tributary areas with loads from any one load group (i.e. Load Group 1, 2 or 3)

(2) Linear interpolation may be made to obtain values of R lying between the listed values.

(3) Live load reduction factor, R is based on the relations: R 0.6 8 A t for Load Group 2 and

R 0.25 14 A t for Load Group 3

2.3.9.3 Determination of Reduced Live Load : The value of the live load reduction factor, R shall depend

on the load group specified in Sec 2.3.9.1 and on the tributary area of the floor or the roof and combination thereof supported by a primary structural member. The reduced live load on a structural member shall be determined using the following steps:

a) Portions of the tributary area pertaining to each of the three load groups specified in Sec 2.3.9.1 shall

be identified and summed up, and a value of the reduction factor R shall be obtained from Table 6.2.7 corresponding to each portion of the tributary area,

b) The reduced live loads or load intensities shall then be obtained for each load group by multiplying

the unreduced live loads or load intensities by the corresponding reduction factors, and finally, c) The total reduced live load on a structural member shall be determined by summing up the reduced

live loads from each load group. 2.4 WIND LOADS 2.4.1 General The minimum design wind load on buildings and components thereof, shall be determined based on the

velocity of the wind, the shape and size of the building and the terrain exposure condition of the site as set forth by the provisions of this section. For the overall design of structures, the general design requirements as specified in Chapter 1 shall also be fulfilled.

2.4.1.1 Scope : Provisions of this section shall apply to the calculation of design wind loads for the

primary framing systems and for the individual structural components and cladding of buildings. The design wind load shall include the effects of the sustained wind velocity component and the fluctuating component due to gusts. For slender buildings, the design wind load shall also include additional loading effects due to wind induced vibrations of the building.

2.4.1.2 Limitations : Provisions of this section shall include forces due to along-wind response of regular-

shaped buildings, caused by the common wind-storms including cyclones, thunder-storms and norwesters. However, the following cases shall remain beyond the scope of these provisions :

a) forces due to cross-wind response of buildings and structures, b) forces, such as torsion etc. generated due to unusual or unsymmetrical geometry of the building, and c) forces generated due to special types of winds, such as tornadoes. For calculation of wind loads arising due to the above special cases and for buildings requiring more

accurate loading information, reference shall be made to reliable literature pertaining to these loads, or specialist advice shall be sought.

2.4.2 Definitions The following definitions shall apply only to the provisions of Sec 2.4. AWNINGS (e.g. PORCH COVER) : A roof-like structure, usually of limited extent, projecting from a wall of

a building. BASIC WIND SPEED, Vb : Fastest-mile wind speed in km/h corresponding to the level of 10 metres above

the ground of terrain Exposure-B defined in Sec 2.4.4 and associated with an annual probability of occurrence of 0.02.

BUILDINGS : Structures that enclose a space and are used for various occupancies.


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CANOPY : A roof adjacent to or attached to a building, generally not enclosed by walls. COMPONENTS AND CLADDING : Structural elements that are either directly loaded by the wind or

receive wind loads originating at relatively close locations and transfer those loads to the primary framing system. Examples include curtain walls, exterior glass windows and panels, roof sheeting, purlins, grits, studs, and roof trusses.

CYCLONE : An intense low-pressure centre accompanied by heavy rain and gale-force winds. It forms

over warm tropical oceans and decays rapidly over land.

DESIGN WIND PRESSURES, p : Equivalent static pressure due to wind including gusts to be used in the determination of wind loads for buildings. The pressure shall be assumed to act in a direction normal to the surface considered and is denoted as:

p

z = pressure that varies with height in accordance with the sustained wind pressure q

z evaluated at

height z, or p

h = pressure that is uniform with respect to height as determined by the sustained wind pressure q

h

evaluated at mean roof height h. ENCLOSED BUILDING : Buildings which have full perimeter wall (nominally sealed) from floor to roof

level. ESSENTIAL FACILITIES : Buildings and structures which are necessary to remain functional during an

emergency or a post disaster period. FASTEST-MILE WIND SPEED : The highest sustained average wind speed in km/h based on the time

required for a mile-long sample of air to pass a fixed point. FREE STANDING ROOF : A roof (of any type) with no enclosing walls underneath, e.g. freestanding

carport. FREESTANDING WALLS : Walls which are exposed to the wind on both sides, with no roof attached, e.g.

fences. GABLED FRAME : A rigid frame having vertical side members and a sloped top with a ridge. GRADIENT HEIGHT: Height from the mean ground level above which the variation of wind speed with

height need not be considered. HOARDING : Free standing (rectangular) signboards, etc., supported clear of the ground. ISOTACH : A line on a map joining points of equal wind speed. MOONSCAPE ROOF : A planar roof with no ridge, which has a constant slope. OPENINGS: Apertures or holes in the exterior walls of a building or structure. Doors or other openings in

exterior walls shall be considered as openings unless such openings and their frames are specifically detailed and designed to resist the wind loads in accordance with the provisions of this section.

PITCHED ROOF : A bi-fold, bi-planar roof with a ridge at its highest point. PRESSURE : Air pressure in excess of ambient. Negative values are less than ambient and positive values

exceed ambient. Net pressures act normal to a surface in the specified direction. PRIMARY FRAMING SYSTEM : An assemblage of major structural elements assigned to provide support

for secondary members and cladding. The system primarily receives wind loading from relatively remote

Chapter 2 Loads


locations. Examples include rigid and braced frames, space trusses, roof and floor diaphragms, shear walls, and rod-braced frames.

SLENDER BUILDINGS AND STRUCTURES : Buildings and structures having a height exceeding five

times the least horizontal dimension, or a fundamental natural frequency less than 1.0 Hz. For those cases in which the horizontal dimensions vary with height, the least horizontal dimension at midheight shall be used.

STRUCTURES : See Sec 1.2.2. STRUCTURE IMPORTANCE COEFFICIENT, CI : A factor that accounts for the degree of hazard to human

life and damage to property.

SUSTAINED WIND PRESSURE, q : The theoretically computed incident pressure of a uniform air stream (fastest-mile speed) of known density, evaluated at a given height above ground level, for a specific terrain exposure condition and for a known occupancy of a building.

TERRAIN : The surface roughness condition when considering the size and arrangement of obstructions to

wind. TOPOGRAPHY : Major land surface features comprising hills, valleys and plains which strongly influence

wind flow patterns. TORNADO : A violently rotating column of air, pendant from the base of a connective cloud, and often

observable as a funnel cloud attached to the cloud base. TRIBUTARY AREA : That portion of the surface area receiving wind loads assigned to be supported by the

structural element considered. TROUGH ROOF : A bi-fold, bi-planar roof with a valley at its lowest point. UNENCLOSED BUILDING OR STOREY : A building or storey which has 85% or more openings on all

sides.


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2.4.3 Symbols and Notation The following symbols and notation shall apply to the provisions of Sec 2.4 only : A = tributary area, square metres.

