wind pressure -asian code

5/28/2018 Wind Pressure -Asian Code

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Document No. :: IITK-GSDMA-Wind02-V5.0:: IITK-GSDMA-Wind04-V3.0

Final Report :: B - Wind CodesIITK-GSDMA Project on Building Codes

IS: 875(Part3): Wind Loads on Buildingsand Structures

-Proposed Draft & CommentaryBy

Dr.Prem KrishnaDr. Krishen KumarDr. N.M. Bhandari

Department of Civil EngineeringIndian Institute of Technology Roorkee

Roorkee


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This document has been developed under the project on BuildingCodes sponsored by Gujarat State Disaster Management Authority,Gandhinagar at Indian Institute of Technology Kanpur.

The views and opinions expressed are those of the authors and not

necessarily of the GSDMA, the World Bank, IIT Kanpur, or theBureau of Indian Standards.

Comments and feedbacks may please be forwarded to:Prof. Sudhir K Jain, Dept. of Civil Engineering, IIT Kanpur,Kanpur 208016, email: [email protected]
mailto:[email protected]:[email protected]


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Code & Commentary IS 875 (Part 3)

CODE COMMENTARY

Foreword0.1 This Indian Standard IS:875 (Part 3)

(Third Revision) was adopted by theBureau of Indian Standards on________(Date), after the draft finalizedby the Structural Safety SectionalCommittee had been approved by theCivil Engineering Division Council.

0.2 A building or a structure in general has toperform many functions satisfactorily.Amongst these functions are the utility ofthe building or the structure for theintended use and occupancy, structuralsafety, fire safety and compliance with

hygienic, sanitation, ventilation anddaylight standards. The design of thebuilding is dependent upon the minimumrequirements prescribed for each of theabove functions. The minimumrequirements pertaining to the structuralsafety of buildings are being covered inloading Codes by way of laying downminimum design loads which have to beassumed for dead loads, imposed loads,wind loads and other external loads, thestructure would be required to bear. Strictconformity to loading standards, it is

hoped, will not only ensure the structuralsafety of the buildings and structures,which are being designed andconstructed in the country and therebyreduce the risk to life and propertycaused by unsafe structures, but alsoreduce the wastage caused by assumingunnecessarily heavy loadings withoutproper assessment.

0.3 This standard was first published in 1957for the guidance of civil engineers,designers and architects associated with

the planning and design of buildings. Itincluded the provisions for the basicdesign loads (dead loads, live loads,wind loads and seismic loads) to beassumed in the design of the buildings. Inits first revision in 1964, the windpressure provisions were modified on thebasis of studies of wind phenomenon andits effect on structures, undertaken by thespecial committee in consultation with theIndian Meteorological Department. Inaddition to this, new clauses on windloads for butterfly type structures were

included; wind pressure coefficients for

IITK-GSDMA-Wind02-V5.0 3 IITK-GSDMA-Wind04-V3.0


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CODE COMMENTARYsheeted roofs, both curved and slopingwere modified; seismic load provisionswere deleted (separate Code having

been prepared) and metric system ofweights and measurements wasadopted.

0.3.1 With the increased adoption of thisCode, a number of comments werereceived on provisions on live loadvalues adopted for differentoccupancies. Live load surveys havebeen carried out in America, Canada,UK and in India to arrive at realistic liveloads based on actual determination ofloading (movable and immovable) in

different occupancies. Keeping this inview and other developments in thefield of wind engineering, the StructuralSafety Sectional Committee decided toprepare the second revision of IS: 875in the following five parts:

Part 1: Dead loads

Part 2: Imposed loads

Part 3: Wind loads

Part 4: Snow loads

Part 5: Special loads and loadcombinations

Earthquake load being covered in aseparate standard, namely,IS:1893(Part 1)- 2002*, should beconsidered along with the above loads.

0.3.2 This part (Part 3) deals with wind loadsto be considered when designingbuildings, structures and componentsthereof. In its second revision in 1987,the following important modificationswere made from those covered in the

1964 version of IS: 875:

*Criteria for Earthquake Resistant Design of Structures (2002 revision).



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CODE COMMENTARY

(a) The earlier wind pressure maps

(one giving winds of shorterduration and other excluding windsof shorter duration) were replacedby a single wind map giving basicmaximum wind speed in m/s (peakgust speed averaged over a shorttime interval of about 3 secondsduration). The wind speeds wereworked out for 50 years returnperiod based on the up-to-datewind data of 43 dines pressuretube (DPT) anemograph stationsand study of other related works

available on the subject since1964. The map and relatedrecommendations were provided inthe Code with the activecooperation of IndianMeteorological Department (IMD).Isotachs (lines of equal windspeed) were not given, as in theopinion of the committee there wasstill not enough extensivemeteorological data at closeenough stations in the country tojustify drawing of isotachs.

