Is+875+(Part 3)+Code+of+Practice+for+Design+Loads+(Other+Tha

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I ndian St andardIS:875 (Part 3) - 1987( Renfficd 1997 )

CODEOFPRACTICEFORDESIGNLOADS(OTHERTHANEARTHQUAKE)FORBUILDINGSANDSTRUCTURES

PART 3 WIND COADS( Second Rev i si onSixth Reprint NOVEMBER 1998

UDC 624-042-41

@J Copy ri ght 1989BUREAU OF INDIAN STANDARDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG


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0.1.2.3.4.5.5.15.25.3

CONTENTSFOREWORD . . .SCOPE . . .NOTATIONS . .TERMINOLOGYGENERALWIND SPEEDAND PRESSURENature of Wind in AtmosphereBasic Wind SpeedDesign Wind Speed ( V, )

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5.3.1 Risk Coefficient ( kr Factor ) . . . . .53.2 Terrain, Height and Structure Size Factor ( kt Factor )5.3.3 Topography ( kS Factor ) . . . . .

5.4 Design Wind Pressure . .5.5 Off-Shore Wind Velocity . . .-.6. WIND PRESSURESND FORCESON BUIL DXNCSISTRUCTURES6.1 General . . . . . .6.2 Pressure Coefficients . . . . .

6.2.1 Wind Load on Individual Members ,.. . . .6.2.2 External Pressure Coefficients . . . . .6.2.3 Internal Pressure Coefficients . . .

6.3 Force Coefficients . . .6.3.1 Frictional Drag . . . . .6.3.2 Force Coefficients for Clad Buildings ._. . .6.3.3 Force Coefficients for Unclad Buildings __. . .

7. DYNAMICEP~ECTS . . . . . .7.1 General 1.. . .7.2 Motion Due to Vortex Shedding . . . . . .

7.2.1 Slender Structures . . . . .

IS : 875 ( Part 3 ) - 1987

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4. Gust Factor ( GF ) or Gust Effectiveness Factor ( GEF] Method . . .8.1 Application . . . . . . . . .8.2 Hourly Mean Wind . . . . . . . . .

8.2.1 Variation of Hourly Mean Wind Speed with Height . . .8.3 Along Wind Load . . . . . . l . .APPENDIK A BASICWIND SPEEDAT 10 m HE IGHTFOR SOME IM PORTANTCrrrxs/TowNs . . . . . . . . . . . . . .APPENDIX B CHANGES N TERRAIN CATEGORIES i.. . . . .APPENDIX C EFFECT OF A CLIFF OR ESCARPMENTON EQUIVALENTHEI GHT ABOVE GROUND( k3 FACTOR . . . . . .APPENDIX D WIND FORCEON CIRCUL ARSECTIONS . . . . . .

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IS t 875 ( Part 3 ) - 1987

I ndian St andardCODEOFPRACTICEFORDESIGNLOADS

(OTHERTHANEARTHQUAKE)FORBUILDINGSANDSTRUCTURESPART 3 WIND LOADS

( Second Rev i si on)6). FOREWORD

0.1 This Indian Standard ( Part 3 ) ( SecondRevision ) was adopted by the Bureau of IndianStandards on 13 November 1987, after the draftfinalized by the Structural Safety Sectional Com-mittee had been approved by the Civil Engineer-ing Division Council.0.2 A building has to perform many functionssatisfactorily. Amongst these functions are theutility of the building for the intended use andoccupancy, structural safety, fire safety and com-pliance with hygienic, sanitation, ventilation anddaylight standards. The design of the building isdependent upon the minimum requirementsprescribed for each of the above functions. Theminimum requirements pertaining to the structuralsafety of buildings are being covered in loadingcodes by way of laying down minimum designloads which have to be assumed for dead loads,imposed loads, wind loads and other externalloads, the structure would be required to bear.Strict conformity to loading standards, it is. hoped,will not only ensure the structural safety of thebuildings and structures which are being designedand constructed in the country and therebyreduce the hazards to life and property caused byunsafe structures, but also eliminate the wastagecaused by assuming unnecessarily heavy loadingswithout proper assessment.0.3 This standard was first published in 1957 forthe guidance of civil engineers, designers andarchitects associated with the planning and designof buildings. It included the provisions for thebasic design loads ( dead loads, live loads, windloads and seismic loads ) to be assumed in thedesign of the buildings. In its first revision in1964, the wind pressure provisions were modifiedon the basis of studies of wind phenomenon andits effect on structures, undertaken by the specialcommittee in consultation with the Indian Mete-orological Department. In addition to this, newclauses on wind loads for butterfly type structureswere included; wind pressure coefficients for

sheeted roofs, both curved and sloping weremodified; seismic load provisions were deleted( separate code having been prepared ) and metricsystem of weights and measurements was adopted.0.3.1 With the increased adoption of this Code,a number of comments were received on provi-sions on live load values adopted for. differentoccupancies. Simultaneously, live load surveyshave been carried out in America and Canada toarrive at realistic live loads based on actual deter-mination of loading ( movable and immovable )in different occupancies. Keeping this in view and

other developments in the field of wind engineer-ing, the Structural Safety Sectional Committeedecided to prepare the second revision of IS : 875in the following five parts:Part 1 Dead loadsPart 2 Imposed loadsPart 3 Wind loadsPart 4 Snow loadsPart 5 Special loads and load combinationsEarthquake load is covered in a separatestandard, namely, IS : 1893-1984* which shouldbe considered along with the above loads.

0.3.2 This Part ( Part 3 ) deals with windloads to be considered when designing buildings,structures and components thereof. In thisrevision, the following important modificationshave been made from those covered in the 1964version of IS : 875:a) The earlier wind pressure maps ( onegiving winds of shorter duration and an-other excluding winds of shorter duration )

*Criteria for earthquake resistant design of structures(fourlh recision ).


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IS : 875 ( Part 3 ) - 1987

b)

4d)

4f 1g)

h)

3W

have been replaced by a single wind mapgiving basic maximum wind speed in m/s( peak gust velocity averaged over a shorttime interval of about 3 seconds duration ).The wind speeds have been worked outfor 50 years return period based on the up-to-date wind data of 43 dines pressuretube ( DPT ) anemograph stations andstudy of other related works available onthe subject since 1964. The map andrelated recommendations have been provi-ded in the code with the active coopera-tion of Indian Meteorological Department( IMD ). Isotachs ( lines of equal velocity )have not been given as in the opinion ofthe committee, there is still not enoughextensive meteorological data at closeenough stations in the country to justifydrawing of isotachs.Modification factors to modify the basicwind velocity to take into account theeffects of terrain, local topography, size ofstructure, etc, are included.Terrain is now classified into four catego-ries based on characteristics of the groundsurface irregularities.Force and pressure coefficients have beenincluded for a large range of clad andunclad buildings and for individual struc-tural elements.Force coefficients ( drag coefficients ) aregiven for frames, lattice towers, walls andhoardings.The calculation of force on circular sectionsis included incorporating the effects ofReynolds number and surface roughness.The external and internal pressure coeffi-cients for gable roofs, lean-to roofs, curvedroofs, canopy roofs ( butterfly type struc-tures ) and multi-span roofs have beenrationalised.Pressure coefficients are given for combinedroofs, roofs with sky light, circular siIos,cylindrical elevated structures, grandstands,etc.Some requirements regarding study ofdynamic effects in flexible slender structuresare included.Use of gust energy method to arrive at thedesign wind load on the whole structure isnow permitted.

0.3.3 The Committee responsible for therevision of wind maps while reviewing available

meteorological wind data and response of struc-tures to wind, felt the paucity of data on which tobase wind maps for Indian conditions on statisti-cal analysis. The Committee, therefore, recomm-ends to all individuals and organizationsresponsible for putting-up of tall structures to,provide instrumentation in. their existing andnew structures ( transmission towers, chimneys,cooling towers, buildings, etc ) at different eleva-tions ( at least at two levels ) to continuouslymeasure and monitor wind data. The instrumentsare required to collect data on wind direction,wind speed and structural response of the struc-ture due to wind ( with the help of accelerometer,strain gauges, etc ). It is also the opinion of thecommittee that such instrumentation in tall struc-tures will not in any way affect or alter thefunctional behaviour of such structures. The dataso collected will be very valuable in evolving moreaccurate wind loading of structures.

0.4 The Sectional Committee responsible for thepreparation of this standard has taken intoaccount the prevailing practice in regard to load-ing standards followed in this country by thevarious authorities and has also taken note of thedevelopments in a number of other countries.In the preparation of this code, the followingoverseas standards have also been examined:a) BSCP 3 : 1973 Code of basic data fordesign of buildings: Chapter V Loading,Part 2 Wind loads.b) AS 1170, Part 2-1983 SAA Loading codePart 2 - Wind forces.c) NZS 4203-1976 Code of practice forgeneral structural design loading forbuildings.d) ANSI A58.1-1972 American StandardBuilding code requirements for minimumdesign loads in buildings and otherstructures.e) Wind resistant design regulations, A WorldList. Association for Science Documents

Information, Tokyo.0.5 For the purpose of deciding whether a parti-cular requirement of this standard is compliedwith, the final value, observed or calculated,expressing the result of a test or analysis, shall berounded off in accordance with IS : 2-1960*. Thenumber of significant places retained in therounded off value should be the same as that ofthe specified value in this standard.

*Rules for roundingoff numerical values ( rcoiscd .4


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IS : 875 ( Part 3 ) - 19871. SCOPE1.1 This standard gives wind forces and theireffects ( static and dynamic ) that should hetaken into account when designing buildings,structures and components thereof.

1.1.1 It is believed that ultimately wind loadestimation will be made by taking into accountthe random variation of wind speed with timebut available theoretical methods have notmatured sufficiently at present for use in the code.For this season, static wind method of loadestimation which implies a steady wind speed,which has proved to be satisfactory for normal,short and heavy structures, is given in 5 and 6.However, a beginning has been made to takeaccount of the random nature of the wind speedby requiring that the along-wind or drag load onstructures which are prone to wind induced osci-llations, be also determined by the gust factor

method ( see 8 ) and the more severe of the twoestimates be taken for design.A large majority of structures met with inpractice do not however, suffer wind inducedoscillations and generally do not require to beexamined for the dynamic effects of wind, includ-ing use of gust factor method, Nevertheless, thereare various types of structures or their componentssuch as some tall buildings, chimneys, latticedtowers, cooling towers, transmission towers, guyedmasts, communication towers, long span bridges,partially or completely solid faced antenna dish,etc, which require investigation of wind induced

oscillations. The use of 7 shall be made for i.denti-fying and analysing such structures.1.1.2 This code also applies to buildings orother structures during erection/construction andthe same shall be considered carefully duringvarious stages of erection/construction. In loca-tions where the strongest winds and icing mayoccur simultaneously, loads on structural members,cables and ropes shall be calculated by assumingan ice covering based on climatic and localexperience.1.1.3 In the design of special structures, such

as chimneys, overhead transmission line towers,etc, specific requirements as specified in therespective codes shall be adopted in conjunctionwith the provisions of this code as far as they areapplicable. Some of the Indian Standards avail-able for the design of special structurers are:IS : 4998 ( Part 1 )-1975 Criteria for designof reinforced concrete chimneys: Part 1Design criteria ( jirst revi sion )IS : 6533-1971 Code of practice for design andconstruction of steel chimneysIS : 5613 ( Part l/Set 1 )-I970 Code of prac-tice for design, installation and maintenanceof overhead power lines: Part 1 Lines up toand including 11 kV, Section 1 Design

IS : 802 ( Part 1 )-I977 Code of practice foruse of structural steel in overhead transmi-ssion line towers: Part 1 Loads and permissi-ble stresses ( smmd revi sion )IS : 11504-1985 Criteria for structural designof reinforced concrete natural draught cool-ing towers

NOTE 1 - This standard does not apply to build-ings or structures with unconventional shapes, unusuallocations, and abnormal environmental conditions thathave not been covered in this code. Special investiga-tions are necessary in such cases to establish wind loadsand their effects. Wind tunnel studies may aiso berequired in such situations.NOTE - In the case of tall structures withunsymmetrical geometry, the designs may have to bechecked for torsional effects due to wind pressure.