A = area of other structures or components and cladding thereof projected on a plane normal to wind direction, square metres.

a = width of pressure coefficient zone used in Fig 6.2.7 and 6.2.8, metres B = horizontal dimension of buildings and structures measured normal to wind direction, metres. c = average horizontal dimension of the building or structure in a direction normal to the wind,

metres.

Cc = velocity-to-pressure conversion coefficient = 47.2x10-6

CG = gust coefficient

CI = structure importance coefficient

Cp = pressure coefficient to be used for determination of wind loads on buildings and structures.

= Cpe external pressure coefficients for surfaces of buildings and structures to be used for wind

loads on primary framing systems using Method 1 in Sec 2.4.6.4 (a).

= C p overall pressure coefficient for buildings and structures to be used for

wind loads on primary framing systems using Method 2 in Sec 2.4.6.4 (b).

C pe = external peak pressure coefficients to be used for wind loads on building components and

cladding.

C pi = internal peak pressure coefficient to be used for wind loads on buildings and components.

Ct = local topographic coefficient given in Sec 2.4.6.8.

Cz = combined height and exposure coefficient for a building at height z above ground

D = diameter of a circular structure or member, metres d = diameter of a circular structure or member, metres D = depth of protruding elements (ribs or spoilers), metres Do = surface drag coefficient given in Table 6.2.12.

f = fundamental frequency of buildings or structures in a direction parallel to the wind, Hz F1, F2 = design wind forces on primary framing system, kN

F = design wind forces on components and cladding, kN

G = gust response factor for primary framing systems of flexible buildings and structures G

h = gust response factor for primary framing systems evaluated at height z = h

Gz = gust response factor for components and cladding evaluated at height z above ground

h = mean roof height or height to top of parapet whichever is greater of a building or structure, except that eaves height may be used for roof slope of less than 10 degrees, metres.

J = pressure profile factor as a function of ratio L = horizontal dimension of a building or structure measured parallel to wind direction, metres M = larger dimension of a sign, metres N = smaller dimension of a sign, metres

p = design pressure to be used in determination of wind loads for buildings, kN/m2

ph = design pressure evaluated at height z =h, kN/m2

pi = internal pressure, kN/m2

pz = design wind pressure evaluated at height z above ground, kN/m2

q = sustained wind pressure, kN/m2

qh = sustained wind pressure evaluated at height z=h, kN/m2

qz = sustained wind pressure evaluated at height z above ground, in kN/m2

r = rise-to-span ratio for arched roofs s = surface friction factor given in Table 6.2.12 S = structure size factor given in Fig 6.2.4 TI = turbulence intensity factor evaluated at two-thirds of the mean roof height or parapet height

of the structure (see Eq 2.4.11)

Chapter 2 Loads


V

= basic wind speed, km/h X = distance to centre of pressure from windward edge, metres Y = response factor as a function of the ratio and the ratio c/h given in Fig 6.2.3

z = height above ground level, metres zg = gradient height given in Table 6.2.12, metres

= power-law coefficient given in Table 6.2.12

= structural damping coefficient (fraction of critical damping)

= ratio obtained from Table 6.2.12

= ratio of solid area to gross area for open sign face of a trussed tower, or lattice structure

= angle of the plane of roof from horizontal, degrees = height-to-width ratio for sign or hoarding

= angle between wind direction and chord of tower guy, degrees. 2.4.4 Terrain Exposure A terrain exposure category that adequately reflects the surface roughness characteristics of the ground

shall be determined for the building site, taking into account the variations in ground roughness arising from existing natural topography, vegetation and manmade constructions.

2.4.4.1 Exposure Category : The terrain exposure in which a building or structure is to be sited shall be

assessed as being one of the following categories: a) Exposure A : Urban and sub-urban areas, industrial areas, wooded areas, hilly or other terrain

covering at least 20 per cent of the area with obstructions of 6 metres or more in height and extending from the site at least 500 metres or 10 times the height of the structure, whichever is greater.

b) Exposure B : Open terrain with scattered obstructions having heights generally less than 10m

extending 800 m or more from the site in any full quadrant. This category includes air fields, open park lands, sparsely built-up outskirts of towns, flat open country and grasslands.

c) Exposure C : Flat and unobstructed open terrain, coastal areas and riversides facing large bodies of

water, over 1.5 km or more in width. Exposure C extends inland from the shoreline 400 m or 10 times the height of structure, whichever is greater.

2.4.4.2 Selection of Exposure Category for Primary Framing System : Design wind load for primary

framing systems for all buildings and structures shall be determined based on the terrain exposure categories defined in Sec 2.4.4.1.

2.4.4.3 Selection of Exposure Category for Components and Cladding : Design wind load on the

components and cladding of all buildings and structures shall be determined on the basis of the exposure category defined in Sec 2.4.4.1, except that Exposure B shall be assumed for buildings or structures having h 20 m and sited in a terrain with Exposure A.

2.4.5 Basic Wind Speed 2.4.5.1 Basic Wind Speed Map : The Basic Wind Speed Map as shown in Fig 6.2.1 is the map showing the

basic wind speeds in km/h for any location in Bangladesh, having isotachs representing the fastest-mile wind speeds at 10 metres above the ground with terrain Exposure B for a 50-year recurrence interval. The minimum value of the basic wind speed set in the map is 130 km/h. Basic wind speeds for selected locations are also provided in Table 6.2.8.

2.4.5.2 Selection of Basic Wind Speed : Value of the basic wind speed required for any specific location

where a building or structure is sited, shall be obtained as follows: i) When the location is listed in Table 6.2.8, value of the basic wind speed shall be taken from that table. ii) If the location lies within any wind region (shown shaded in the map of Fig 6.2.1), the value marked

for that wind region shall be used.


6-16

iii) For a location lying on any isotach in the map, the value of that isotach shall be taken. iv) For a location lying outside the positions (i) through (iii) above, linear interpolation shall be made

between the adjacent isotachs to obtain the basic wind speed. For areas where local records or terrain conditions indicate higher values of basic wind speeds

(substantiated by site-specific analysis) than those reflected in Fig 6.2.1 and Table 6.2.8, the site-specific values shall be adopted as the minimum basic wind speeds.