(b) Modification factors to modify thebasic wind speed to take intoaccount the effects of terrain, localtopography, size of structures, etc.were included.

(c) Terrain was classified into fourcategories based on characteristicsof the ground surface irregularities.

(d) Force and pressure coefficientswere included for a large range ofclad and unclad buildings and for

individual structural elements.(e) Force coefficients (drag

coefficients) were given for frames,lattice towers, walls and hoardings.

(f) The calculation of force on circularsections was includedincorporating the effects ofReynolds number and surfaceroughness.

(g) The external and internal pressurecoefficients for gable roofs, lean-toroofs, curved roofs, canopy roofs(butterfly type structures) and multi-span roofs were rationalized.



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CODE COMMENTARY(h) Pressure coefficients were given

for combined roofs, roofs with skylight, circular silos, cylindrical

elevated structures, grandstands,etc.

(i) An analysis procedure forevaluating the dynamic response offlexible structures under windloading using gust response factorwas included.

0.3.3 The Committee responsible for therevision of wind maps, while reviewingavailable meteorological wind data andresponse of structures to wind, felt thepaucity of data on which to base windmaps for Indian conditions onstatistical analysis. The Committee,therefore, recommended to allindividuals and organizationsresponsible for putting-up of tallstructures to provide instrumentation intheir existing and new structures(transmission towers, chimneys,cooling towers, buildings, etc.) atdifferent elevations (at least at twolevels) to continuously measure andmonitor wind data. It was noted that

instruments were required to collectdata on wind direction, wind speed andstructural response of the structuredue to wind (with the help ofaccelerometers, strain gauges, etc). Itwas also the opinion of the committeethat such instrumentation in tallstructures will not in any way affect oralter the functional behaviour of suchstructures, and the data so collectedwill be very valuable in assessing moreaccurate wind loading on structures.

0.3.4 It is seen at the time of undertaking the

third revision of this Code (during 2003-2004) that:

(i) Not much progress has yet beenmade in regard toinstrumentation and collection ofdata in India as mentioned in0.3.3 though additional data hasbecome available throughmeasurements of wind speed atthe meteorological stations. Inaddition there is a need toaddress the issue of cyclonic

winds and the damage causedby these winds.



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CODE COMMENTARY(ii) There has been a substantial

research effort on determinationof wind effects on buildings and

structures, the world over, duringthe past couple of decades, thusmaking available additionalinformation of improved quality.

(iii) A better understanding hasdeveloped concerning peaksuctions/pressures.

(iv) There is a better appreciationabout the randomness thatprevails in the directionality ofwind, and the degree ofcorrelation amongst pressuresthat it causes on a surface.

(v) There is a better understandingof the significant influence of theaveraging area used on thepressures evaluated.

(vi) There is an appreciation of thefact that wind loads on differentparts of the structure are not fullycorrelated.

(vii) There is a significant effect

possible on the wind forces in abuilding on account ofinterference between similar ordissimilar buildings.

(viii) It is realized that as a result ofthe second revision, thestandard produced was oncontemporary lines. Changesare therefore warranted onlywhere these would bring aboutan improvement in the quality ofthe standard.

In carrying out this revision, the aboveobservations have been taken intoaccount.

0.4 The Sectional Committee responsiblefor the preparation of this Standard hastaken into account the prevailingpractice in regard to Loading Standardsfollowed in this country by the variousauthorities and has also taken note ofthe developments in a number of othercountries. In the preparation of thisCode, the following overseas

Standards have also been examined:

(a) BS 6399-2:1997 Loading for



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CODE COMMENTARYBuildings, Part 2: Code of Practicefor Wind loads.

(b) AS/NZS1170.2: 2002 StructuralDesign Actions-Part 2: WindActions.

(c) ASCE 7-02 American Society ofCivil Engineers: Minimum DesignLoads for Buildings and OtherStructures.

(d) National Building Code of Canada1995.

(e) Architectural Institute of JapanRecommendations for Loads onBuildings, 1996.

Wind Resistant Design Regulations, AWorld List. Association for ScienceDocuments Information, Tokyo.

0.5 For the purpose of deciding whether aparticular requirement of this Standardis complied with, the final value,observed or calculated, expressing theresult of a test or analysis, shall berounded off in accordance with IS:2-1960*. The number of significantplaces retained in the rounded off

value should be the same as that ofthe specified value in this Standard.

*Rules for Rounding-off Numerical Values (Revised).



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CODE COMMENTARY

1. Scope

1.1 -This Standard gives wind forces and theireffects (static and dynamic) that should betaken into account while designing buildings,structures and components thereof.