2. NOTATIONS2.1 The following notations shall be followedunless otherwise specified in relevant clauses:

A=Ae -Ar, =b =

Cl =Cl, =tit -cf -c, =

CPB =CPl =d-D =F

Fa 1Ft -F' =

h Xh, =

Pd -

surface area of a structure or part ofa Structure;effective frontal area;an area at height z;breadth of a structure or structuralmember normal to the wind streamin the horizontal plane;force coefficient/drag coefficient;normal force coefficient;transverse force coefficient;frictional drag coefficient;pressure coefficient;external pressure coefficient;internal pressure coefficient;depth of a structure or structuralmember parallel to wind stream;diameter of cylinder;force normal to the surface;normal force;transverse force;frictional force;height of structure above meanground level;height of development of a velocityprofile at a distance x down wind from a change in terrain category;multiplication factors;multiplication factor;length of the member or greater hori-zontal dimension of a building;design wind pressure;


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IS : 875 ( Part 3 ) - 1987pz =Pe -Pi -R, =s wvb -v, =rz =W 3

w -X=e sa -B =9+-t=c-

design wind pressure at height


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on the structure at a height corresponding to thatof vz.NOTE - Positive sign of the pressure coefficientindicates pressure acting towards the surface and nega-tive sign indicates pressure acting away from thesurface.

Retur n Peri od - Return period is the numberof years, the reciprocal of which gives the proba-bility of e.xtreme wind exceeding a given windspeed in any one year.Shielding E$ect - Shielding effect or shieldingrefers to the condition where wind has to passalong some structure(s) or structural element(s)located on the upstream wind side, before meet-ing the structure or structural element underconsideration. A factor called shielding factoris used to account for such effects in estimating theforce on the shielded structures.Sucti on - Suction means pressure less than theatmospheric ( static ) pressure and is taken to actaway from the surface.Solid i t y Rat io - Solidity ratio is equal to theeffective area ( projected area of all the individualelements ) of a frame normal to the wind direc-tion divided by the area enclosed by the boundaryof the frame normal to the wind direction.

NOTE - Solidity ratio is to be calculated for indi-vidual frames.Y?-eerrainategory - Terrain category means the

characteristics of the surface irregularities of anarea which arise from natural or constructedfeatures. The categories are numbered in increas-ing order of roughness.Velocit y Profil e - The variation of the horizon-tal component of the atmospheric wind speed atdifferent heights above the mean ground level istermed as velocity profile.Tokography - The nature of the earthssurface as influenced the hill and valley confi-gurations.

4. GENERAL4.1 Wind is air in motion relative to the surfaceof the earth. The primary cause of wind is tracedto earths rotation and differences in terrestrialradiation. The radiation effects are primarilyresponsible for convection either upwards ordownwards. The wind generally blows horizontalto the ground at high wind speeds. Since verticalcomponents of atmospheric motion are relativelysmall, the term wind denotes almost exclusivelythe horizontal wind, vertical winds are alwaysidentified as such. The wind speeds are assessedwith the aid of anemometers or anemographswhich are installed at meteorological observa-tories at heights generally varying from 10 to30 metres above ground.4.2 Very strong winds ( greater than 80 km/h )are generally associated with cyclonic storms,

IS : 875 ( Part 3 ) - 1987thunderstorms, dust storms or vigorous monsoons.A feature of the. cyclonic storms over the Indianarea is that they rapidly weaken after crossingthe coasts and move as depressions/lows inland.The influence of a severe storm after striking thecoast does not, in general exceed about 60 kilo-metres, though sometimes, it may extend even upto 120 kilometres. Very short duration hurricanesof very high wind speeds called Kal Baisaki orNorwesters occur fairly frequently during summermonths over North East India.4.3 The wind speeds recorded at any locality areextremely variable and in addition to steady windat any time, there are effects of gusts which maylast for a few seconds. These gusts cause increasein air pressure but their effect on stability ofthebuilding may not be so important; often, gustsaffect only part of the building and the increasedlocal pressures may be more than balanced by amomentary reduction in the pressure elsewhere.Because of the inertia of the building, short periodgusts may not cause any appreciable increase instress in main components of the buildingalthough the walls, roof sheeting and individualcladding units ( glass panels ) and their support-ing members such as purlins, sheeting rails andglazing bars may be mqre seriously affected.Gusts can also be extremely important for designof structures with high slenderness ratios.4.4 The liability of a building to high wind press-ures depends not only upon the geographicallocation and proximity of other obstructions toair flow but also upon the characteristics of thestructure itself.4.5 The effect of wind on the structure as a wholeis determined by the combined action of externaland internal pressures acting upon it. In all cases,the calculated wind loads act normal to thesurface to which they apply.4.6 The stability calculations as a whole shall bedone considering the combined effect, as well asseparate effects of imposed loads and wind loadson vertical surfaces, roofs and other part of thebuilding above general roof level.4.7 Buildings shall also be designed with dueattention to the effects of wind on the comfort ofpeople inside and outside the buildings.5. WIND SPEED AND PRESSURE5.1 Nature of Wind in Atmosphere - Ingeneral, wind speed in the atmospheric boundarylayer increases with height from zero at groundlevel to a maximum at a height called the gradi-ent height. There is usually a slight change indirection ( Ekman effect ) but this is ignored inthe code. The variation with height dependsprimarily on the terrain conditions. However, thewind speed at any height never remains constantand it has been found convenient to resolve itsinstantaneous magnitude into an average or meanvalue and a fluctuating component around this


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IS : 875 ( Part 3 ) - 1987average vaiue. The average value depends onthe averaging time employed in analysing themeteorological data and this averaging timevaries from a few seconds to several minutes. Themagnitude of fluctuating component of the windspeed which is called gust, depends on the aver-aging time. In general, smaller the averaginginterval, greater is the magnitude of the gustspeed.5.2 Basic Wind Speed - Figure 1 gives basicwind speed map of India, as applicable to 10 mheight above mean ground level for different zonesof the country. Basic wind speed is based on peakgust velocity averaged over a short time intervalof about 3 seconds and corresponds to meanheights above ground level in an open terrain( Category 2 ). Basic wind speeds presented inFig. 1 have been worked out for a 50 year returnperiod. Basic wind speed for some importantcities/towns is also given in Appendix A.,5.3 Design Wind Speed ( V, ) - The basicwind speed ( V, ) for any site shall be obtainedfrom Fig. 1 and shall be modified to include thefollowing effects to get design wind velocity atany height ( V, j for the chosen structure:

a) Risk level;b) Terrain roughness, height and size of struc-ture; andc) Local topography.It can be mathematically expressed as follows:

wherev, = vb kl k~ ks

V, = design wind speed at any heightz in m/s;kl = probability factor ( risk coeffi.cient ) ( see 5.3.1 );ks = terrain, height and structure sizefactor ( see 5.3.2 ); andks = topography factor ( see 5.3.3 ).

NOTE - Design wind speep up to IO m height frommean ground level shall be considered constant.5.3.1 Risk Coejicient ( kI Factor ) - Figure 1gives basic wind speeds for terrain Category 2 asapplicable at 10 m above ground level based on 50years mean return period. The suggested lifeperiod to be assumed in design and the corres-ponding kl factors for different class of structuresfor the purpose of design is given in Table 1. Inthe design of all buildings and structures, aregional basic wind speed having a mean returnperiod of 50 years shall be used except as specifi-ed in the note of Table 1.5.3.2 Terrain, Height and Structure Size Factor( k, Factor )

5.3.2.1 Terrain - Selection of terrain cate-gories shall be made with due regard to the effect

of obstructions which constitute the ground sur-face roughness. The terrain category used in thedesign of a structure may vary depending on thedirection of wind under consideration. Whereversufficient meteorological information is availableabout the nature of wind direction, the orientationof any building or structure may be suitablyplanned.

Terrain in which a specific structure standsshall be assessed as being one of the followingterrain categories:4

b)

Cl

Category 1 - Exposed open terrain withfew or no obstructions and in which theaverage height of any object surroundingthe structure is less than 1.5 m.NOTE - This category includes open sea-coastsand flat treeless plains.

Category 2 - Open terrain with well scatt-ered obstructions having heights generallybetween I.5 to 10 m.

NOTE - This is the criterion for measure-ment of regional basic wind speeds and includesairfields, open parklands and undeveloped spar-sely built-up outskirts of towns and suburbs. Openland adjacent to sea coast may also be classified asCategory 2 due to roughness of large sea waves athigh winds.CategoTy 3 - Terrain with numerous closelyspaced obstructions having the size ofbuilding-structures up to 10 m in heightwith or without a few isolated tall struc-tures.NOTE 1 - This category includes well woodedareas, and shrubs, towns and industrial areas fullor partially developed.

NOTE 2 - It is likely that the next highercategory than this will not exist in most designsituations and that selection of a more severecategory will be deliberate.NOTE 3 - Particular attention must be givento performance of obstructions in areas affected byfully developed tropical cyclones.Vegetation whichis likely to be blown down or defoliated cannot berelied upon to maintain Category 3 conditions.Where such situation may exist, either an inter-

mediate category with velocity multipliers midwaybetween the values for Category 2 and 3 given inTable 2, or Category 2 should be selected havingdue regard to local conditions.d) Category 4 - Terrain with numerous largehigh closely spaced obstructions.

NOTE - This category includes large city cen-tres, generally with obstructions above 25 m andwell developed industrial complexes.5.3.2.2 Variation of wind speed with height fordi@erent sizes of structures in different terrains ( k,factor ) - Table 2 gives multiplying factors ( lir )by which the basic wind speed given in Fig. 1shall be multiplied to obtain the wind speed atdifferent heights, in each terrain category fordifferent sizes of buildings/structures.