2.4.6 Determination of Design Wind Loads 2.4.6.1 Basis of Wind Load Calculation : The minimum design wind load on buildings, structures and

components thereof shall be calculated, within the scope and limitations given in Sec 2.4.1 taking into account the following effects which shall be determined in accordance with the provisions of this section :

a) equivalent static pressure or suction on building surfaces arising due to the sustained or mean wind

velocity, i.e. the fastest-mile wind speed, b) variation of the mean wind velocity, and hence the pressure, along the height above the ground, c) terrain exposure of the building site,

Chapter 2 Loads



6-18

Table 6.2.8 Basic Wind Speeds for Selected Locations in Bangladesh

Location

Basic Wind Speed (km/h)

Location

Basic Wind Speed (km/h)

Angarpota Bagerhat Bandarban Barguna Barisal Bhola Bogra Brahmanbaria Chandpur Chapai Nawabganj Chittagong Chuadanga Comilla Coxs Bazar Dahagram Dhaka Dinajpur Faridpur Feni Gaibandha Gazipur Gopalganj Habiganj Hatiya Ishurdi Joypurhat Jamalpur Jessore Jhalakati Jhenaidah Khagrachhari Khulna Kutubdia Kishoreganj Kurigram Kushtia Lakshmipur

150 252 200 260 256

225 198 180 160 130

260 198 196 260 150

210 130 202 205 210

215 242 172 260 225

180 180 205 260 208

180 238 260 207 210

215 162

Lalmonirhat Madaripur Magura Manikganj Meherpur Maheshkhali Moulvibazar Munshiganj Mymensingh Naogaon Narail Narayanganj Narsinghdi Natore Netrokona Nilphamari Noakhali Pabna Panchagarh Patuakhali Pirojpur Rajbari Rajshahi Rangamati Rangpur Satkhira Shariatpur Sherpur Sirajganj Srimangal St. Martins Island Sunamganj Sylhet Sandwip Tangail Teknaf Thakurgaon

204 220 208 185 185

260 168 184 217 175

222 195 190 198 210

140 184 202 130 260

260 188 155 180 209

183 198 200 160 160

260 195 195 260 160

260 130

d) configuration and dynamic response characteristics of the building or structure, e) occupancy importance of the building, f) magnification of the mean wind pressure due to the effect of the fluctuating component of wind

speed, i.e. gusts, and

Chapter 2 Loads


g) additional load amplification resulting from the dynamic wind-structure interaction effects due to gusts on slender buildings and structures.

2.4.6.2 Sustained Wind Pressure : The sustained wind pressure, q

z on a building surface at any height z

above ground shall be calculated from the following relation :

qz = Cc CI Cz V b2 (2.4.1)

where, qz = sustained wind pressure at height z, kN/m2

CI = structure importance coefficient as given in Table 6.2.9

Cc = velocity-to-pressure conversion coefficient = 47.2x10-6

Cz = combined height and exposure coefficient as given in Table 6.2.10

Vb = basic wind speed in km/h obtained from Sec 2.4.5

If a structure is located within a local topographic zone, qz shall be modified in accordance with Sec 2.4.6.8.

Table 6.2.9

Structure Importance Coefficients, CI for Wind Loads

Structure Importance Category (see Table 6.1.1 for Occupancy)

Structure Importance Coefficient, CI

I

II III IV V

Essential facilities Hazardous facilities Special occupancy structures Standard occupancy structures Low-risk structures

1.25 1.25 1.00 1.00 0.80

Table 6.2.10

Combined Height and Exposure Coefficient, Cz

Height above Coefficient, Cz (1)

ground level, z (metres)

Exposure A Exposure B Exposure C


6-20

0-4.5 6.0 9.0

12.0

15.0 18.0 21.0 24.0

27.0 30.0 35.0 40.0

45.0 50.0 60.0 70.0

80.0 90.0

100.0 110.0

120.0 130.0 140.0 150.0

160.0 170.0 180.0 190.0

200.0 220.0 240.0 260.0

280.0 300.0

0.368 0.415 0.497 0.565

0.624 0.677 0.725 0.769

0.810 0.849 0.909 0.965

1.017 1.065 1.155 1.237

1.313 1.383 1.450 1.513

1.572 1.629 1.684 1.736

1.787 1.835 1.883 1.928

1.973 2.058 2.139 2.217

2.910 2.362

0.801 0.866 0.972 1.055

1.125 1.185 1.238 1.286

1.330 1.371 1.433 1.488

1.539 1.586 1.671 1.746

1.814 1.876 1.934 1.987

2.037 2.084 2.129 2.171

2.212 2.250 2.287 2.323

2.357 2.422 2.483 2.541

2.595 2.647

1.196 1.263 1.370 1.451

1.517 1.573 1.623 1.667

1.706 1.743 1.797 1.846

1.890 1.930 2.002 2.065

2.120 2.171 2.217 2.260

2.299 2.337 2.371 2.404

2.436 2.465 2.494 2.521

2.547 2.596 2.641 2.684

2.724 2.762

Note : (1) Linear interpolation is acceptable for intermediate values of z.

2.4.6.3 Design Wind Pressure : The design wind pressure, pz for a structure or an element of a structure

at any height, z above mean ground level shall be determined from the relation : pz = CG Cp qz (2.4.2)

where, pz = design wind pressure at height z , kN/m2

CG = gust coefficient which shall be Gz , Gh, or G as set forth in Sec 2.4.6.6

Cp = pressure coefficient for structures or components as set forth Sec 2.4.6.7

qz = sustained wind pressure obtain from Eq (2.4.1).

2.4.6.4 Design Wind Load for Buildings and Structures : Design wind load on the main wind force

resisting systems of buildings and structures shall be determined by using one of the following two methods:

Chapter 2 Loads


a) Method 1 (Surface Area Method) : The surface area method shall be used for gabled rigid frames

and single storey rigid frames and may be used of other framing systems. In this method the design wind pressures shall be assumed to act simultaneously normal to all exterior surfaces including roof of buildings or structures. The forces F1 , acting normal to the building surfaces or the roof, shall be

calculated as follows : i) For all framing systems:

F1 = pAz (2.4.3) where, F1 = wind force on primary framing systems acting normal to a surface, or roof, or a

part thereof.

p = design wind pressure on building surfaces, kN/m2 = p

z for windward surfaces as used in Eq (2.4.2)

= ph

for non-windward surfaces as used in Eq (2.4.2)

Az = area of the building surface or roof tributary to the framing system at height z

upon which the design pressure p operates, in square metres. ii) For gabled frames and single-storey rigid frames: In order to obtain the most critical loading condition, gabled frames and other single-storey

rigid frames shall be investigated for both the force F1 obtained from Eq (2.4.8) and that given

by the relation :

F1 p pi A z (2.4.4)

where, p

i = internal pressure =

C pi q

h

C pi = internal peak pressure coefficient as given in Sec 2.4.6.7, and

qh

= sustained wind pressure evaluated at mean roof height, given by Eq (2.4.1).