C1.1

C1.1.1

C1.1.2

This Code provides information on wind effectsfor buildings and structures, and theircomponents. Structures such as chimneys, coolingtowers, transmission line towers and bridges areoutside the scope of this Code. There are IndianStandards dealing with chimneys and coolingtowers separately. Information on bridges (only

static forces) is given in IRS and IRCSpecifications. For aerodynamics of bridges,specialist literature may be consulted. Withsubstantial work being done worldwide in the area

of wind engineering, there is growing body ofnew information. The user of this Code is advised

to consult specialist literature for the design oflarge or important projects involving varioustypes of structures.

1.1.1Wind causes a random time-dependent load,which can be seen as a mean plus afluctuating component. Strictly speaking allstructures will experience dynamicoscillations due to the fluctuating component(gustiness) of wind. In short rigid structures

these oscillations are insignificant, andtherefore can be satisfactorily treated ashaving an equivalent static pressure. This isthe approach taken by most Codes andStandards, as is also the case in thisStandard. A structure may be deemed to beshort and rigid if its natural time period is lessthan one second. The more flexible systemssuch as tall buildings undergo a dynamicresponse to the gustiness of wind. Methodsfor computing the dynamic effect of wind onbuildings have been introduced in thisStandard.

Apart from tall buildings there are severalother structural forms (though outside thescope of this Standard) such as tall latticedtowers, chimneys, guyed masts that need tobe examined for aerodynamic effects.

Wind is not a steady phenomena due to naturalturbulence and gustiness present in it. However,when averaged over a sufficiently long timeduration (from a few minutes to an hour), a mean

component of wind speed can be defined whichwould produce a static force on a structure.

Superimposed on the mean/static component isthe time varying component having multiplefrequencies spread over a wide band.

1.1.2 This Code also applies to buildings or otherstructures during erection/ construction andthe same shall be considered carefully duringvarious stages of erection/construction. Inlocations where the strongest winds and icingmay occur simultaneously, loads on

structural members, cables and ropes shallbe calculated by assuming an ice covering

The construction period of a structure is muchsmaller than its expected life. Therefore, a smaller

return period of 5 to 10 years or longer may beconsidered for arriving at the design factor (factork1) for construction stages/period of a structure

depending on its importance. In snowfall areaswhere icing occurs, wind loads have to be



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CODE COMMENTARYassessed accordingly. Elements such as cables andropes can undergo a dynamic response in suchcases and have to be examined accordingly.

based on climatic and local experience.

1.1.3In the design of special structures, such aschimneys, overhead transmission line towers,etc., specific requirements as given in therespective Codes shall be adopted inconjunction with the provisions of this Codeas far as they are applicable. Some of theIndian Standards available for the design ofspecial structures are:

IS: 4998 (Part 1) 1992 Criteria for design ofreinforced concrete chimneys: Part 1 -Design Criteria (first revision)

IS:6533 1989 Code of practice for designand construction of steel chimneys

IS:5613 (Part 1/Sec 1)- 1985 Code ofpractice for design, installation andmaintenance of overhead power lines:Part 1 Lines up to and including 11 kV,Section 1 Design

IS:802 (Part 1)-1995 Code of practice for useof structural steel in overheadtransmission line towers: Part 1 Loads

and permissible stresses (secondrevision)

IS:11504-1985 Criteria for structural designof reinforced concrete natural draughtcooling towers

NOTE: 1 This standard IS:875 (Part 3)-1987does not apply to buildings or structures withunconventional shapes, unusual locations,and abnormal environmental conditions thathave not been covered in this Code. Specialinvestigations are necessary in such cases toestablish wind loads and their effects. Windtunnel studies may also be required in suchsituations.

NOTE: 2 In the case of tall structures withunsymmetrical geometry, the designs oughtto be checked for torsional effects due to windpressure.

C1.1.3 See C1.1



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CODE COMMENTARY

2. Notations

2.1

The following notations shall be followedunless otherwise specified in relevantclauses. Notions have been defined in thetext at their first appearance. A few of thenotations have more than one definition,having been used for denoting differentvariables :

A = Surface area of a structure orpart of a structure

Ae= Effective frontal area

Az= Frontal contributory area atheight z

b = Breadth of a structure orstructural member normal to thewind stream in the horizontalplane

BBs= Background factor

CD= Drag coefficient

CL= Lift coefficient

Cf= Force coefficient

Cfn= Normal force coefficient

Cft= Transverse force coefficient

Cf= Frictional drag coefficient

Cdyn= Dynamic response factor

Cp= Pressure coefficient

Cpe= External pressure coefficient

Cpi= Internal pressure coefficient

Cfs= Cross-wind force spectrumcoefficient

d = Depth of a structure orstructural member parallel towind stream in the horizontalplane

D = Diameter of cylinder or sphere;Depth of structure

E = Wind energy factor

f0= First mode natural frequency ofvibration

F = Force on a surface

Fn= Normal force

Ft= Transverse force

F= Frictional force



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CODE COMMENTARY

Notations have been defined also in the text at

their first appearance. A few of the notations havemore than one definition, having been used fordenoting different parameters.