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IS : 875 ( Part 3 ) - 1387The buildings/structures are classified into thefollowing three different classes depending upontheir size:Class A - Structures and/or their componentssuch as cladding, glaxing, roofing, etc, havingmaximum dimension ( greatest horizontal orvertical dimension ) less than 20 m.Class B - Structures and/or their com-

ponents such as claddinp, glazing, roofing,etc, having maximum dimension ( greatesthorizontal or vertical dimension ) between 20and 50 m.c1a.U C - Structures and/or their componentssuch as cladding, glazing, roofing, etc, havingmaximum dimension ( greatest horizontal orvertical dimension ) greater than 50 m.

TABLE 1 RISK COEFFICIENTS.FOR DIFFERENT CLASSES OF STRUCTURES INDIFFERENT WIND SPEED ZONES( Clause 5.3.1 )

CLASS OF STRUCTWZE MEAN PROBABLE k, FACTOR BOB BASIC WIND SPEEDDESIGN LIFE OF (m/s ) 0~STRUCTURE IN r-------- ---_--__7YEARS 33 39 44 47 50 5550 -1.0 10 1.0 10 1.0 105 0.82 0.76 0.73 071 0.70 067

25 0.94 0.92 0.91 0.90 090 089

All general buildings and structuresTemporary sheds, structures such asthose used during construction

operations ( for example, form-work and falsework ), structuresduring construction stages andboundary wallsBuildings and structures presentinga low degree of hazard to life andproperty in the event of failure,such as isolated towers in woodedareas, farm buildings other thanresidential buildingsImportant buildings and structuressuch as hospitals communicationbuildings / towers, power plantstructures

100 105 I 06 10: 107 I 08 1.08

NOTE - The factor kt is based on statistical concepts which take account of the degree of reliability requiredand period of time in years during which these wil l be exposure to wind, that is, life of the structure. Whateverwind speed is adopted for design purposes, there is always a probability ( however small ) that it may be exceededin a storm of exceptional violence; the greater the period of years over which these will be exposure to the wind,the greater is the probability. Higher return periods ranging from 100 to 1 000 years ( implying lower risk level ) inassociation with greater periods of exposure may have to be selected for exceptionally important structures, suchas, nuclear power reactors and satellite communication towers. Equation given below may be used in such casesto estimate k, factors for different periods of exposure and chosen probability of exceedance ( risk level ). Theprobabil ity level of 063 is normally considered sufficient for design of buildings and structures against wind effectsand the values of k, corresponding to this risk level are given above.XN, Pkl = x5O, 0.63

*-L+*{-+ql-P$Jz---- A + 4Bwhere

N = mean probable design life of structure in years;PN - risk level in N consecutive years ( probabil ity that the design wind speed is exceeded at least once inN successive years ), nominal value = 063;

X N,P = extreme wind speed for given values of Nand PN; andx5O, 063 = extreme wind speed for N = 50 years and PN = 063.

A and B are coefficients having the following values for different basic wind speed zones:Zone A B

33 m/s 832 9239 m/s 840 14044 m/s 88,O 18047 m/s 88.0 20550 m/s 888 22855 m/s 90.8 27.3

11


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IS : 875 ( Part 3 ) - 1987

HEIGHT

m(1)IO:o3050100150200250300350400459500

TABLE 2 k, FACTORS TO OBTAIN DESIGN WIND SPEED VARIATION WITH HEIGHT INDIFFERENT TERRAINS FOR DIFFERENT CLASSES OF BUILDINGS/STRUCTURES( ClaUJ C.3.2.2 )

TEBRAIN CATEQORY 1CLASSI---_*--1A B c(2) (5) (4)105 103 0.991.09 107 103112 1.10 106115 113 1091-20 118 114126 124 120130 1.28 124132 130 126134 132 128135 134 130137 135 131138 136 1.32139 137 133140 1.38 134

TERRAIN CATEC+ORYCLbSSr---_h-_--~A B c

(5) (6) (7)100 0.98 0.93105 102 0.971.07 105112 110 ::z117 115 110124 122 1.17128 1.25 1.21130 128 124132 131 1261.34 1 32 1.28136 134 129137 135 130138 136 131139 137 132

TEERAIN CATEQO~Y 3CLASSc--_-~--_-~A B c

(8) (9) (10)091 088 082097 0% 087101 :% 091106 * 096112 109 1.02120 117 110124 121 115127 1.24 118x 126 1201.28 122132 130 124134 131 1.25135 132 1261~36 133 1.28

NOTE 1 - Se6 5.3.2.2 for definitions of Class A, Class B and Class C structures.NOTE 2 - Intermediate values may be obtainedconstant wind speed between 2 heights for simplicity.by linear interpolation, if desired,

TEP.BAIN CATECJ ORPCLASSt-_-*---~A B c(11) (12) (1310.80 0.76 0670.80 076 0.670.80 076 067OY7 093 083110 105 095120 115 105124 120 1.10127 122 113128 1.24 116130 126 I.171.31 1.27 1191.32 1.28 1201.33 129 121134 1.30 122

It is permissible to assume

5.3.2.3 Terrai n categori es in r elat ion to th e dir ec-t ion of w ind - The terrain category used in thedesign of a structure may vary depending on thedirection of wind under consideration. Wheresufficient meteorological information is available,the basic wind speed may be varied for specificwind direcion.

5.3.2.4 Changes in terra in categori es - Thevelocity profile for a given terrain category doesnot develop to full height immediately with thecommencement of that terrain category butdevelop gradually to height ( h, ) which increa-ses with the fetch or upwind distance (x).

a) Fetch and develobed height relationship - Therelation between the developed height (h,)and the fetch (x) for wind-flow over eachof the four terrain categories may be takenas given in Table 3.b) For structures of heights greater than thedeveloped height (h,) in Table 3, thevelocity profile may be determined in

accordance with the following:i) The les3 or least rough terrain, or

ii) The method described in Appendix B.5.3.3 Tojography ( ks Factor ) - The basicwind speed Vb given in Fig. 1 takes account ofthe general level of site above sea level. This doesnot allow for local topographic features such ashills, valleys, cliffs, escarpments, or ridges whichcan significantly affect wind speed in their vici-nity. The effect of topography is to accelerate windnear the summits of hills or crestsof cliffs, escarp-ments or ridges and decelerate the wind in valleys

or near the foot of cli%, steep escarpments, orridges.12

TABLE 3 FETCH AND DEVELOPED HEIGHTRELATIONSHIP( C1UUS6.3.2.4 )

FE?: (x) DEVELOPEDHEIGHT, hx IN METRES,--__--h_Terrain Terrain Terrain ----yCategory Terrain1 Category 2 Category 3 Category 4(1) (2) (3) (4) (5)02 12 20 35 6005 20 30 35 9.51 25 45 80 1302 35 65 110 1905 60 100 170 30010 80 140 25C 45020 120 200 350 50050 180 300 400 500

5.3.3.1 The effect of topography will besignificzt at a site when the upwind slope (6) isgreater than about 3, and below that, the valueof ks may be taken to be equal to 1-O. The valueof ks is confined in the range of 1-O to 1.36 forslopes greater than 3. A method of evaluating thevalue of ks for values greater than 1.0 is given inAppendix C. It may be noted that the value ofks varies with height above ground level, at amaximum near the ground, and reducing to 1.0at higher levels.5.4 Design Wind Pressure - The design windpressure at any height above mean ground levelshall be obtained by the following relationshipbetween wind pressure and wind velocity:

pz = 0.6 r-i


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IS : 875 ( Part 3 ) - 1987where

pz = design wind pressure in N/ms atheight z, andv, - design wind velocity in m/s atheight 2.

NOTE - The coefficient 06 (in SI units ) in theabove formula depends on a number of factors aodmainly on the atmospheric pressure and air tempera-ture. The value chosen corresponds to the averageappropriate Indian atmospheric conditions.

5.5 Off Shore Wind Velocity - Cyclonicstorms form far away from the sea coast andgradually reduce in speed as they approach thesea coast. Cyclonic storms generally extend up toabout 60 kilometres inland after striking the coast.Their effect on land is already reflected in basicwind speeds specified in Fig. 1. The influence ofwind speed off the coast up to a distance of about200 kilometres may be taken as 1.15 times thevalue on the nearest coast in the absence of anydefinite wind data.6. WIND PRESSURES AND FORCES ONBUILDINGS/STRUCTURES6.1 General - The wind load on a buildingshall be calculated for:

a) The building as a whole,b) Individual structural elements as roofs andwalls, andc) Individual cladding units including glazingand their fixings.

6.2 Pressure Coefficients - The pressurecoefficients are always given for a particular sur-face or part of the surface of a building. The windload acting normal to a surface is obtained bymultiplying the area of that surface or its appro-priate portion by the pressure coefficient (C,) andthe design wind pressure at the height of the sur-face from the ground. The average values of thesepressure coefficients for some building shapes aregiven in 6.2.2 and 6.2.3.Average values of pressure coefficients aregiven for critical wind directions in one or more

quadrants. In order to determine the maximumwind load on the building, the total load shouldbe calculated for each of the critical directionsshown from all quadrants. Where considerablevariation of pressure occurs over a surface, it hasbeen subdivided atid mean pressure coefficientsgiven for each of its several parts.In addition, areas of high local suction( negative pressure concentration ) frequentlyoccurring near the edges of walls and roofs areseparately shown. Coefficients for the local effectsshould only be used for calculation of forces onthese local areas affecting roof sheeting, glasspanels, individual cladding units including theirfixtures. They should not be used for calculatingforce on entire structural elements such as roof,walls or structure as a whole.

NOTE 1 - The coefficients given different tables have k!ey?%tained mainly from me;gurements on models in wind- tunnels, ahd the greatmajority C.of data available has been obtained in con-ditions of ielatively smooth flow. Where sufficient fielddata exists as in the case of rectangular buildings,values have been obtained to allow for turbulent flow.NOTE 2 - In recent years, wall glazing and clad-

ding design has been a source of major concern. Althoughof less consequence than the collapse of main struc-tures. damage to glass can be hazardous and causeconsiderable financial losses.NOTE3 - For pressure coefficients for structuresnot covered here, reference may be made to specialistliterature on the subject or advise may be sought fromspecialists in the subject.

6.2.1 Wi nd Load on Indi vi dual M embers - Whencalculating the wind load on individual strcuturalelements such as roofs and walls, and individualcladding units and their fittings, it is essential totake account of the pressure difference betweenopposite faces of such elements or units. For cladstructures, it is, therefore, necessary to know theinternal pressure as well as the external pressure.Then the wind load, F, acting in a directionnormal to the individual structural element orcladding unit is:

F=(G~---C~~)AP~wherec De = external pressure coefficient,c Di = internal pressurecoefficient,

A = surface area of structuralor cladding unit, andPd = design wind pressure.

element

NOTE 1 - If the surface design pressure varies withheight, the surface areas of the structural element maybe sub-divided so that the specified pressures are takenover appropriate areas.NOTE 2 - Positive wind load indicates the forceacting towards the structural element and negativeaway from it.