The resultant force of the complete framing system of the building shall be taken to be the

summation of forces F1 due to the effects of the pressures on all surfaces of the building. For the

maximum force on the building, forces along all critical directions shall be investigated. b) Method 2 (Projected Area Method) : This method may be used for any building or structure as a

whole except those specified in a(ii) above. In the projected area method, the horizontal pressure shall be assumed to act upon the full vertical projected area of the structure and the vertical pressure shall be assumed to act simultaneously upon the full horizontal projected area, except where the pressure coefficients are given for the surface area, e.g. Table 6.2.17. According to Method 2, the total wind force on the primary framing system of a building or a structure shall be calculated using the formula :

F2 =

pz A z (2.4.5) where, F

2 = total wind force on the framed system of the building in a specified direction, kN

pz = design wind pressure, in kN/m

2, for use with the overall pressure coefficient Cp for

the cross-sectional shapes provided in Tables 6.2.15 to 6.2.21

A z = Projected frontal area normal to wind tributary to the framing system at height z, in

square metres. In the projected area method, the overall pressure coefficients Cp provided in Tables 6.2.15 to 6.2.21

for various cross-sectional shapes, shall be used for the total height of the building or the structure having a particular cross-sectional shape. In order to determine the most critical loads, the total wind


6-22

force F2 shall be calculated for each wind direction for which the overall pressure coefficient

C p is

provided. 2.4.6.5 Design Wind Loads for Components and Cladding : Design wind load on individual structural

components such as roofs, walls, and individual cladding units and their immediate supporting members and fixings etc., of enclosed buildings and structures shall be determined in accordance with the following relation:

F =

C peq C piqi

Az

(2.4.6)

where, F = total wind force on a building component perpendicular to the surface, kN

C pe = external peak pressure coefficient for components, see Fig 6.2.7 and 6.2.8 for rectangular

building

C pi = internal peak pressure coefficient as given in Table 6.2.14

q = sustained wind pressure acting on external surfaces of a building q

i = wind pressure developed at the interior of the building.

The pressures q and q

i shall be determined as follows :

For h 18 m: q = qh

and qi = q

h

For h > 18 m: q = qz

for ( + ve) values of

C pe , and

q = qh

for ( ve) values of

C pe ,

qi = q

z for all values of Cpe.

If the peak pressure coefficients

C pe and C pi are not provided in Fig 6.2.7 and 6.2.8 and in Table 6.2.14, the

following equation may be used for determining the wind forces on structural components :

F = 1.25 pz A z (2.4.7)

where, pz = design wind pressure for components as given in Eq (2.4.2), kN/m

2

A z = Projected area of the component normal to wind at level, z above ground, in square metres.

2.4.6.6 Wind Gust Effects : Wind gusts cause additional loading effects due to turbulence over the

sustained wind speed. For slender buildings and structures, this additional loading gets further amplified due to dynamic wind structure interaction effects. A slender or wind-sensitive building shall be one having (i) a height exceeding five times the least horizontal dimension, or (ii) a fundamental natural frequency less than 1.0 Hz. Gust coefficient, C

G as included in Eq (2.4.2) shall account for such additional gust loading

effects on non-slender and slender buildings and shall be set equal to the Gust Response Factors, Gh

, Gz

or G as set forth below : a) Gust Response Factor, G

h for Non-slender Buildings and Structures : For the main wind force

resisting systems of non-slender buildings and structures, the value of the gust response factor, Gh

shall be determined from Table 6.2.11 evaluated at height h above mean ground level of the building or structure. Height h shall be defined as the mean roof level or the top of the parapet, whichever is greater.

b) Gust Response Factor, Gz for Building Components : For components and cladding of all buildings

and structures, the value of the gust response factor Gz shall be determined from Table 6.2.11

evaluated at the height above the ground, z at which the component or cladding under consideration is located on the structure.

Chapter 2 Loads



6-24

Table 6.2.11

Gust Response Factors, Gh and Gz(1)

Height above Gh (2)

and Gz

ground level (metres)

Exposure A Exposure B Exposure C

0-4.5 6.0 9.0

12.0

15.0 18.0 21.0 24.0

27.0 30.0 35.0 40.0

45.0 50.0 60.0 70.0

80.0 90.0

100.0 110.0

120.0 130.0 140.0 150.0

160.0 170.0 180.0 190.0

200.0 220.0 240.0

260.0 280.0 300.0

1.654 1.592 1.511 1.457

1.418 1.388 1.363 1.342

1.324 1.309 1.287 1.268

1.252 1.238 1.215 1.196

1.180 1.166 1.154 1.114

1.134 1.126 1.118 1.111

1.104 1.098 1.092 1.087

1.082 1.073 1.065

1.058 1.051 1.045

1.321 1.294 1.258 1.233

1.215 1.201 1.189 1.178

1.170 1.162 1.151 1.141

1.133 1.126 1.114 1.103

1.095 1.087 1.081 1.075

1.070 1.065 1.061 1.057

1.053 1.049 1.046 1.043

1.040 1.035 1.030

1.026 1.022 1.018

1.154 1.140 1.121 1.107

1.097 1.089 1.082 1.077

1.072 1.067 1.061 1.055

1.051 1.046 1.039 1.033

1.028 1.024 1.020 1.016

1.013 1.010 1.008 1.005

1.003 1.001 1.000 1.000

1.000 1.000 1.000

1.000 1.000 1.000

Note : (1)

(2)

For main wind-force resisting systems, use building or structure height h for z. Linear interpolation is acceptable for intermediate values of z.

c) Gust Response Factor, G for Slender Buildings and Structures : Gust response factor, G for the primary framing systems of slender buildings and structures shall be calculated by a rational analysis incorporating the dynamic properties of the primary framing system as given by the following relations.

Chapter 2 Loads


G =

0.65P

11.0T I2S

1 kc

(2.4.8)

where, P = f J (2.4.9)

f =

55.44fh

sVb (2.4.10)

TI =

2.35 Do

h 13.72 and (2.4.11)

f = fundamental natural frequency of the building or structure, Hz

= structural damping coefficient (fraction of critical damping)

h = mean roof height or height to parapet, metre c = average horizontal dimension of the building or structure normal to wind, metre V

b = basic wind speed, km/h

k = 0.00656 for building or structure = 0.00328 for open framework (lattice) structure J = pressure profile factor given in Fig 6.2.2

resonance factor given in Fig 6.2.3 S = structure size factor provided in Fig 6.2.4

Other parameters of Eq (2.4.8) through (2.4.11) are defined in Sec 2.4.2. Values of the parameters ,

Do , s and shall be those given in Table 6.2.12.

The gust response factor G as determined by this provision shall account for the load magnification effect caused by the wind gusts in resonance with along-wind oscillations of the structure, but shall not provide allowances for any cross-wind response such as that due to vortex shedding, galloping, flutter and ovalling nor for any torsional loading effect resulting from such response. Cases where cross-wind or torsional loading is possible, specialist advice shall be sought for further analysis, or wind tunnel tests specified in Sec 1.5.3.5 shall be made for determining such effects.