gR= Peak factor for resonantresponse

gv= Peak factor for upwind velocityfluctuations

h = Height of structure above meanground level

hx= Height of development of aspeed profile at distance xdownwind from a change interrain category

hp = Height of parapet

Hs= Height factor for resonantresponse

Ih= Turbulence intensity

IF Interference factor

k = Mode shape power exponent

k1

k2

k3

k4

Wind speed multiplicationfactors

K = Force coefficient multiplicationfactor for members of finitelength

Ka= Area averaging factor

Kc= Combination factor

Kd= Wind directionality factor

Km= Mode shape correction factor

l = Length of a member or greaterhorizontal dimension of abuilding

L = Actual length of upwind slope

Le= Effective length of upwind slope

Lh= Integral turbulence length scale

N = Reduced frequency

pd= Design wind pressure

pz= Wind pressure at height zpe= External wind pressure

pi= Internal wind pressure

Re= Reynolds number

S = Size reduction factor

Sr= Strouhal number

T = Fundamental time period ofvibration

Vb= Regional basic wind speed

Vh= Design wind speed at height h

Vz= Design wind speed at height z



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CODE COMMENTARY

zV =Hourly mean wind speed atheight z

W = Lesser horizontal dimension ofa building in plan, or in thecross-section a structuralmember;

W= Bay width in a multi-baybuilding;

We = Equivalent cross-wind staticforce

X = Distance downwind from achange in terrain category;fetch length

Z = Height above average ground

level= Inclination of roof to the

horizontal plane

= Effective solidity ratio; Dampingratio

= Average height of surfaceroughness

= Solidity ratio

= Shielding factor or eddyshedding frequency

= Wind direction in plan from agiven axis; upwind ground / hillslope



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CODE COMMENTARY

3. Terminology

For the purpose of this Code, the followingdefinitions shall apply.

Angle of Attack / Incidence () - Angle invertical plane between the direction ofwind and a reference axis of thestructure.

Breadth (b) Breadth means horizontaldimension of the building measurednormal to the direction of wind.

Depth (D) Depth means the horizontaldimension of the building measured in

the direction of the wind.Note Breadth and depth are dimensionsmeasured in relation to the direction of wind,whereas length and width are dimensions related tothe plan.

Developed Height Developed height is theheight of upward penetration of the windspeed profile in a new terrain. At largefetch lengths, such penetration reachesthe gradient height, above which the windspeed may be taken to be constant. Atlesser fetch lengths, a wind speed profileof a smaller height but similar to that of

the fully developed profile of the terraincategory has to be taken, with theadditional provision that wind speed atthe top of this shorter profile equals thatof the unpenetrated earlier profile at thatheight.

Effective Frontal Area (Ae) The projectedarea of the structure normal to thedirection of the wind.

Element of Surface Area The area ofsurface over which the pressurecoefficient is taken to be constant.

Force Coefficient (Cf) - A non-dimensionalcoefficient such that the total wind forceon a body is the product of the forcecoefficient, the dynamic pressure due tothe incident design wind speed and thereference area over which the force isrequired.

NOTE When the force is in thedirection of the incident wind, the non-dimensional coefficient will be called asdrag coefficient (CD). When the force isperpendicular to the direction of incidentwind, the non-dimensional coefficient willbe called as lift coefficient (CL).



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CODE COMMENTARY

Ground Roughness The nature of the

earths surface as influenced by smallscale obstructions such as trees andbuildings (as distinct from topography) iscalled ground roughness.

Gust A positive or negative departure ofwind speed from its mean value, lastingfor not more than, say, 2 minutes with thepeak occurring over a specified interval oftime. For example, 3 second gust windspeed.

Peak Gust Peak gust or peak gust speed isthe wind speed associated with the

maximum value.

Fetch Length (X) Fetch length is thedistance measured along the wind from aboundary at which a change in the type ofterrain occurs. When the changes interrain types are encountered (such as,the boundary of a town or city, forest,etc.), the wind profile changes incharacter but such changes are gradualand start at ground level, spreading orpenetrating upwards with increasing fetchlength.

Gradient Height Gradient height is theheight above the mean ground level atwhich the gradient wind blows as a resultof balance among pressure gradientforce, Coriolis force and centrifugal force.For the purpose of this Code, thegradient height is taken as the heightabove the mean ground level, abovewhich the variation of wind speed withheight need not be considered.

Interference Factor Ratio of the value of atypical response parameter for a

structure due to interference divided bythe corresponding value in the standalone case.

Mean Ground Level The mean ground levelis the average horizontal plane of thearea in the close vicinity and immediatelysurrounding the structure.

Pressure Coefficient Pressure coefficient isthe ratio of the difference between thepressure acting at a point on a surfaceand the static pressure of the incidentwind to the design wind pressure, where

the static and design wind pressures aredetermined at the height of the pointconsidered after taking into account the



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CODE COMMENTARYgeographical location, terrain conditionsand shielding effect.