6.2.2 Ext ernal Pr essure Coefi cient s6.2.2.1 Wal l s - The average externalpressure coefficient for the walls of clad buildingsof rectangular plan shall be as given in Table 4.In addition, local pressure concentration coeffi-cients are also given.6.2.2.2 Pi tched rbofs of rectangular clad buil d-

ings - The average external pressure coefficientsand pressure concentration coeecients for pitchedroofs of rectangular clad building shall be asgiven in Table 5. Where no pressure concentrationcoefficients are given, the average coefficientsshall apply. The pressure coefficients on the under -side of any overhanging roof shall be taken inaccordance with 6.2.2.7.NOTE 1 - The pressure concentration shall beassumed to act outward ( suction pressure ) at the

ridges, eaves, cornices and 90 degree corners of roofs( see 6.2.2.7 ).NOTE 2 - The pressure concentration shall not beincluded with the net external pressure when comput-ing overall loads.


15/63

IS : 875 ( Part 3 ) - 1987TABLE 4 EXTERNAL PRESSURE COEFFICIENTS ( Cpe ) FOR WALLS OF RECTANGULARCLAD BUILDINGS

( clause 6.2.2.1 ) -I-

_-

WINDANGLE0

BUILDINUHEIGHTRATIO

ELEVATION PLAN Cpe FOR SURFACE / LOCAL CpeBUILDINGPLANRATIO

-A B

--D

--

+0.7-0.5

-0.2-05

-05i-0.7

-05-02

-I-+0.7 -0.25 -0.6 -06-05 -0.5 +0.7 -01

-i_- -- --+0.7-06

-02 -06-06 +0*7

-0.6-02

_j.- 0.3-0.5

_-_

.-

--02 -0.8-0.8 +08

.-I--

--

--

-

I

- .

degrees0

30

030

090

090

0

90

c3a 78 1A -iD

-0.7-0.1

Ic -08,I

1:

-1.0

-l> -1'1J

-I} -11J

+


16/63

TABLE 5 EXTERNAL PRESSURE COEFFICIE NTS ( cp, ) FOR PITCHED ROOFS OF RECTANGULAR CLAD BUIL DINGS( Clause 6.2.2.2 )

ik;Il>lD1N0 RlX!FHEIGHT AKaLERATIO CL

k---W -_1n- 0 -08 -06 -10 -06 -20-11-0907 -0.6-050.6 -08-0.908 -0606-06 - 205-20 -2.0-2015 -15 -12-15 -I.010I 30_ I -02 -0.5 I -08 -0.a I -I0 l_pp___m / -_ / -10

WIND ANGLE 8 WIND ANQLE O0 900

EF GH EG FH

- 08 -04 -08 -0.4-09 -04 - 08 -04-12 -0.4 -08 -06-04 -04 -07 -060 -0.4 -0.7 -0.6+0*3 -0.5 -07 -06+0*7 -0.6 -07 -0.6I

-20- 14-14- 10-08

LOCAL COEFFICIENTS

--10-12- 12- 11-11- 11

+o 2 -05 -0.8 -08 -+06 -05 -08 -08-- - .-

0 I -.0.7 -06 -0.9 -0.7 -9.n -3.n -9.n_. , h ,. I IL -08 -0.6 -08 -08r5;;


17/63


18/63

18:875(Part3)-1987

TABLE 6 EXTERNAL PRESSURE COEFFICIENTS ( C,, ) FOR MONOSLOPE ROOFS FORRECTANGULAR CLAD BUIILDINGS WITH $ < 2

( Clause 6.2.2.3 )

y = h or 015 W, whicheveris the lesser.

NOTE - Area Hand area L refer to the whole quadrant.

ROOFAIGQLEOL 0 45O

WIND ANQLE 13

90 135O 180

LOCAL Cpe

Degree H L H L H&LH&L H L H L Hi Hs Lz Ls He Leem*3%%g %$.I& o, .L .5!a -z E;a%*


19/63

IS : 875 ( Part 3 ) - 1987

TABLE 7 PRESSURE COEFFICIENTS FOR MDNOSLOPE FREE RQOFS( Clause 6.2.2.4 )

h1Rooy ANGLE( DECUUUES SOLIDITY RATIO

MAXINUY ( LARQEST + VE ) AKD MINI MTJ IU LARGEST - VE ) PRESSURECOEFFICIENTSOverall Local CoefficientsCoefficients

-05

10152025300

5

10

15

20

25

All values ofd

-d=O4- l4- O4- l4=04=1

4- o4- Ib-04- l4- o4- l

1 BzzzB N+0-z +0*5 +1*8 +1-l+0*4 +08 +2-l + 3+0*5 +1*2 +24 + 6+0*7 + 14 +27 +18-l-O8 +1*7 +2*9 +2*1+1-o +2-o +3*1 +23+1-z f22 +32 +24-.-05 -06 -13 - 14-10 -12 - 18 -19-0.7 - 1.1 -17 1.8-11 -1.6 -2.2 -23-0.9 -15 -2.0 -2.1-13 -21 -2.6 -2.7-1.1 -18 -24 -25-14 -23 -2.9 -30-1.3 -22 -28 -29-1.5 -26 -31 -32-1.6 -26 -62 -32-17 -28 -3.5 -35

30 4- o -18 -3.0 -3.8 -364=1 - 18 -30 -38 -3.6

NOTE - For monopitch canopies the centre of pressure should be taken to act at 03 UJ from the windwardedge.

19


20/63

KS : 875 ( Part 3 ) - 1987TABLE 8 PRESSURE COEFFICIENTS FOR FBEE STANDING DOUBLE SLOPED ROOFS

( Clause 6.2.2.4 )-c,

-CP .-Cn F 10I

I--ve ROCF ANGLE +ve ROOF ANGLE

h

-IRoos Xsa~n 11 SOLIDITY MAXIMOX ( LAB~EST+VE ) AYD MI NI~X ( LARGE ST - VE ) Pn~aacnn: DEc;lIEZ% ) RATI O CO~FFI~~~NTS

! 1 Overall Local Coefficients! Coefficients

I / liz%@zl--Cl +0*7+0.5 -i-O% + 6 / +06 +1*7-15 +06 +1.5 + 07 + 4- 10 $-O-4 +06 + 4 +08 + 1-5 +03 +1*5 i-0.8 +087-5 +0.3 :x,.j Ail values of +0.4 + 18 +1*3 +04f 10+15 ! +0*7 + 8 +1*4 +0*4i20 I 9 +0*4+06 +0.9 +1.9 +14 +0*4! :x:;

+1*1 +1*9 +1*5 +0.43:3; + 2 +1*9

f16 -!-05+ 3 +1*9 +16 +0*7/I -0.7 --20 I$=0 -0.9-09 -13 -16 -06+=1 - 12 -17 -19 -_12o-0 -06 I--:5 / -08 -13 -164-l -0.8 -06-1.1 -17 -19 - 12

-10 I _,y * -06 1 -08-0.8 j_ -1.3 -1.5 -0.6-11 -17 -19 -1:3$10 ! -0.5-5 -07 -13 -1.6: -08 ; -0.6-15 -17 -19 -14/ -I+5 K:, -06 / -0.6 -1.4 1 -14 -11-09 -13 -18 -1.8 -21

+ 10 f=-0.7 -07 1 -15 Al.4 -1.4/ = _ -ll 1 -1.4 -20 -1.8 -24/+ 15 / = ;f=Y -08 j -0.9 - 17 -14 -18-12 - 15 -22 -1.9 -2%

i20 $I:, -09 -1 -18 - 1.4 -20-13 / -1.7 -23 -1.9 -301 Ii-25 $w& 1 -1.0 1 -1.4 i -19 - 14 -20I -14 I -!9 -24 -21 -30_L___ -_ ; ---.-_

i30 $1; -10 -1-4 -14 -20- 14 -21 1 I::?_b -22 -3.0Each slope of a duopitch canopy should he able to withstand forces using both the maximum and the mmimurnoefficients, and the whole canopy should be able to support forces using one slope at the maximum coefficient with theIther slope at the minimum coeffictent. For duopitch canopies the cenrre of pressure should be taken to act at the centrePeach slope.

20


21/63


22/63

YS : 875 ( Pars 3 ) - Y987

TABLE 9 PRESSURE COEFFICIENTS ( TOP AND BOTTOM ) FOR PXTCHED ROOFS, a +e 3tP( &uw 6.2.2.4 )

--9

045O9o

_-A-4590

7

-Ij_II-

-Ti 1 TI

1 E 1i

L; G zI__ ---- _____: JI cRoof sIope a 0 30e - 0 - 450, D, D, E, E :x1:length9 = 90, D, D, E, E pr r !engzhb, thereafter Cp = 0

D 1 DIII06 ! -10

0.1 ; -0.3-03 j -C4

, ----I1 1 End SurfacesE ) E j c j c / c;I /j I --05 / -0.3 1!

-06 / i I-0.3 /10.3 1 -0.4 I -0*3 / 0.8 /: I 03

-7I

.j_G

IForj : Cp top = -iO; Cp bottom = -0.2Tangentially acting friction: ROOo ip 005 pdbd


23/63

IS I 875 ( Part 3 ) - 1387

i

.-

TABLE 10 PRESSURE COEFFI CIENTS ( TOP AND BOTTOM ) FOR PITCHED FREE ROOFS,a = 300 WITH EFFECTS OF TRAIN OR STORED MAIBRIAL S( Clause 6.2.2.4 )

!,b:5C

III!

-

.I-._G__cE

- --_

&d --I

Roof slope LY= 300Efftctz of trains or storedmaterials:0 a 0 -45, or 135 -180,D, D. E, F full lqngth6 - ;;,.$, D , E, E par t& = 0 b, thereafter

PRESSURE COEFFI CIENTS,cpcl / End Surfaces

D D E E c c G G

0 01 08 -07 0945O -01 05 -08 0590 -04 -05 -04 -05 -03 08 03 -04

180 -03 -06 04 -06-

45 Forj : Cp top = - 15; C, bottom Q 05go0 Tangentially acting fri ction: &a = 005 pdbd

22


24/63

IS I 875 ( Part 3 ) - 1987

TABLE 11 PRESSURE COEFFICIENTS (TOPANDBOTTOM)FORPlTCHEDF~~ BOOFS,am 10"( lause.2.2.4)

b=Sd

fb=d

1

Roof slope (L = IO8 = 0 - 45, D, D, E, E full length0 = 90, D, D, E, E par1 length b,thereafter Cp = 0

PRESSURECOEFFICIENTS, CD

e End SurfacesD D E E

c I C G 1 G-- -~ .00 -1.0 03 -0.5 0.2 ,45" -0'3 0.1 -0'3 0190 -0.3 0 -0.3 0 -0'4 0.8 09 -0.6

-0" Forf: Cp top = -110; Cp bottom = 04

0 - 90 Tangentially acting friction, RIO = O1pdbd


25/63

TABU I2 PRESSURE COEFFICIFiNTS (*OP AND BOTTOM ) FOR PITCRBD FBE ROOFSir - 10 WITH EFFECTS OF TRAIN OR STORED MATJZItIAL8( CIaw 6.2.2.4 )

ii

!