6-26

Chapter 2 Loads


Table 6.2.12 Building Exposure Parameters

Building Exposure Do s

A B C

0.222 0.143 0.100

0.010 0.005 0.003

1.33 1.00 0.85

1.0/h 0.07/h 0.0061/h

2.4.6.7 Pressure Coefficients for Buildings, Structures and Components : The pressure coefficients Cp to

be used in Eq (2.4.2) for the determination of design wind pressure shall be equal to the values described below:


6-28

a) Cpe : external pressure coefficient as given in Fig 6.2.5 and Fig 6.2.6 and in Table 6.2.13 for

external surfaces of buildings or structures. This coefficient shall be used with Method 1 given in Sec 2.4.6.4a(i).

b)

C pi : internal peak pressure coefficients as given in Table 6.2.14 for internal surfaces of

buildings. These coefficients shall be used along with the coefficients Cpe for design

wind load on components, or with Cpe for design wind load on buildings as per

provisions of Sec 2.4.6.4a (ii)

Chapter 2 Loads


Notation : B : Horizontal dimension of building, in metres measured

normal to wind direction C

G : Gust response coefficient

h : Mean roof height, in metres except that eave height may be used for 10 degrees

L : Horizontal dimension of building, in metres measured parallel to wind direction

ph : Design wind pressure

qh, qz : Sustained wind pressure, in kN/m2 evaluated at respective

heights z : Height above ground in metres : Roof slope from horizontal, degrees

External Pressure Coefficient Cpe for Walls *

Surface L/B Cpe For use with

Windward wall

all values

0.8

pz = CG Cpe qz

Leeward wall

0.10 0.65 1.00 2.00 4.00

0.5 0.6 0.5 0.3 0.2

pz = CG Cpe qh

Side wall all values - 0.7 pz = CG Cpeqz

* These coefficients may be used when h/B 5.0. Alternatively, use Table 6.2.15 and Method 2, Sec 2.4.6.4(b)

External Pressure Coefficients, Cpefor Roof

Windward Side

Wind (degrees) Leeward

Direction h/L 0 10-15 20 30 40 50 > 60 Side

Normal to ridge

0.3

0.5 1.0

1.5

- 0.7

- 0.7 - 0.7 - 0.7

0.2* - 0.9* - 0.9 - 0.9 - 0.9

0.2

- 0.75 - 0.75 - 0.9

0.3

- 0.2 - 0.2 - 0.9

0.4

0.3 0.3

- 0.35

0.5

0.5 0.5 0.2

0.01

0.01 0.01 0.01

- 0.7 for all

values of h/L

and

Parallel to ridge

h/B or h/L 2.5

h/B or h/L > 2.5

- 0.7

- 0.8

- 0.7

- 0.8

Coefficients are to be used with ph= CG Cpe qh , see Sec 2.4.6.6(a)

* Both values of Cpe shall be used for load calculations. Note : (1) These coefficients shall be used with Method 1, Sec 2.4.6.4.(a).


6-30

(2) Refer to Table 6.2.13 for arched roofs.

(3) For flexible buildings and structures, use appropriate G as determined by Sec 2.4.6.6 (c). (4) Plus and minus signs signify pressures acting toward and away from the surfaces, respectively.

(5) Linear interpolation may be made for values of h/L, and L/B ratios other than listed.

Fig 6.2.5 External Pressure Coefficients, Cpe for Primary Framing

Systems of Rectangular Buildings

Chapter 2 Loads



6-32

c)

C pe : external peak pressure coefficient as given in Fig 6.2.7 and 6.2.8 to be applied on external

surfaces of buildings to obtain design wind load on individual components and cladding in accordance with Sec 2.4.6.5.

d) Cp : overall pressure coefficient as given in Tables 6.2.15 through 6.2.21 for various cross-

sectional shapes to be used with the projected area of buildings or structures when Method 2 in Sec 2.4.6.4(b) is used.

If pressure coefficients Cpe , C pi ,

C pe or Cp are not provided herein for certain buildings, structures or

components, reliable references shall be followed or specialist advice shall be sought. Table 6.2.13 External Pressure Coefficients, Cpe for Arched Roofs

Cpe

Condition Rise-to-span Ratio, r

Windward Quarter

Centre half

Leeward Quarter

Roofs on elevated structures Roofs springing from ground level

0 < r < 0.2 0.2 < r < 0.3* 0.3 < r < 0.6 0 < r < 0.6

0.9 1.5 r 0.3 2.75 r 0.7 1.4 r

0.7 r 0.7 r 0.7 r 0.7 r

0.5 0.5 0.5 0.5

* When the rise-to-span ratio is 0.2 < r < 0.3 alternate coefficients given by (6r 2.1) shall also be used for the windward quarter.

Notes: (1)

(2) (3)

Values listed are for the determination of average loads on primary framing system. Plus and minus signs signify pressures acting toward and away from the surfaces, respectively. For components and cladding :

a)

b)

At roof perimeter, use the external pressure coefficients in Fig 6.2.7 with based on spring-line slope and q

h based on Exposure B.

For remaining roof area, use external pressure coefficients of this table multiplied by 1.2 and q

h based on Exposure B.

Table 6.2.14

Internal Peak Pressure Coefficients for Buildings, Cpi

Condition Cpi

Percentage of total wall area occupied by openings in one wall exceeds that of all other walls by 10% or more and openings in all other walls do not exceed 20% of respective wall area.

+ 0.75 and - 0.25

All other cases

0.25

Chapter 2 Loads


Notes: (1)

(2) (3) (4)

Values are to be used with q

z or qh as specified in Sec 2.4.6.4 a(ii) and 2.4.6.5.

Plus and minus signs signify pressures acting toward and away from the surfaces, respectively.

Appropriate positive and negative values of Cpi shall be considered when determining the

controlling load requirement. Percentage of openings is based on gross area of wall.


6-34

Note : (1) Vertical scale denotes

C pe to be used with qh based on Exposure B .

(2) The horizontal scale denotes tributary area in square metres.

(3) External pressure coefficients for walls may be reduced by 10% when 10 degrees. (4) Plus and minus signs signify pressures acting toward and away from the surfaces, respectively. (5) Each component shall be designed for maximum positive and negative pressures.

(6) Roof overhangs shall have

C pe given in Fig (b) to be applied at the top surface plus a C pe = + 0.8 applied at the

bottom surface.

Fig 6.2.7 External Peak Pressure Coefficients C p e

for Loads on Building Components and Cladding for Buildings with Mean Roof Height, h of 18 metres or Less

Chapter 2 Loads


Note : (1) Vertical scale denotes

C pe to be used with appropriate qz or qh

(2) Horizontal scale denotes tributary area A in square metres

(3) Use qh with negative values of C pe

(4) Each component shall be designed for maximum positive and negative pressures (5) If a parapet is provided around the roof perimeter, zones (3) and (4) may be treated as zone (2)

(6) For roofs with a slope of more than 10 degrees, use

C pe from Fig 6.2.7 and qh based on Exposure B

(7) Plus and minus signs signify pressures acting toward and away from the surfaces, respectively.

(8) Roof overhangs shall have

C pe given in Fig (b) to be applied at the top surface plus a C pe = + 0.8 applied at the

bottom surface.

(9) For parapet use

C pe = 1.3

Notation:

a : 5% of minimum width or 0.5h, whichever is smaller h : Mean roof height in metres z : Height above ground in metres.