NOTE: Positive sign of the pressure coefficientindicates pressure acting towards the surfaceand negative sign indicates pressure actingaway from the surface.

Return Period Return period is the numberof years, the reciprocal of which gives theprobability of an extreme wind exceedinga given speed in any one year.

Shielding Effect Shielding effect orshielding refers to the condition wherewind has to pass along some structure(s)or structural element(s) located on theupstream wind side, before meeting the

structure or structural element underconsideration. A factor called shieldingfactoris used to account for such effectsin estimating the force on the shieldedstructure(s).

Speed Profile The variation of thehorizontal component of the atmosphericwind speed with height above the meanground level is termed as speed profile.

Suction Suction means pressure less thanthe atmospheric (static) pressure and isconsidered to act away from the surface.

Solidity Ratio Solidity ratio is equal to theeffective area (projected area of all theindividual elements) of a frame normal tothe wind direction divided by the areaenclosed by the boundary of the framenormal to the wind direction.

NOTE Solidity ratio is to be calculated forindividual frames.

Terrain Category Terrain category meansthe characteristics of the surfaceirregularities of an area, which arise fromnatural or constructed features. The

categories are numbered in increasingorder of roughness.

Topography The nature of the earthssurface as influenced by the hill andvalley configurations in the vicinity of theexisting / proposed structure.



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CODE COMMENTARY

4. GENERAL

4.1-

Wind is air in motion relative to the surfaceof the earth. The primary cause of wind istraced to earths rotation and differences interrestrial radiation. The radiation effects aremainly responsible for convection currenteither upwards or downwards. The windgenerally blows horizontal to the ground athigh speeds. Since vertical components ofatmospheric motion are relatively small, theterm wind denotes almost exclusively thehorizontal wind while vertical winds are

always identified as such. The wind speedsare assessed with the aid of anemometersor anemographs, which are installed atmeteorological observatories at heightsgenerally varying from 10 to 30 metersabove ground.

4.2

Very strong winds are generally associatedwith cyclonic storms, thunderstorms, duststorms or vigorous monsoons. A feature of

the cyclonic storms over the Indian region isthat they rapidly weaken after crossing thecoasts and move as depressions/ lowsinland. The influence of a severe storm afterstriking the coast does not, in general exceedabout 60 kilometers, though sometimes, itmay extend even up to 120 kilometers. Veryshort duration hurricanes of very high windspeeds called Kal Baisaki or Norwestersoccur fairly frequently during summer monthsover North East India.

4.3

The wind speeds recorded at any locality areextremely variable and in addition to steadywind at any time, there are effects of gusts,which may last for a few seconds. These

gusts cause increase in air pressure but theireffect on stability of the building may not be

C4.1 -

For the purpose of this Code wind speed has beenconsidered as that occurring at 10 m height abovethe general ground level. Several new recording

stations have been established in the country bythe Indian Meteorological Department over thelast two decades, the information from which canhelp upgrade the wind zoning map of India.However, more extensive data are needed to makethis exercise meaningful.

C4.2 -

Several atmospheric phenomena are responsiblefor wind storms. Cyclonic storms, that hit some

of the coastal regions of India, are the most

devastating due to extremely high wind speeds inthese storms accompanied by sea surge andflooding. These can last several hours. Thecurrent revised draft has recognized the fact thatthe high wind speeds that occur in cyclones far

exceed the wind speeds for design given in theCode at present, and addresses the problem vis--

vis the 60 km strip in the east coast and theGujarat coast by including suitable factors toenhance the design wind speed, keeping in viewthe importance of the structure.

Tornados, which are a narrow band phenomenonof limited time duration, often occur during the

summer, mostly in Northern parts of India.These, however, have extremely high windspeeds, often higher than in the severest cyclones.

C4.3 -

Higher the intensity of a gust, lower is its

duration. The Code specifies the basic windspeed as that of a gust of 3 second duration; or inother words, the wind speed averaged over a 3-second period. The effect of reduction in theaverage wind pressure with increase in the areaover which the pressure is considered (the



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CODE COMMENTARY

so important; often, gusts affect only part ofthe building and the increased local

pressures may be more than balanced by amomentary reduction in the pressureelsewhere. Because of the inertia of thebuilding, short period gusts may not causeany appreciable increase in stress in maincomponents of the building although thewalls, roof sheeting and individual claddingunits (glass panels) and their supportingmembers such as purlins, sheeting rails andglazing bars may be more seriously affected.Gusts can also be extremely important fordesign of structures with high slendernessratios.

tributary area) is accounted for by the AreaAveraging Factor, Ka defined in Section 5.4.2. A

maximum reduction of 20% in wind pressures isspecified for tributary area beyond 100 m2.