-Th=O$th_A_

Roof slope m - IO0EAacts of trains or stored materials:e-o.=- 45,or 135 - 180,D, D , E, E full length0 = 90*, D, D, E, E part length b ,thereafter CD = 0

Gi!1I/1 -04 0.8 03!I

G

-0%iI I

i

0 ForJ: I;, top = -15; Cp bottom = 0.90 - I!$ / Tangentially acting friction: R,o = 0.1 p& j

24


26/63

1sr875(Part3)-1987

TABLE 13 EXTERNAL PRESSURE COEFTZCXENTS FOR TROUGHED FRER ROOPS, a = IO( Clause 6.2.2.4 )

Roof slope a - 109 = 0 -45, D. D, E. E fulliengthA = 90*, D,_ D, E, E Fatt lengthb, thereafter Cp I 9

04Y90

ID D 1 E, / E I/

03 -07 I 02 ! -09I0 -02 , 01 j -03/

-01 0.1 -01 I 010 Forf : CD top = 04; Cp bottom = - i-1

0 -90 Tangentially acting friction Rgo = Gi &bi

P&EssUnE cOEFFICIEK?K3, cp


27/63

ISr875( Part3)-1987

TAtWE 14 PRESSURE COEFFICIENTS ( TOP AND BOTTOM ) FOR TROUGHED FREE ROOFS,a = IO WITH EFFECTS OF TRAINS OR STORED MATERIALS( Clause6.2.2.4 )

b= 5d

E

TfLmi------i Roof slope (I = 10Effects of trains or storedmaterials:13= 0 - 450, or 135 - 180,D, D, E , E full length13 = go, D, D, E, E , partlength b thereafter Cp = 0e

0045O90

180

0O- 180

D

-07-04-0.1-04

PRESSURE COEFFICIENTS, Cp

D E E

08 -06 0603 -02 0201 -01 01

-0.2 -0.6 - 03

Forf: Cp top = - 11; CD bottom = 09Tangentially acting friction: &,o = 01 pabd

26


28/63

6.2.2.5 Curved roofs - For curved roofs, theexternal pressure coefficients shall be as given inTable 15. Allowance for local effects shall be-made in accordance with Table 5.6.2.2.6 Pitched and saw-tooth roofs of multi-span buildings - For pitched and saw-toothroofs of multi-span buildings, the external average

pressure coefficients and pressure concentrationcoefficients shall be. as given in Tables 16 and 17respectively. provided that all spans shall be equaland the height to the eaves shall not exceed thespan.NOTE- Evidence on multi-span buildings isfragmentary; any departure given in Tables 16 and 17should be investigated separately.

6.2.2.7 Pressure coeficients on overhangs fromroofs - The pressure coefficients on the top over-hanging portion of the roofs shall be taken to bethe same as that of the nearest top portion of thenon-overhanging portion of the roofs. The pressurecoefficients for the underside surface of the over-hanging portions shall be taken as follows andshall be taken as positive if the overhangingportion is on the windward side:

a) 1.25 if the overhanging slopes,b) 1.00 if the overhanging isShorizontal, andc) 0.75 if the overhanging slopes upwards.For overhanging portions on sides other thanthe windward side, the average pressure coeffi-cients on adjoining walls may be used.

6.2.2.8 Cylindrical structures - For the pur-pose of calculating the wind pressure distributionaround a cylindrical structure of circular cross-section, the value of external pressure coefficientsgiven in Table 18 may be used provided that theReynolds number is greater than 10 000. Theymay be used for wind blowing normal to the axisof cylinders having axis normal to the groundplane ( that is, chimneys and silos ) and cylindershaving their axis parallel to the ground plane( that is, horizontal tanks ) provided that theclearance between the tank and the ground is notless than the diameter of the cylinder.h is height of a vertical cylinder or length of ahorizontal cylinder. Where there is a free flow ofair around both ends, h is to be taken as half thelength when calculating h/D ratio.In the calculation of the resultant load on theperiphery of the cylinder, the value of C,t shallbe taken into account. For open ended cylinders,C,i shall be taken as follows:a) 0.8 where h/D is not less than 0.3, andb) 0.5 where h/D is less than 0.3.

6.2.2.9 Roofs and bottoms of cylindrical elevatedstructures - The external pressure coefficients forroofs and bottoms of cylindrical elevated structuresshall be as given in Table 19 ( see also Fig. 2 ).

IS : 875 ( Part 3 ) - 1987The total resultant load (P) acting on the roofof the structure is given by the following formula:

P = 0.785 D ( _!q - C,, pa)The resultant of Pfor roofs lies at 0.1 D fromthe centre of the roof on the windword side.

6.2.2.10 Combined roofs and roofs with a skylight - The average external pressure coefficientsfor combined roofs and roofs with a sky light isshown in Table 20.6.2.2.11 Grandstands - The pressure coeffi-cients on the roof ( top and bottom ) and rearwall of a typical grandstand roof which is openon three sides is given in Table 21. The pressurecoefficients are valid for a particular ratio ofdimensions as specified in Table 21 but may beused for deviations up to 20 percent. In general,the maximum wind load occurs when the wind is

blowing into the open front of the stand, causingpositive pressure under the roof and negativepressure on the roof.6.2.2.12 Upper surface of round silos andtanks - The pressure coefficients on the uppersurface of round silos and tanks standing onground shall be as given in Fig. 2.6.2.2.13 Spheres - The. external pressurecoefficients for spheres shall be as given inTable 22.

6.2.3 Internal Pressure Coejicients - Internal airpressure in a building depends upon the degreeof permeability of cladding to the flow of air.The internal air pressure may be positive ornegative depending on the direction of flow ofair in relation to openings in the buildings.

6.2.3.1 In the case of buildings where thecladdings permit the flow of air with openings notmore than about 5 percent of the wall area butwhere there are no large openings, it is necessaryto consider the possibility of the internal pressurebeing positive or negative. Two design conditionsshall be examined, one with an internal pressurecoefficient of +0.2 and another with an internalpressure coefficient of -0.2.

The internal pressure coefficient is algebrai-cally added to the external pressure coefficientand the analysis which indicates greater distress ofthe member shall be adopted. In most situationsa simple inspection of the sign of external pressurewill at once indicate the proper sign of the inter-nal pressure coefficient to be taken for design.NOTE - The term normal permeability relates t*the flow of air commonly aftorded by claddings notonly through open windows and doors, but also through

the slits round the closed wincows 2nd doors and thro-ugh chimneys, ventilators and through the joints bet-ween roof coverings, the total open area being less than5 percent of area of the walls having the openings.


29/63

TABLE 15 EXTERNAL PRESSURE COEFFICIENTS FOR CURVED ROOFS( Clause 6.2.2.5 )

l------~-----la) Roof springing from ground level

b) Roof on elevated structure

-0.6

Values of C, Cl and C2

-CL_0'1 -08 +0*1_-0.2 -09 +0*3___-03 -1.0 +0*4p_--04 -11 +06-- -~0.5 -12 +0.7

c2

-08-0.7

.~ -0.3jp +0*4

i-o.7

NOTE - fihen the wind is blowingnormal to gable ends, Cpe may betaken as equal to -0.7 for the fullwidth of the roof.over a length of l/2from the gable ends and -0.5 for theremaining portion.

rCENTRAL HALF (Cl

fiGkIfCiN OF ROOF EELTHIS LINE TO BETREAIED AS ANEXTENSION ofVERTICAL SUPPORTS

c) Doubly curved roofs7 0 0.6-

GUARTE4 i R


30/63

ISr875(Part3)-19a7

TABLE 16 EXTERNAL PRESSURE COEFFICIENTS ( C b iOR PlTCHED ROeFS OPMULTISPAN BUILDINGS (ALL SPANS EQ&lp, WITH h > w( Ckusc 6.2.2.6 )

I w J_ w J_ w _1_ w _I_ w I w* 1I- -l- I-ROOF PLAN y=h or 0-1~WHICHEVER IS LESSh,= h,=h

iISECTION

ROOF WIND FIRST SPAN FIRST OTHER END SPANANRLE ANQLE INT~YIcDIATE INT~R~~EDIATESPAN SPAN-- -- c----ta e --7 C d m n x 2

degrees degrees5 0 -09 -0.6 -04 -03 -03 -03 -0.3 -03 I

10 -11 -0.6 -04 -03 -03 -0.3 -03 -04 I

20 -07 -06 -04 -03 -03 -03 -0.3 -0.3 \30 -0.2 -06 -0.4 -03 -0.2 -03 -02 -05 )45 +0*3 -0.6 -0.6 -04 -02 -0.4 -02 -0.5 J

Distancer---- -- h-P---- __-Roof Wind hx ha h3Angle Angled;reea 8degreesup to 45 90 -08 -06 -02

LOCAL ~RFPIOUNT

-20 -15

Frictional drag: When wind angle 0 - O,horizontal forcesdue to frictional drag are allowed for in the aboyevalues; andwhen wind angle 0 = 90, allow for frictional drag in accordance with 6.3.1.

NOTE - Evidence on these buildings is fragmentary and any departure from the casu given should bainvestigated reparately.


31/63

IS : 875 ( Part 3 ) - 1987

TABLE 17 EXTERNAL PRESSURE COEFFICIENTS C,e FOR SAW-TOOTH ROOFS OF MUL TI-SPAN BUILDINGS (ALL SPANS EQUAL ) WITH h > w( Clause 6.2.2.6 )

ROOF PLAN Y =hor 01 UI which-ever is the lesshl=hB = h

SECTION

WIND FIRST SPAN FIRST OTHER END SPANS LOCAL COEFFICIENTANC+LE INTER~~~EDIATE INTERMEDIATEe SPAN SPANSc----Y r--hw-y r---h_-~ C--h--7a b c d m R x t

degrees0 +06 -0.7 -07 -0.4 -0.3 -02 -0.1 -03 1 -15180 -05 -0.3 -0.3 -0.3 -0.4 -0.6 J -20-06 -01

DISTANCEc------------ -+.L----_-----~WIND h ha haANGLE 0degrees

90 -0.8 -0% -02210 Similarly, but handed

Frictional drag: When wind angle 0 = O, horizontal forces due to frictional dragvalues; and are allowed for in the abovewhen wind angle 8 I 90, allow for frictional drag in accordance with 6.3.1.