Fig 6.2.8 External Peak Pressure Coefficients C p e

for Loads on Building Components and Cladding for Buildings with Mean Roof Height, h Greater Than 18 metres


6-36

Table 6.2.15 (1) Overall Pressure Coefficients,Cp

(2) for Rectangular Buildings with Flat Roofs

h/B L/B

0.1 0.5 0.65 1.0 2.0 > 3.0

0.167 ) All 1.0 1.2 1.4

Round (D

qz > 0.167): Moderately smooth 0.5 0.6 0.7

Rough (D/D 0.02) 0.7 0.8 0.9 Very rough(D/D 0.08) 0.8 1.0 1.2

Round (D

qz 0.167): All 0.7 0.8 1.2

Notes: 1)

2) 3)

The design wind force shall be calculated based on the area of the structure projected on a plane normal to the wind direction. The force shall be assumed to act parallel to the wind direction. Linear interpolation may be used for h/D values other than those shown. Notation :

D:

D: h:

diameter or least horizontal dimension, metres. depth of protruding elements such as ribs and spoilers, metres. height of structure, metres.

Table 6.2.17

Overall Pressure CoefficientsCp for Monoslope Roofs Over Unenclosed Buildings and Structures

L/B

(degrees) 5 3 2 1 1/2 1/3 1/5

10 15 20 25 30

0.2 0.35 0.5 0.7 0.9

0.25 0.45 0.6 0.8 1.0

0.3 0.5

0.75 0.95 1.2

0.45 0.7 0.9

1.15 1.3

0.55 0.85 1.0 1.1 1.2

0.7 0.9

0.95 1.05 1.1

0.75 0.85 0.9

0.95 1.0

Location of centre of pressure, X/L, for L/B values of :

2 to 5 1 1/5 to 1/2

10 to 20 25 30

0.35 0.35 0.35

0.3 0.35 0.4

0.3 0.4

0.45

Chapter 2 Loads


Note: 1) 2) 3)

Wind forces act normal to the surface and shall be directed inward or outward. Wind shall be assumed to deviate by 10 degrees from horizontal. Notation :

B: L: X: Q:

dimension of roof measured normal to wind direction, metres dimension of roof measured parallel to wind direction, metres distance to centre of pressure from windward edge of roof, metres angle of plane of roof from horizontal, degrees.


6-38

Table 6.2.18

Overall Pressure Coefficients,Cp for Solid Signs

At Ground Level Above Ground Level

Cp M/N Cp

3 5 8 10 20 30 40

1.2 1.3 1.4 1.5

1.75 1.85 2.00

6 10 16 20 40 60 80

1.2 1.3 1.4 1.5

1.75 1.85 2.00

Note :1)

2)

3)

Signs with openings comprising less than 30% of the gross area shall be considered as solid signs. Signs for which the distance from the ground to the bottom edge is less than 0.25 times the vertical dimension shall be considered to be at ground level. To allow for both normal and oblique wind directions, two cases shall be considered :

a) b)

Resultant force acts normal to sign at geometric centre, and Resultant force acts normal to sign at level of geometric centre and at a distance from windward edge of 0.3 times the horizontal dimension.

4) Notation:

M:N

Ratio of height to width Larger dimension of sign, metres Smaller dimension of sign, metres.

Table 6.2.19

Overall Pressure Coefficients Cp for Open Signs and Lattice Frameworks

Cp

Flat-sided Round Members

Members D q

z 0.167

D q

z > 0.167

< 0.1 0.1 to 0.29 0.3 to 0.7

2.0 1.8 1.6

1.2 1.3 1.5

0.8 0.9 1.1

Notes: 1)

2)

3)

Signs with openings comprising 30% or more of the gross area are classified as open signs. The calculation of the design wind forces shall be based on the area of all exposed members and elements projected on a plane normal to the wind direction. Forces shall be assumed to act parallel to the wind direction. Notation :

: Ratio of solid area to gross area D : Diameter of a typical round member, in metres.

Chapter 2 Loads


Table 6.2.20

Overall Pressure Coefficients, Cp for Trussed Towers

Cp

Square Towers Triangular Towers

< 0.025 0.025 to 0.44 0.45 to 0.69

0.7 to 1.0

4.0

4.1 5.2 1.8

1.3 + 0.7

3.6

3.7 4.5 1.7

1.0 + Note : 1) 2)

3) 4)

5) 6) 7) 8)

Force coefficients are given for towers with structural angles or similar flat-sided members. For towers with rounded members, the design wind force shall be determined using the values in the above table multiplied by the following factors:

For < 0.29: factor = 0.67

For 0.3


6-40

Notes: 1) The force coefficients shall be used in conjunction with exposed area of the tower guy in square metre, calculated as chord length multiplied by guy diameter.

2) Notation: Cp,D :

Cp,L :

Force coefficient for the component of force acting in direction of the wind. Force coefficient for the component of force acting normal to direction of the wind and in the

plane containing the angle . Angle between wind direction and chord of the guy, in degrees.

2.4.6.8 Effect of Local Topography : If a structure or any portion thereof is located within a local

topographic zone, such as regions around hills and ridges as shown in Fig 6.2.9, the sustained wind pressure obtained from Sec 2.4.6.2 shall be modified by multiplying by a local topographic coefficient, Ct .

Value of the coefficient, Ct shall be obtained from Fig 6.2.9.

Local Topographic Coefficient, Ct at Crest

Upwind slope

(tan

Coefficient, Ct

0.05 0.1 0.2 0.3

1.19 1.39 1.85 2.37

Chapter 2 Loads


Legend:

tan = the upwind slope,

H

2Lu

tand = the average downwind slope, measured from the crest of a hill or ridge or to the

ground level at a distance of 5H. H = the height of the hill or ridge in meters Lu = the horizontal distance upwind from the crest to a level half the height below the

crest in meters. Notes: (1) For intermediate values of upwind slope, linear interpolation is permitted. (2) Ct = 1.0 for a point at or out side the boundary of the local topographic zones as shown in the figure .

For any point within the local topographic zone, value of the coefficient, Ct shall be obtained by

interpolation from the value at crest given in the table and the value of Ct=1 at the boundary of the

zone. The interpolation shall be linear with horizontal distance from the crest, and with height above the local ground level.

Fig 6.2.9 Local Topographic Coefficient, Ct for Hills and Ridges. 2.5 EARTHQUAKE LOADS 2.5.1 General Minimum design earthquake forces for buildings, structures or components thereof shall be determined in

accordance with the provisions of this section. For primary framing systems of buildings or structures, the design seismic lateral forces shall be calculated either by the Equivalent Static Force Method or by the Dynamic Response Method based on the criteria set forth in Sec 2.5.5.1. Overall design of buildings and structures to resist seismic ground motion and other forces shall comply with the applicable design requirements given in Chapter 1.