4.4

The response of a building to high windpressures depends not only upon thegeographical location and proximity of otherobstructions to airflow but also upon thecharacteristics of the structure itself.

4.5

The effect of wind on the structure as a wholeis determined by the combined action ofexternal and internal pressures acting uponit. In all cases, the calculated wind loads actnormal to the surface to which they apply.

4.6

The stability calculations as a whole shall bedone with and without the wind loads onvertical surfaces, roofs and other parts of the

building above average roof level.

Contrary to this, one may consider wind effectsover a limited (small) area of the surface. This isparticularly important near the edges and ridge ofa structure or sharp corners elsewhere in abuilding, where large suctions occur due toseparation of flow and generation of eddies. The

area of influence being small, there is bettercorrelation within these areas. These local areaeffects are treated elsewhere in the Code.

C4.4 -

The dynamic characteristics of a flexible structuredefined by its time period of vibration and

damping would affect its response to the gustinessor turbulence in wind, which itself gets modifieddue to presence of other structures/ obstructions,particularly those in the close vicinity of thestructure. The effect of the latter is difficult toevaluate and a simplified approach has been

added for limited cases for the first time in the

Code to approximate these so called interferenceeffects in Section 7.

C4.5-

The pressures created inside a building due toaccess of wind through openings could be suction

(negative) or pressure (positive) of the same orderof intensity while those outside may also vary in

magnitude with possible reversals. Thus thedesign value shall be taken as the algebraic sumof the two in appropriate/concerned direction.

Furthermore, the external pressures (or forces)

acting on different parts of a framework do notcorrelate fully. Hence there is a reduction in theoverall effect. This has been allowed for in clause6.2.3.13.

C4.6 -

The stability of a structure shall be checked bothwith and without the wind loads, as there may bereversal of the forces under wind besides a

reduced factor of safety considered with the windloads.



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CODE COMMENTARY

C4.7 -4.7

Comfort of the inhabitants of a tall flexiblebuilding can be affected by large wind induceddeflections or accelerations, particularly the latter.There is no criterion included in this Code forcontrol on these parameters. Since there is no realtall building activity yet in India, the problem has

not attained importance. Likewise, at the plazalevel around a tall building, there may be

accentuated flow conditions, particularly if thebuilding has other similar structures adjacent to it.Thus the pedestrians at the plaza level can be putto inconvenience. A wind tunnel model study is

required to determine the flow pattern and to

carryout the design accordingly.

Buildings shall also be designed with dueattention to the effects of wind on the comfortof people inside and outside the buildings.



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CODE COMMENTARY

5. WIND SPEED AND

PRESSURE

5.1- Nature of wind in Atmosphere

In general, wind speed in the atmosphericboundary layer increases with height fromzero at ground level to a maximum at aheight called the gradient height. There isusually a slight change in direction (Ekmaneffect) but this is ignored in the Code. Thevariation with height depends primarily on theterrain conditions. However, the wind speed

at any height never remains constant and ithas been found convenient to resolve itsinstantaneous magnitude into an average ormean value and a fluctuating componentaround this average value. The averagevalue depends on the averaging timeemployed in analyzing the meteorologicaldata and this averaging time can be taken tobe from a few seconds to several minutes.The magnitude of fluctuating component ofthe wind speed, which represents thegustiness of wind, depends on the averagingtime. Smaller the averaging interval, greater

is the magnitude of the wind speed.

C5.1 -

As is explained in Code, wind speed can be takento comprise of a static (mean) component and afluctuating component, with the magnitude of thelatter varying with time interval over which thegust is averaged. Thus with reduction in theaveraging time, the fluctuating wind speed would

increase. The fluctuating velocity is normallyexpressed in terms of turbulence intensity which

is the ratio of the standard deviation to the meanwind speed and is expressed in percentage.

5.2 Basic Wind Speed (Vb)

Figure 1 gives basic wind speed map ofIndia, as applicable at 10 m height abovemean ground level for different zones of thecountry. Basic wind speed is based on peakgust speed averaged over a short timeinterval of about 3 seconds and correspondsto 10m height above the mean ground levelin an open terrain (Category 2). Basic windspeeds presented in Fig. 1 have been

worked out for a 50-year return period. Thebasic wind speed for some importantcities/towns is also given in Appendix A.

C5.2 -

Code defines the basic wind speed as the peakgust wind speed averaged over a period of 3seconds. It includes both the mean and the

fluctuating components of the turbulent wind. Toobtain hourly mean wind speed, the 3-second

value may be multiplied by factor 0.65. In theopen terrain category, since wind speed varieswith height, ground roughness, local topographyand return period of the storm, besides the region

of the country, the conditions for which Vb isdefined have been specified in this clause. Thecountry has been divided into six wind zones and

certain coastal regions affected by cyclonic stormsas defined in clause 5.3.4.