NOTE -separately. Evidence on these buildings is fragmentary and any departures from the cases given should be investigated

30


32/63

18:875(P8rt3)-1987

TABLE I8 EXTERNAL PRESSURE DISTRIBUTION COEFPICIENTS AROuN6 CTLiNDkWWLsTRucTURm3 ( CIaucs6.2.2.8 )

POSITION OFPEBIPHERY, 0IX DEQREEB

PRESSUI~E COEFFICIENT,Cm

h/D = 25 h/D = 7

0 10 1.015 O-8 0830 0.1 0145 -09 -0860 -19 -1775 -25 -2.290 -26 -22

105 - 1.9 -17120 -09 -08135 -0.7 -0.6150 -06 -0.5165 -06 -05180 -0.6 -0.5

--I h/D = 1

100801

-07-1;2- 1.6-17-1.2-0.7-0.5-04-04-04 I--


33/63

IS -I 875 ( Part 3 ) - 1987

T-LB 19 =TBRNAL PRESSURE COE@FICIENTS FOR ROOFS AND BOTTOMS OFCYLINDRICAL BUILDINGS( Clause6.2.2.9 )

POIREC?TIONOf WIN0

(bl

(cl

COS~FICIE~ OF EXTERXAL PREBSURE, CpsSTRUCTURE ACCOBDIITG O SEAPE

a,budc dHID Roof (z/H) -1 Roof05 -0.65 100 -075

130 -100 125 -075

I 2.00 - 100 150 -075Total force acting on the roof of the structure, P 1 0785 Da ( pi - CpePd )The resultant of P lier ecceotricdly, # a OID

_-

Bottom

-08

-0.7

-0.6

32


34/63

IS:875(Part5)-1987TABLE 28 EXTERNAL PRESSURE COEFFICIENTS, Cw FOR COMBINED ROOFS AND ROOFSWITH A SKY LIGHT

( Clause 6.2.2.10 )

a) Combined Roofs

-0. 8

POETION DIRECTION 1 DIRECTION 2

a From the Diagram

b

I candd

VALUE0 OP cpe

Cpe = -05, - < 15hr -04

Cpe = -07, _!!!_ > I.5he

See Table 5

see 6.2.2.7( Co n f i n u r d

33


35/63

IS : 875 ( Part 3 ) - 1987

TABLE 20 EXTERNAL PRESSURE COEFFICIENTS, -Cpe FOR COMBINED ROOFS AND ROOFSWITH A SKY LIGHT - Contd

b) .Roofs with a Sky Light

WIN0

PORTION

Ge I

b; ; b, bl < bs0 b a and b

----0.6 $07 See Table for combined roofs

34


36/63

IS t 875 ( Part 3 ) - 1987TABLE 21 PRESSURE COEFF ICIENTS AT TOP AND BOTTOM ROOF OF GRAND STANDSOPEN THREE SIDES ( ROOF ANGLE UP TO 5 )

( Clause 6.2.2.11 )( A b : = 0.8 : 1 : 22 )

G 1I0H

i-----b4( Shaded area to scale )

10

0*

45O

135

-

/ 180-0.6

KM777

7Mw

FRONT AND BACK OF WALL

8 3 x L---

0* -l-O9 -0.5 +0.9-45 +0.8 -06 +0*4

135O - 11 +06 - 1.0-_

180~ -0.3 co.9 -03-

60 Mw - CpofK = -1060 Mw - c, Of.3 = + 10

--M

-0.5

-04

+0*4

+0.9

TOP AND BOTTOM OF ROOF

B c D E-- -.-

+0*9 -1.0 +0.9 -0.7

$07 -07 -CO4 -0.5

-1.1 -07 -10 -0.9N_

-03 -0.6 -0.3 -06

+09~-+08

CO7 f09---05 f03

-0.9 -10--.-

-06 -03

45O

45

MR - cp ( top ) = -2.0-

MB - Cp ( bottom ) = + 10

35


37/63

I8 : 875 ( Part 3 ) - 1987

FIQ. 2

T-.5 j.0 a 0.5h

0.20


38/63

iS I 875 ( Part 3 ) - 1987

TARLE !Z2 =TRRNAL PRESSURE DISTRIRUTION COEFFICIENTS AROdNDSPHERICAL STRUCTURES( Chse 6.2.2.13 )

1-

0 4-1' 015 +0.930 -to*545 -0160 -0.775 --I190 - 1.2

105 - 10120 -0.6135 -0.2150 +0*1165 +0*3180 +0*4

-REMAIIKS

Ct = 0.5 for Dl;d < 7= 0.2 for DVa > 7

6.3.1 Frictional Drag - In certain buildings ofspecial shape, a force due to .frictional drag shallbe taken into account in addition to those loadsspecified in 6.2. For rectangular clad buildings,this addition is necessary only where the ratio

C, - 0.02 for surfaces with corrugationsacross the wind direction, andCf = 0.04 for surfaces with ribs across thewind direction.

d d- orh F is greater than 4. The frictional drag For other buildings, the frictional drag hasforce, F , in the direction of the wind is given by been indicated, where necessary, in the tables ofthe following formulae: pressure coefficients and force coefficients.Ifh< b,F=C,(d-4h)b@,

s Cr ( d - 4h ) 2 h i&, and 6.3.2 Force Corfi cients or Ciad Bui ldingsif A > b, F - ;& -j 4b ) bjd

- 4b ) 2 h& . 6.3.2.1 Clad buildings of uni form section -The first term in each case gives the drag on The overall force coefficients for rectangular cladburldthe roof and the second on the walls. The value mgs of uniform section with Aat roofs inof Cr has the following values: uniform flow shall be as given in Fig. 4 and forother clad buildings of uniform section ( withoutC, - 0.01 for smooth surfaces without corru- projections, except-where otherwise sho& ) shallgations or ribs across the wind direction, be as given in Table 23.

37


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IS : 875 ( Part 3 ) - 1987

(C) For F = I, use average valuesArrows indicate d irection of wind.

FIG. 3 LARGE OPENINQ IN-BUILDINGS VALUESOF COEFFICIENTS F INTERNAL PRESXJRE )WITM TOP CLOSED6.3.2.2 Buil dings of circular shajcs - Force surface varying linearly from a maximum of l-7coefficients for buildings circular cross-section Cr at the up wind edge to 044 Ci at the downshapes shall be as given in Table 23. However,more precise estimation of force coefficients for wind edge.circular shapes of infinite length can be obtained The wind load on appurtenances and supportsfrom Fig. 5 taking into account the average for hoardings shall be accounted for separately byheight of surface roughness E. When the length using the appropriate net pressure coefficients.is finite, the values obtained from Fig, 5 shall be Allowance shall be made for shielding effects ofreduced by the multiplication factor K ( see also one element or another.

Table 25 and Appendix D ). 6.3.2.4 Soli d circul ar shajes mount ed on a6.3.2.3 Lox w all s and hoardings - Force surface - The force coefficients for solid circularcoefficients for low walls and hoardings less than shapes mounted on a surface shall be as given in15 m high shall be as given in Table 21 provided Fig. 6.the height shall be measured from the ground to 6.3.3 Force Coeji cients or Unclad Bui ldi ngsthe top of the walls or hoarding, and providedthat for walls or hoardings above ground the 6.3.3.1 General - This section applies to.clearance between the wall or hoarding and the permanently unclad buildings and to frameworksground shall be not less than 0.25 times the verti- of buildings while temporarily unclad. In the casecal dimension of the wall or hoarding. of buildings whose surfaces are well rounded, suchas those with elliptic, circular or ovoid cross-To allow for oblique winds, the design shall sections, the total force can be more at windalso be checked for net pressure normal to the speeds much less than the maximum due to38


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ztransition in the nature of boundary layer OII them.Although this phenomenon is well known in thecase of circular cylinders, the same phenomenonexists in the case of many other well-rounded:structures, and this possibility must be checked.6.3.3.2 Ind iv idual members

a) The coefficients refer to the members ofinfinite length. For members of finite length,the coefficients should be multiplied by afactor K that depends on the ratio I / bwhere 1 is the length of the member and5 is the width across the direction or wind.Table 25 gives the required values of K.The foliowing special cases must be notedwhile estimating K.

b)

i) Where any member abuts onto a plate orwall in such a way that free flow of airaround that end of the member is pre-vented, then the ratio of l / b shall bedoubled fat the purpose of determiningK; and

ii) When both ends of a member are so

c)

d)

tc

h-_=ab\\I 701 i I I

IS : 875 ( Part 3 ) - 1987obstructed, the ratio l / b shall be taken asinfinity for the purpose of determining K_

Fl at-sided members - Force coefficients forwind normal to the longitudinal axis offlat-sided structural members shall be asgiven in Table 26.The force coeficients are given for twomutually perpendicular directions relativeto a reference axis on the structural mem-ber. They are designated as CI, and Cft,give the forces normal and transverse,respectively to the relerence plane as shownin Table 26.Normal force, F, = C,, pd A1 bTransverse force, Ft = Cft pa K 1 bCi rcu la r secti ons - Force coefficients formembers of circular section shall be asgiven in Table 23 ( seealso Appendix D ).Force coefficients for wires and cables shallbe as given in Table 27 according to thediamater (D), the design wind speed ( fti)and the surface roughness.

a

a/b -4A Values of Cr versus -I for $ 2 1

4B Values of Cc versus -: for -a < 1FI~J. 4 FORCE COEFFICIENTB OR RECTANGULAR CLAC BUILDINGS N UNIPBRM FLO~V


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IS:873(Part3)-1987TABLE 23 FORCE COEFFI CIENTS Cf FOR CLAD BUIL DINGS OF UNIF ORM SECTION( ACTING IN THE DIRECTION OF WIND )

[ Clauses 6.3.2.1,6.3.2.2 and 6.3.3.2(c) ]-i-1,!-;1Ij.1

- i-,I1,.j-_I--

Cr POX HEIOET/BEEADTH RATJO

._

- T- 1.IIIII!

r_I. !I

/-1

3pro1/2j 1 5 10 f 20!,- -II

I.I.!Ij-

2

07

oa

12

06

i-- I08 09

c-5 j 0.6 i 0.6-I--

All surfaces 6

I0'7 0-i

Ij-o-5 , 0.5 05--

05

See aim Apppendix c Snzooth >6 _I10---

> 10

05 I 05 0' 7- -

0.2

17

15

I02 O- 2

10

10

-!Ellipseb/d - 2

8

0.8 / 08_-

08 u-8

o-9 1-l i i.309 I 11 1.3

-0

(4r b/d = 1r/b i= l/3

34

-- --_/___0.8 ) 08

--O-5 05

--10

i Ia0--06 / 0%

066 06-_

0.4 0.4--

0.7 0:8--

0.5 05--

07

04

0.9

0.5

@3

0.2

- _

)_04 0.5

13b/d = 1r\e - lJ 6

< 10> 10

08

05 G.6

04-1.

s

03 I 0.3-- i0.2 , 0.2

03 i 0.3.30.2

05

1.0 ;

I

--

05 05/d = l/2r/b = l/6 Allvalues 0.5 0.6 06 07

-]-t d

-ni b/d - 2 Al l

d ri b = l /12 values 0.9 o-9 11II!-( Chlintrcd )

40


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IS t 875 ( Part 3 ) - 1987TABLE 23 FORCE COEFFICIENTS Ci FOR CLAD BUILDINGS OF UNIFORM SECTION( ACTING IN THE DIRECTION OF WIND) - Contd