2.5.2 Definitions The following definitions of terms shall be applicable only to the provisions of Sec 2.5 : BASE : The level at which the earthquake motions are considered to be imparted to the structures or the

level at which the structure as a dynamic vibrator is supported. BASE SHEAR : Total design lateral force or shear at the base of a structure. BEARING WALL SYSTEM : A structural system without a complete vertical load carrying space frame, see

Sec 1.3.2.

BRACED FRAME : An essentially vertical truss system of the concentric or eccentric type which is provided

to resist lateral forces. BUILDING FRAME SYSTEM : An essentially complete space frame which provides support for gravity

loads, see Sec 1.3.2. DIAPHRAGM : A horizontal or nearly horizontal system of structures acting to transmit lateral forces to the

vertical resisting elements. The term "diaphragm" includes horizontal bracing systems. DUAL SYSTEM : A combination of a Special or Intermediate Moment Resisting Frame and Shear Walls or

Braced Frames designed in accordance with the criteria of Sec 1.3.2. ECCENTRIC BRACED FRAME (EBF) : A steel braced frame designed in conformance with Sec 1.8. ESSENTIAL FACILITIES : Buildings and structures which are necessary to remain functional during an

emergency or a post disaster period.


6-42

FLEXIBLE DIAPHRAGM : A floor or roof diaphragm shall be considered flexible, for purposes of this provision, when the maximum lateral deformation of the diaphragm is more than two times the average storey drift of the associated storey. This may be determined by comparing the computed midpoint in-plane deflection of the diaphragm under lateral load with the storey drift of adjoining vertical resisting elements under equivalent tributary lateral load.

FLEXIBLE ELEMENT OR SYSTEM : An element or system whose deformation under lateral load is

significantly larger than adjoining parts of the system. FLEXIBLY SUPPORTED EQUIPMENT : Non-rigid or flexibly supported equipment is a system having a

fundamental period, including the equipment, greater than 0.06 second. HORIZONTAL BRACING SYSTEM : A horizontal truss system that serves the same function as a floor or

roof diaphragm. INTERMEDIATE MOMENT RESISTING FRAME (IMRF) : A concrete or steel frame designed in accordance

with Sec 8.3 or 10.5.17 respectively. MOMENT RESISTING FRAME : A frame in which members and joints are capable of resisting forces

primarily by flexure. ORDINARY MOMENT RESISTING FRAME (OMRF) : A moment resisting frame not meeting special

detailing requirements for ductile behaviour. PRIMARY FRAMING SYSTEM : That part of the structural system assigned to resist lateral forces. RIGIDLY SUPPORTED EQUIPMENT : A rigid or rigidly supported equipment is a system having a

fundamental period less than or equal to 0.06 second. SHEAR WALL : A wall designed to resist lateral forces parallel to the plane of the wall (sometimes referred

to as a vertical diaphragm or a structural wall). SOFT STOREY : Storey in which the lateral stiffness is less than 70 per cent of the stiffness of the storey

above. SPACE FRAME : A three-dimensional structural system without bearing walls composed of members

interconnected so as to function as a complete self contained unit with or without the aid of horizontal diaphragms or floor bracing systems.

SPECIAL MOMENT RESISTING FRAME (SMRF) : A moment resisting frame specially detailed to provide

ductile behaviour complying with the seismic requirements provided in Chapters 8 and 10 for concrete and steel frames respectively.

SPECIAL STRUCTURAL SYSTEM : A structural system not listed in Table 6.2.24. STOREY : The space between floor levels. Storey-x is the storey below level-x. STOREY SHEAR, Vx : The summation of design lateral forces above the storey under consideration.

STRENGTH : The usable capacity of an element or a member to resist the load as prescribed in these

provisions. STRUCTURE : An assemblage of framing members designed to support gravity loads and resist lateral

forces. Structures may be categorized as building and non-building structures as defined in Sec 1.2.2. TOWER : A tall, slim vertical structure. VERTICAL LOAD-CARRYING FRAME : A space frame designed to carry all vertical gravity loads.

Chapter 2 Loads


WEAK STOREY : Storey in which the lateral strength is less than 80 per cent of that of the storey above. 2.5.3 Symbols and Notation The following symbols and notation shall apply to the provisions of this section : A

c = the combined effective area, in square metres of the shear walls in the first storey of the

structure. A

e = the effective horizontal cross-sectional area, in square metres of a shear wall in the first

storey of the structure. A

x = the torsion amplification factor at level-x.

C = numerical coefficient specified in Sec 2.5.6.1.

C = numerical coefficient specified in Sec 2.5.8 and given in Table 6.2.26. Ct = numerical coefficient given in Sec 2.5.6.2.

De = the length in metres of a shear wall element in the first storey in the direction parallel to the

applied forces. ft = lateral force at level -i for use in Eq (2.5.5).

Fi,F

n,F

x = lateral force applied to level-i, -n, or -x respectively.

F = lateral forces on an element or component or on equipment supports. F

t = that portion of the base shear V, considered concentrated at the top of the structure in

addition to Fn.

F x = force on floor- or roof-diaphragm.

g = acceleration due to gravity. hi, hn, hx = height in metres above the base to level i, -n or -x respectively.

I = structure importance coefficient given in Table 6.2.23. I = structure importance coefficient specified in Sec 2.5.8 for structural and non-structural

components and equipment. Level-i = level of the structure referred to by the subscript i, e.g., i = 1 designates the first level above

the base. Level-n = the uppermost level in the main portion of the structure. Level-x = the level under consideration e.g., x = 1 designates the first level above the base. R = response modification coefficient for structural systems given in Table 6.2.24. S = site coefficient for soil characteristics given in Table 6.2.25. T = fundamental period of vibration, in seconds, of the structure in the direction under

consideration. V = the total design lateral force or shear at the base Vx = the design storey shear in storey x

W = the total seismic dead load defined in Sec 2.5.5.2 wi , wx = that portion of W which is located at or assigned to level -i or -x respectively

w x = the weight of the diaphragm and the elements tributary thereto at level-x, including

applicable portions of other loads defined in Sec 2.5.5.2. W = the weight of an element or component Z = seismic zone coefficient given in Table 6.2.22.

i = horizontal displacement at level-i relative to the base due to applied lateral forces, in metre,

for use in Eq (2.5.5). 2.5.4 Seismic Zoning 2.5.4.1 Seismic Zoning Map : The seismic zoning map of Bangladesh is provided in Fig 6.2.10. Based on

the severity of the probable intensity of seismic ground motion and damages, Bangladesh has been divided into three seismic zones, i.e. Zone 1, Zone 2 and Zone 3 as shown in Fig 6.2.10 with Zone 3 being the most severe.

2.5.4.2 Selection of Seismic Zone and Zone Coefficient : Seismic zone for a building site shall be

determined based on the location of the site on the Seismic Zoning Map provided in Fig 6.2.10. Each


6-44

building or structure shall be assigned a Seismic Zone Coefficient, Z corresponding to the seismic zone of the site as set forth in Table 6.2.22.