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CODE COMMENTARY

5.3Design Wind Speed (Vz)The basic wind speed for any site shall beobtained from Fig. 1 and shall be modified toinclude the following effects to get designwind speed, Vz at any height, Z for thechosen structure: (a) Risk level, (b) Terrain

roughness and height of structure, (c) Localtopography, and (d) Importance factor for thecyclonic region. It can be mathematicallyexpressed as follows:

Vz= Vbk1k2k3k4,

where

Vz= design wind speed at any height z inm/s,

k1= probability factor (risk coefficient) (see5.3.1),

k2= terrain roughness and height factor(see 5.3.2),

k3= topography factor (see 5.3.3), andk4= importance factor for the cyclonic region

(see 5.3.4).

NOTE: The wind speed may be taken as constant upto aheight of 10 m. However, pressures for buildingsless than 10m high may be reduced by 20% forstability and design of the framing.

5.3.1 Risk Coefficient (k1)Fig. 1 gives basic wind speeds for terraincategory 2 as applicable at 10 m heightabove mean ground level based on 50 yearsmean return period. The suggested life spanto be assumed in design and thecorresponding k1factors for different class of

structures for the purpose of design are givenin Table 1. In the design of all buildings andstructures, a regional basic wind speedhaving a mean return period of 50 years shallbe used except as specified in the note ofTable 1.

C5.3 -The basic wind speed, Vbcorresponds to certainreference conditions. Hence, to account forvarious effects governing the design wind speed

in any terrain condition, modifications in the formof factors k1, k2, k3, and k4are specified.

C5.3.1 -The peak wind speed considered for design isbased on the probability of occurrence of themaximum/severest storm over the design life of

the structure. It is known that storms of greaterseverity are less frequent, that is, such stormshave a longer return period. Thus for economical

design of structures, the design wind speed hasbeen related to the return-period of storms, withVbdefined for 50-years return period consideringthe generally acceptable value of probability ofexceedence as 0.63 for the design wind speed

over the life of the structure. This has been termedas the risk level PNin N consecutive years (Table

1) and the corresponding value of the riskcoefficient, k1, for N taken as 50 years, would be1.0. The values of k1for N taken as 5, 25 and 100years, and for various zones of the country, aregiven in Table-1. The designer may, however, usea higher value of N or k1, if it is considered

necessary to reduce the risk level of an importantstructure.



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Figure 1: Basic wind speed in m/s (based on 50 year return period)



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Table 1: Risk coefficients for di fferent classes of st ructures in dif ferent wind speedzones [Clause 5.3.1]

k1factor for Basic Wind Speed (m/s)of

Class of Structure

Mean Probabledesign life ofstructure in

years

33 39 44 47 50 55

All general buildings and structures 50 1.0 1.0 1.0 1.0 1.0 1.0

Temporary sheds, structures such as thoseused during construction operations (forexample, formwork and false work),structures during construction stages, andboundary walls

5 0.82 0.76 0.73 0.71 0.70 0.67

Buildings and structures presenting a lowdegree of hazard to life and property in theevent of failure, such as isolated towers inwooded areas, farm buildings other thanresidential buildings, etc.

25 0.94 0.92 0.91 0.90 0.90 0.89

Important buildings and structures such ashospitals, communication buildings, towersand power plant structures

100 1.05 1.06 1.07 1.07 1.08 1.08

NOTE The factor k1 is based on statistical concepts, which take account of the degree of reliabilityrequired, and period of time in years during which there will be exposure to wind, that is, life of thestructure. Whatever wind speed is adopted for design purposes, there is always a probability(howsoever small) that it may be exceeded in a storm of exceptional violence; the greater thenumber of years over which there will be exposure to wind, the greater is the probability. High returnperiods ranging from 100 to 1000 years (implying lower risk level) in association with greater periodof exposure may have to be selected for exceptionally important structures, such as, nuclear powerreactors and satellite communication towers. Equation given below may be used in such cases toestimate k1 factors for different periods of exposure and chosen probability of exceedence (risk

level). The probability level of 0.63 is normally considered sufficient for design of buildings andstructures against wind effects and the values of k1corresponding to this risk level are given above.

( )

B4A

NP1lnN

1lnBA

63.0,50X

NP,NX

1k

+

==

where

N = mean probable design life of the structure in years;

PN = risk level in N consecutive years (probability that the design wind speed is exceeded at least

once in N successive years), nominal value = 0.63;

XN,P = extreme wind speed for given value of N and PN; and

X50,0.63= extreme wind speed for N = 50 years and PN= 0.63

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

Zone A B

33 m/s 83.2 9.2

39 m/s 84.0 14.0

44 m/s 88.0 18.0

47 m/s 88.0 20.5

50 m/s 88.8 22.8

55 m/s 90.8 27.3



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Table 2: k2 factors to obtain design wind speed variation with height in differentterrains [Clause 5.3.2.2]

Terrain and height mul tiplier (k2)Height (z)

(m)