P~ax SRAPE Vdb

m2;s

Cf FOR HEIGHT/BREADTH RATIO !--I10 20 ICC /

-

. -I/.I_

2 5to 1;2

0.7

1

08 0.8 0.9_-0.5 05 0.5

10

_/--.-J-_-____, I6

I !I10 I 12 1 16

__...+__/-I- _

.I-

. _

_

I

05 O-6 j 06I

0.5

11

.I-_-

I

_-

-

_ -

-I

-I-

-i-,-

.-

- _

--

_ _

-/-

-

-

_-

_-

_-

__/_

_

-

- -

_-/--I-0 r (10u r/a=113 _va

710

08 0.8 139

0.5

09

0.9

15

06.5 1--

0.9

05 0' 6.5

0.9--

0.9

0.5

1.10 All-~ r/a = l/12 values 12 13 16-09 1.1 12 :3

10

16

12---

0.5

(11r/b = l/4 ~

711

097 O-7 0.8

04 0.4 04

0.8 08

0.5

--

12

04

10

_-

08 09

O-5

11

--

1.0

-I08 1.4

-_

0.7 11 1.3.7

0.7

04

--

0.7

--

0.8 0.9 10 I.1-.__

0.5

13

04-/- _I-------

0.4 1 05- I-

0.4 0.5


43/63

IS : 875 ( Part 3 ) - R987

TABLE 23 FORCE COEFFICIENTS cf FOB CLAD BUILDINGS OF UNIFORM SECTION( ACTING IN THE DIRECTION OF WIND ) - Contd

P&AN SHAPE Vd Cr FOR HEI ~ET/BREADTH RATIO

up to l/2 I 2 5 10msls I

20 cc_----- I----

- D 4:z~ llvalues 1.2 1.2 1.2 14 16--0.7-cl


44/63

18:875(Part3)-1987

@6

014l6 2 3 L 5 6 8 106- -2 3 L 5 6 8 107 2 3 L56 81

Fro. 5 VARIATION OF Cf WITH R, ( 3 x 10 ) FOR CIRCULAR SECTIONS

TABLE 24 FORCE COEFFICIENTS FOR LOW WALLS OR HOARDINGS ( < 15m HIGH )( Clause 6.3.2.3 )

t - - blI I

ABOVE GROUND h>,O-25h ONE EDGE ON GRUUNDWind normal to face

WIDTH TO HEIGHT RATIO, b/h

Wall Above Ground Wall on Ground

From 05 to 6 From 1 to 12 l-210 20 1316 32 1420 40 l-540 80 1.7560 120 1880 or more 160 or more 20

-

1

-

DRAG COEFFICIENT, Cf

43


45/63

IS : 875 ( Part 3 ) - 1987

SIOE ELEVATION DESCRIPTION OF SHAPE

CIRCULAR OISC

HEMISPHERICALBOWL

HEMISPHERICALBOWL

HEMISPHERICALSOLID

SPHERICALSOLID

06 FOR V,,O


46/63


47/63

D I 875 ( Part 3 ) - 1987

TABLE 27 FORCE COEFF ICI ENTS FORWIRES AND CABLES ( I/D = 100 )[ Clause 6.3.3.2(d) ]

FLOW REW.IE FORCE COEFFICIENT, Cr FOR~_--_-~-~--_---~Smooth Moder- Fine ThickSurface ately Stranded StrandedSmooth Cables CablesWire(Galvani-

cf sub a

c t iilbt =

A clrc sub -ht =

force coefficient for subcritica)circular members as given in.Table 28 or Appendix D,force coefficient for the flatsided members as given inTable 28,

zed orPainted)(1) (2) (3) (4) (5)DVa < 06 me/s - - 1.2 1.3

QVa 2 06 ma/s - - 09 11Dvd < 0.6 ml/s 12 12 - -Dvd 2 cj mjs 0.5 0.7 - -

A

effective area of subcriticalcircular members,effective area of flat-side&members,

+ub= & rc Bub + Amty andArea of the frame in aY = supercritical flow >Ae

6.3.3.3 Singleframes - Force coefficients fora single frame having either:a) all flat sided members, orb) all circular members in which all the

members of the frame have either:i) D va less than 6 ms/s, orii) DVa greater than 6 ml/s.

shall be as given in Table 28 according to thetype of the member, the diameter (D), the designwind speed (v,J) and the solidity ratio (+).TABLE 28 FORCE COEFF ICI ENTS FORSINGLE FRAMES

SOLIDITY FORCE COEFFICIENTS, Q, FORRATIO Q r-___-_--*--_____-~Fiat-sided Circular SectionsMembers ~--_--~~---~-~

(1) (2)01 1.90.2 100.3 1704 I .705 i.6075 I.6100 2.0

Subcri- Super-tical flow critical flow(DVdC6 ms/s) (Dv&% ma/s)

(3) (4)12 0.71.2 0.812 0.81.1 0.81-l 0.8I .5 1420 2.0

Linear interpolation between the values is permitted.

Force coefficients for a single frame not com-plying with the above requirements shall becalculated as follows:

+ (1 - Y) + crlatsubwhereC super = force coefficient for the super-critical circular members asgiven in Table 28 or Appen-dix D,

6.3.3.4 Mu&h frame buildings - Thissection applies to structures having two or more.parallel frames where the windward frames mayhave a shielding effect upon the~frames to leewardside. The windward frame and any unshield partsof other frames shall be calculated in accordancewith 6.3.3.3, but the wind load on the parts offrames that are sheltered should be multiplied bya shielding factor which is dependent upon thesolidity ratio of the windward frame, the types ofthe members comprising the frame and the spac-ing ratio of the frames. The values of the shieldingfactors are given in Table 29.TABLE 29 SHIE LDI NG FACTOR q FORMULTIPLE FRAMES

EFFECTIVE FRAME SPACIXGRATIOSorJnrTY c_--_______*-_-.40- ___RATIO, fl ~05 10 20 * >a.0

(1) (2) (3) (4) (5) (6)0 1.0 10 10 10 100.1 09 1.0 1.0 1.0 100.2 0.8 0.9 10 10 1003 07 0.8 10 10 1004 0.6 07 10 1.0 1005 05 0.6 09 10 100.7 0.3 0.6 0.8 o-9 101.0 03 06 06 0.8 10

Linear interpolation between values is permitted.

Where there are more than two frames ofsimilar geometry and spacing, the wind load onthe third and subsequent frames should be takenas equal to that on the second frame. The loads.on the various frames shall be added to obtaintotal load on the structure.a) The frame spacing ratio is equal to thedistance, centre to centre of the frames,beams or girders divided by the leastoverall dimension of the frame, beam orgirder measured at right angles to thedirection of the wind. For triangular framedstructures or rectangular framed structuresdiagonal to the wind, the spacing ratio

46


48/63

b)should be calculated from the mean dis-tance between the frames in the directionof the wind.Effective solidity ratio, p:p = CJ for flat-sided members.@ is to be obtained from Fig. 7 formembers of circular cross-sections.

0 0.1O-2 0.3 04 05 06 0 7 0 8SOLI DI TY RATIO. 9

FIG..~ EFFECTIVE SOLIDITY RATIO, pFOR ROUND SECTION MEMBERS

6.3.3.5 Lattice towersa) Force coefficient for lattice towers of squareor equilateral triangle section with flat-sided members for wind blowing against anyface shall be as given in Table 30.

TABLE 30 OVERALL FORCE COEFFICIENT FORTOWERS COMPOSED OF FLAT-SIDED MEMBERSSOLIDITY RATIO FORGE COEEFICIENT BORcm-_-_-.-- s-s-7

4 Square Towers Equilateral Tri-angular Towers(1) (2) (3)0.1 38 3.102 3.3 270.3 2.8 2.30.4 23 1905 21 15

b)

4

4

For square lattice towers with flat-sidedmembers the maximum load, which occurswhen the wind blows into a corner shall betaken as 1.2 times the load for the windblowing against a face.For equilateral-triangle lattice towers withflat-sided members, the load may be assu-m ed to be constant for any inclination ofwind to a face.Force coefficients for lattice towers ofsquare section with circular members, all inthe same flow regime, may be as given inTable 31.

IS t 875 ( Part 3 ) - 1987Force coefficients for lattice towers ofequilateral-triangle section with circularmembers all in the same flow ragime maybe as given in Table 32.

TABLE 31 OVERALL FORCE COEFFICIENT FORSQUARE TOWERS COMPOSED OFROUNDED MEMBERS[ Clause 6.3.3.5(d) ]SOLIDITY

RATIO OF FORCE COEFFICIENT FORr-----------FRONT FACE Subcritical Flowh-- _____ --~

(Dvd < 6 mr/s) Supercritical Flow( DVd 2 6 d/s 1r-__*_-_y r---hOnto face --7Onto Onto face Ontocorner corner(1) (2) (3) (4) ,(5)005 24 2.5 11 1201 22 23 12 1302 19 2.1 13 1603 17 1S 14 16

04 16 19 1.4 1.60.5 14 19 14 16

TABLE 32 OVERALL FORCE COEFFICIENT FOREQUILATERAL-TRIANGULAR TOWERSCOMPOSED OF ROUNDED MEMBERS[ Clause 6.3.3.5(e) ]

SOLIDITY RATIO FORCE COEFFICIENT FOBOF FRONT FACE I----- ----s+

--_-_--_-~Subcritical Flow Supercritcial Flow(Dvd < 6 m*/s) (Dvd < 6 ms/s)c__-*-_~ r-__A-__yAll wind All winddirections directions

(1) !2) (3)005 18 0.801 l-7 0.80.2 16 1103 15 1104 1.5 1105 14 12

6.3.3.6 Tower a@rtenanccs - The windloading on tower appurtenances, such as ladders,conduits, lights, elevators, etc, shall be calculatedusing appropriate net pressure coefficients forthese elements. Allowance may be made forshielding effect from other elements.7. DYNAMIC EFFECTS7.1 General - Flexible slender structures andstructural elements shall be investigated to ascer-tain the importance of wind induized oscillationsor excitations along and across the direction ofwind.

In general, the following guidelines may beused for examining the problems of wind inducedoscillations:a) Buildings and closed structures with aheight to minimum lateral dimension ratio

of more than about 5.0. and47


49/63

IS : 875 ( Part 3 ) - 1987b) Buildings and closed structures whosenatural frequency in the first mode -is lessthan 1-O Hz.Any building or structure which does notsatisfy either of the above two criteria shall beexamined for dynamic effects of wind.

NOTE 1 - The fundamental time period (I) mayeither be established by experimental observations onsimilar buildings or calculated by any rational methodof analysis. In the absence of such data, T may bedetermined as follows for multi-storeyed buildings:4

b)

For moment .resisting frames without bracing orshear walls for resisting the lateral loadsz-=0*1 nwhere

n = number of storeys including basement sto-reys; andFor all others

== 009 Hd/dwhere

H - total height of the main structure of thebuilding in metres, andd = maximum base dimension of building inmetrcs in a direction parallel to the appliedwind force.