2.5.5 Design Earthquake Forces for Primary Framing Systems The design earthquake lateral forces on the primary framing systems of every building or structure shall be

calculated based on the provisions set forth in this section. The design seismic forces shall be assumed to act nonconcurrently in the direction of each principal axis of the building or the structure, except otherwise required by the provisions of Sec 1.5.4 and 1.7.

2.5.5.1 Selection of Lateral Force Method : Seismic lateral forces on primary framing systems shall be

determined by using either the Equivalent Static Force Method provided in Sec 2.5.6, or the Dynamic Response Method given in Sec 2.5.7 complying with the restrictions given below :

a) The Equivalent Static Force Method of Sec 2.5.6 may be used for the following structures : i) All structures, regular or irregular, in Seismic Zone 1 and in Structure Importance Category IV

in Seismic Zone 2, except case b(iv) below. ii) Regular structures under 75 metres in height with lateral force resistance provided by

structural systems listed in Table 6.2.24. except case b(iv) below. iii) Irregular structures not more than 20 metres in height. iv) A tower like building or structure having a flexible upper portion supported on a rigid lower

portion where: 1) both portions of the structure considered separately can be classified as regular structures, 2) the average storey stiffness of the lower portion is at least ten times the average storey

stiffness of the upper portion, and 3) the period of the entire structure is not greater than 1.1 times the period of the upper

portion considered as a separate structure fixed at the base. b) The Dynamic Response Method as given in Sec 2.5.7 may be used for all classes of structure, but shall

be used for structures of the following types. i) Structures 75 metres or more in height except as permitted by case a(i) above. ii) Structures having a stiffness, weight or geometric vertical irregularity of Type I, II, or III as

defined in Table 6.1.3. or structures having irregular features not described in either Table 6.1.3 or 6.1.4.

iii) Structures over 20 metres in height in Seismic Zone 3 not having the same structural system throughout their height except as permitted by Sec 1.6.4.

iv) Structures, regular or irregular, located on Soil Profile Type S4 as given in Table 6.2.25, which

have a period greater than 0.7 second. The analysis shall include the effects of the soils at the site and shall conform to Sec 2.5.7.1 (c).

2.5.5.2 Seismic Dead Load : Seismic dead load, W, is the total dead load of a building or a structure,

including permanent partitions, and applicable portions of other loads listed below : a) In storage and warehouse occupancies, a minimum of 25 per cent of the floor live load shall be

applicable. b) Where an allowance for partition load is included in the floor design in accordance with Sec 2.3.3.3,

all such loads but not less than 0.6 kN/m2 shall be applicable. c) Total weight of permanent equipment shall be included.

Chapter 2 Loads



6-46

2.5.6 Equivalent Static Force Method This method may be used for calculation of seismic lateral forces for all structures specified in Sec 2.5.5.1(a) 2.5.6.1 Design Base Shear : The total design base shear in a given direction shall be determined from the

following relation :

V

ZIC

RW (2.5.1)

where, Z = Seismic zone coefficient given in Table 6.2.22 I = Structure importance coefficient given in Table 6.2.23 R = Response modification coefficient for structural systems given in Table 6.2.24 W = The total seismic dead load defined in Sec 2.5.5.2 C = Numerical coefficient given by the relation :

C

1.25S

T 2 / 3 (2.5.2)

S = Site coefficient for soil characteristics as provided in Table 6.2.25 T = Fundamental period of vibration in seconds, of the structure for the direction under

consideration as determined by the provisions of Sec 2.5.6.2. The value of C need not exceed 2.75 and this value may be used for any structure without regard to soil

type or structure period. Except for those requirements where Code prescribed forces are scaled up by 0.375R, the minimum value of the ratio C/R shall be 0.075.

Table 6.2.22 Table 6.2.23

Seismic Zone Coefficients, Z Structure Importance Coefficients I, I

Seismic Zone

(see Fig 6.2.10)

Zone

Coefficient

Structure Importance Category (see Table 6.1.1 for occupancy)

Structure

Importance

Coefficient

I I I Essential facilities 1.25 1.50

1 0.075 II Hazardous facilities 1.25 1.50 2 0.15 III Special occupancy structures 1.00 1.00 3 0.25 IV Standard occupancy structures 1.00 1.00 V Low-risk Structures 1.00 1.00

2.5.6.2 Structure Period : The value of the fundamental period, T of the structure shall be determined

from one of the following methods : a) Method A : For all buildings the value of T may be approximated by the following formula :

T = Ct (hn) 3/4 (2.5.3)

where, Ct = 0.083 for steel moment resisting frames

= 0.073 for reinforced concrete moment resisting frames, and eccentric braced steel frames

= 0.049 for all other structural systems hn = Height in metres above the base to level n.

Alternatively, the value of Ct for buildings with concrete or masonry shear walls may be taken as

0.03 1 Ac . The value of Ac shall be obtained from the relation :

Ac =

Ae 0.2 De hn 2 (2.5.4)

Chapter 2 Loads


where, Ac = The combined effective area, in square metres, of the shear walls in the first

storey of the structure. Ae = The effective horizontal cross-sectional area, in square metres of a shear wall in

the first storey of the structure. De = The length, in metre of a shear wall element in the first storey in the direction

parallel to the applied forces. The value of De /hn for use in Eq ( 2.5.4) shall not exceed 0.9.


6-48

Table 6.2.24

Response Modification Coefficient for Structural Systems, R

Basic Structural System(1) Description of Lateral Force Resisting System R

(2)

a. Bearing Wall System

1. Light framed walls with shear panels i) Plywood walls for structures, 3 storeys or less ii) All other light framed walls 2. Shear walls i) Concrete ii) Masonry 3. Light steel framed bearing walls with tension only bracing 4. Braced frames where bracing carries gravity loads i) Steel

ii) Concrete (3)

iii) Heavy timber

8 6

6 6 4

6 4

4

b. Building Frame System

1. Steel eccentric braced frame (EBF) 2. Light framed walls with shear panels i) Plywood walls for structures 3-storeys or less ii) All other light framed walls 3. Shear walls i) Concrete ii) Masonry 4. Concentric braced frames (CBF) i) Steel

ii) Concrete (3)

iii) Heavy timber

10

9 7

8 8

8 8 8

c. Moment Resisting Frame System

1. Special moment resisting frames (SMRF) i) Steel ii) Concrete

2. Intermediate moment resisting frames (IMRF), concrete(4)

3. Ordinary moment resisting frames (OMRF)

i) Steel

ii) Concrete (5)

12 12 8

6

5

d. Dual System 1. Shear walls i) Concrete with steel or concrete SMRF ii) Concrete with steel OMRF

iii) Concrete with concrete IMRF (4)

iv) Masonry with steel or concrete SMRF v) Masonry with steel OMRF

vi) Masonry with concrete IMR

bnbc 93 chapter 2

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