NOTE 2 - If preliminary studies indicate thatwind-induced oscillations are likely to be rignificant,investigations should be persuade with the aid of analy-tical methods or, if necessary, by means oi wind tunneltests on models.NOTE - CrossLwind motions may by due tolateral gustiness of the wind, unsteady wake flow (forexample, vortex shedding ), negative aerodynamicdamping or to a combination of these effects. Thesecross-wind motions, can become critical in the design of

tall buildings/structures.NOTE 4 - Motions in the direction of wind (knownalso as buffeting) are caused by fluctuating wind forceassociated with gusts. The excitations depend on gustenergy available at the resonant frequency.NOTE 5 - The wake shed from an upstream bodymay intensify motions in the direction of the wind, andmay also affect crosswind motions.NOTE6-The designer must be aware of thefollowing three forms of wind induced motion whichare characterized by increasing amplitude of oscillationwith the increase of wind speed.a) Galloping - Galloping is transverse oscillationsof some structures due to the development ofaerodynamic forces which are in phase with themotion. It is characterized by the progressively

increasing amplitude of transverse vibrationwith increase of wind speed. The cross-sectionwhich are particularly prone to this type ofexcitation include the following:i) All structures with non-circular cross-sections,such as triangular, square, polygons, as wellas angles, crosses, and T-sections,ii) Twisted cables and cables with ice encrusta-tions.

b) Flu tter - Flutter is unstable oscillatory motionof a structure due to coupling between aerody-namic force and elastic deformation of thestructure. Perhaps the most common form isoscillatory motion due to combined bendingand torsion. Although oscillatory motions ineach degree of frebdom may be damped, insta-bility can set in due to energy transfer from onemode of oscillation to another, and the structureis seen to execute sustained or divergent oscilla-

Cl

tions with a type of motion which is a combina-tion of the individual modes of motion. Suchenergy transfer takes place when the naturalfrequencies of modes, taken individually, areclose to each other ( ratio. being typically lessthan 20 ). Flutter can set in at wind speedsmuch less than those required for exciting theindividual modes of motion. Long spansuspension bridge decks or any member of astructure with large values of d/t ( where d i sthe depth of a structure or structural memberparallel to wind stream and t is the least lateraldimension of a member ) are prone to low speedflutter. Wind tunnel testing is required to.determine critical flutter speeds and the likelystructural response. Other types of flutter aresingle degree of freedom stall flutter, torsionalflutter, etc.Ovafiing- This walled structures with open endsat one or both ends such as oil storage tanks,and natural draught cooling towers in which theratio of the diameter of minimum lateral dimen-sion to the wall thickness is of the order of !OOor more, are prone to ovalling oscillations.These oscillations are characterized by periodicradial deformation of the hollow structure.

NATE 7 -Buildings and structures that may besubjected to serious wind excited oscillations requirecareful investigation. It is to be noted that wind induc-ed oscillations may occur at wind speeds lower than thestatic design wind speed for the location.NOTE - Analytical methods for the response ofdynamic structures to wind loading can be found in thefollowing publications:i) Engineering Science Data, Wind EngineeringSub-Series ( 4 volumes ), London, ESDU Inter-national.

ii) Wind Engineering in the Eighties, Construc-tion Industry Research and Information Associ-ation, 1981, London.iii) Wind Effects on Structures by E. Simiu andR.H. Scanlan, New York, John Wiley andSons, 1978.iv) Supplement to the National Building Code ofCanada. 1980. NRCC, No. 17724, Ottawa, Nati-onal Research Council of Canada, 1980.v) Wind forces on structures by Peter Sachs. Per-gamon press.

vi) Flow induced vibration by Robert D. Clevins,Van Nostrand Reinfold Co.vii) Appropriate Indian Standards ( see 1.1.3 ).

NOTE 9 - In assessing wind loads due to such dy-namic phenomenon as galloping, flutter and ovalling, ifthe required information is not available either in thereferences of Note 8 or other literature, specialist adviseshall be sought, including experiments on models inwind tunnels.

7.2 Motion Due to Vortex Shedding7.2.1 Slender Structures - For a structure, theshedding frequency, 3 shall be determined by thefollowing formula:

whereS = Strouhal number,

v#j = design wind velocity, andb = breadth of a structure or structuralmembers in the horizontal planenormal to the wind direction.

48


50/63

IS : 875 ( Part 3 ) - 19878.2.1 Variation of Hourb Mean Wind Speed withHeight - The variation of hourly mean windspeed with height shall cbe calculated as follows:

Vz = Vb h ha kswhereP, = hourly mean wind speed in m/s,at height e;vb = regional basic wind speed in m/s(see Fig. 1 );kl = probability factor ( see 5.3.1 );& = terrain and height factor ( seeTable 33 ); andA-s topography factor ( see 5.3.3 ).

a) Circular Structures - For structures circularin cross-section:S = 0.20 for bV, not greater than 7,and

= 0.25 for bV, greater than 7.b) Rectangular Structures - For structures ofrectangular cross-section:

S = O-15 for all values of b V,.NOTE 1 - Significant cross wind motions may beproduced by vortex shedding if the natural frequencyof the structure or structural element is equal to thefrequency of the vortex shedding within the range ofexpected wind velocities. In such cases, further analysisshould be carried out on the basis of references given inNote 8 of 7.1.NOTE 2 - Unlined welded steel chimney stacksand similar structures are prone to excitation by vortexshedding.NOTE 3 - Intensification of the effects of periodicvortex shedding has been reported in cases where twoor more similar structures are located in close proxi-mity. for example, at less than 20 b


51/63

IS : 875 ( Part 3 ) - 1987 .where

& = peak factor defined as the ratio of theexpected peak value to the root meanvalue of afluctuating load, andY = roughness factor which is dependent onthe size of the structure in relation tothe ground roughness.

The, value of (gfr is given in Fig. 8,B = background factor indicating a measureof slowly varying component of fluctuat-ing wind load and is obtained fromFig. 9,SE-eP measure of the resonant component of thefluctuating wind load,

S = size reduction factor ( see Fig. 10 ),E = measure of available energy in the windstream at the natural frequency of thestructure ( see Fig. 11 ),/3= damping coefficient ( as a fraction ofcritical damping ) of the structure ( seeTable 34 ), and

grr 0-d= 4 and is to be accounted onlyfor buildings less than 75 m high interrain Category 4 and for buildings .lessthan 25 m high in terrain Cateiory 3,and is to be taken as zero in all othercases.

BUILDING HEIGHT,mFro 8 VALUES OF&r AND L (h)

0. 80.6

0.01 -02 -04 .06 .l .2 .3 .L .5 .f! 1 2 6 810CZh/L(h)


52/63


53/63

IS t 875 ( Part 3 ) - 1987

0.2W$ 0.150cLI 0 .!QlLgJ 0.05, O.OL= 0.03; 0.022

0.01

fo L(h:/vh

Fro. 11 GUST ENERGYFACTOR,EIn figures 8 to 11,

wherec, = lateral correlation constant which maybe taken as 10 in the absence of moreprecise load data,Ca = longitudinal correlation constantwhich may be taken as 12 in theabsence of more precise load data,b = breadth of a structure normal to the

TABLE 34 SUGGESTED VALUES OF DAMPINGCOEFFICIENT( Clause 8.3 )

N ATUBE 0~ STRIJCTURE DAMPINGCOEFFICIENT, @(1) (2)

Welded steel structures 0010Bolted steel structures 0020Reinforced concrete structures 0016

8.3.1 The peak acceleration along the winddirection at the top of the structure is given bythe following formula:wind stream,h = height of a structure,

.pb = v, = hourly mean wind speed at height t, wherez==f,, = natural frequency of the structure, andLul) = a measure of turbulence length scale( see Fig. 9 ).

mean deflection at the positionwhere the acceleration is required.Other notations are same as givenin 8.3.

52


54/63

APPENDIX A( Clause 5.2 )

IS t 875 ( Part 3 ) - 1987

BASIC WIND SPEED AT 10 m HEIGHT FOR SOME IMPORTANT CITIES/TOWNSCi t y/ Town Basic Wi nd Seed ( m/s )

AgraAhmadabadAjmerAlmoraAmritsarAsansolAurangabadBahraichBangaloreBarauniBareillyBhatindaBhilaiBhopalBhubaneshwarBhujBikanerBokaroBombayCalcuttaCalicutChandigarhCoimbatoreCuttackDarbhangaDarjeelingDehra DunDelhiDurgapurGangtokGauhatiGayaGorakhpurHyderabadImphalJ abalpurJ aipurJ a.mshedpur

4739474747473947334747473939505047474450394739505547474747475039474447474747

Ci t y/ own Basic Wi nd Speed m/s )J hansi 47J odhpur 47Kanpur 47Kohima 44Kurnool 39Lakshadweep 39Lucknow 47Ludhiana 47Madras 50Madurai 39Mandi 39Mangalore 39Moradabad 47Mysore 33Nagpur 44Nainital 47Nasik 39Nellore 50Panjim 39Patiala 47Patna 47Pondicherry 50Port Blair 44Pune 39Raipur 39Rajkot 39Ranchi 39Roorkee 39R ourkela 39Simla 39Srinagar 39Surat 44Tiruchchirrappalli 47Trivandrum 39Udaipur 47Vadodara 44Varanasi 47Vi jaywada 50Visakhapatnam 50


55/63

IS a 875 ( Part 3 ) - 1987APPENDIX B

[ Clau.se5.3.2.4(b)(ii) ]CHANGES IN TERRAIN CATEGORIES

B-1. LOW TO HfGH NUMBERB-l.1 In cases of transition from a low categorynumber ( corresponding to a low terrain rough-ness ) to a higher category number ( correspond-ing to a rougher terrain ), the velocity profileover the rougher terrain shall be determined asfollows:

a) Below height h,, the velocities shall bedetermined in relation to the rougherterrain; andb) Above height h,, the velocities shall bedetermined in relation to the less rough( more distant ) terrain.

B-2. HIGH TO LOW NUMBERB-2.1 In cases of transition from a more rough toa less rough terrain, the velocity profile shall bedetermined as follows:

a) Above height h,, the velocities shall be

b)determined in accordance with the rougher( more distant ) terrain; andBelow height h,, the velocity shall be takenas the lesser of the following:i) that determined in accordance with theless rough terrain, and

ii) the velocity at height h, as determined.in relation to the rougher terrain.NOTE - Examples of determination of velocityprofiles in the vicinity of a change in terrain categoryare shown in Fig. 12A and 12B.

B-3. MORE THAN O&E CATEGORYB-3.1 Terrain changes involving more than onecategory shall be treated in similar fashion tothat described in B-1 and B-2.

NOTE- Examples nvolvingthree terrain catego-riesare shown n Fig. 12C.

x,=FETCH,h, = HEIGHT FOR CATEGORY 4-..,. e PROFILE FOR CATEGORY6-----. PROFILE FOR CATEGORY 2- DESIGN PROFILE AT A

WINDDIRECTION

CATEGORY 2

12A Determination of Velocity Profile Near a Change in Terrain Category ( less rough to more rough )

x2=FETCH, h2=HEIGHT FOR CATEGORY 2..--..PROFIL E FOR CATEGORY .4

- --- PROFILE FOR CATEGORY 2-DESIGN PROFILE AT A I

LIII

WINDDIRECTION

//I-- ACATEGORY L CATEGdRY 2x2 -*128 Determination of Velocity PioRle Near a Change in Terrain Category (more rough to less rough)Fro. 12 VELOCITY PROFILE N THE VICIIVITYOF A CHANGE IN TERRAIN CATEGORY - Co&

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