TerrainCategory 1

TerrainCategory 2

TerrainCategory 3

TerrainCategory 4

10 1.05 1.00 0.91 0.80

15 1.09 1.05 0.97 0.80

20 1.12 1.07 1.01 0.80

30 1.15 1.12 1.06 0.97

50 1.20 1.17 1.12 1.10

100 1.26 1.24 1.20 1.20

150 1.30 1.28 1.24 1.24

200 1.32 1.30 1.27 1.27

250 1.34 1.32 1.29 1.28

300 1.35 1.34 1.31 1.30

350 1.37 1.36 1.32 1.31

400 1.38 1.37 1.34 1.32

450 1.39 1.38 1.35 1.33

500 1.40 1.39 1.36 1.34

NOTE: For intermediate values of height z and terrain category, use linearinterpolation.



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CODE COMMENTARY

5.3.2 Terrain and Height Factor(k2)

5.3.2.1

Terrain Selection of terrain categories shallbe made with due regard to the effect ofobstructions which constitute the groundsurface roughness. The terrain category usedin the design of a structure may varydepending on the direction of wind underconsideration. Wherever sufficientmeteorological information is available aboutthe wind direction, the orientation of anybuilding or structure may be suitably planned.

Terrain in which a specific structure stands

shall be assessed as being one of thefollowing terrain categories:

a) Category 1 Exposed open terrainwith a few or no obstructions and inwhich the average height of any objectsurrounding the structure is less than1.5 m.

NOTE This category includes open sea coastsand flat treeless plains.

b) Category 2 Open terrain with well-scattered obstructions having height

generally between 1.5 and 10 m.

NOTE This is the criterion for measurement ofregional basic wind speeds and includesairfields, open parklands and undevelopedsparsely built-up outskirts of towns andsuburbs. Open land adjacent to seacoastmay also be classified as Category 2 dueto roughness of large sea waves at highwinds.

c) Category 3 Terrain with numerousclosely spaced obstructions having thesize of building-structures up to 10 min height with or without a few isolated

tall structures.NOTE 1 This category includes well-wooded

areas, and shrubs, towns and industrialareas fully or partially developed.

NOTE 2 It is likely that the next higher categorythan this will not exist in most designsituations and that selection of a moresevere category will be deliberate.

NOTE 3 Particular attention must be given toperformance of obstructions in areasaffected by fully developed tropicalcyclones. Vegetation, which is likely to beblown down or defoliated, cannot be relied

upon to maintain Category 3 conditions.Where such a situation exists, either an

C5.3.2.1-The Code defines 4 types of terrains and explainsthat a structure may effectively lie in two differenttypes of terrain for two different wind directions.

In addition, the designer shall keep in mind, thefuture development of the surrounding area whichmay alter the ground roughness and hence theterrain category. It may be noted that Category 2has been considered as the datum with respect towhich the other terrain categories have beendefined. In a given situation, the effect of terraincondition, if deviated from the above reference

terrain, is accounted for through the factor, k2.

Photographs CP1 to CP4 (Cook 1985) are givento demonstrate how terrain categories 1 to 4 maybe assigned. This is merely for guidance purpose.



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CODE COMMENTARY

intermediate category with speed

multipliers midway between the values forCategory 2 and 3 given in Table 2 may beused, or Category 2 be selected havingdue regard to local conditions.

d) Category 4 Terrain with numerouslarge high closely spaced obstructions.

NOTE This category includes large city centers,generally with obstructions taller than 25 mand well-developed industrial complexes.

5.3.2.2

Variation of wind speed with height fordifferent terrains (k

2 factor) Table 2 gives

multiplying factor (k2) by which the basic windspeed given in Fig. 1 shall be multiplied toobtain the wind speed at different heights, ineach terrain category.

5.3.2.3

Terrain categories in relation to the directionof wind As also mentioned in 5.3.2.1, the

terrain category used in the design of a

C5.3.2.2-

The variation of wind speed with height is alsodependent upon the ground roughness and is thus

different for each terrain category, as can bevisualized from Fig. C1. Wind blows at a given

height, with lesser speeds in rougher terrains andwith higher speeds in smoother terrains. Further,

in any terrain, wind speed increases along theheight upto the gradient height and the values ofthe gradient heights are higher for rougherterrains. By definition, wind speeds beyond

gradient heights in all terrains are equal. At anyheight in a given terrain, the magnitude of wind

speed depends on the averaging time. Shorter theaveraging time, the higher is the mean windspeed. Also it takes quite a distance, called fetchlength, for wind to travel over a typical terrain tofully develop the speed profile idealized for thatterrain category.

Fig. C 1 Boundary Layer Profile for

Different Approach Terrains

C5.3.2.3-

Ground obstructions in the path of wind may bedifferent for different directions of the wind.



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CODE COMMENTARY

structure may vary depending on thedirection of wind under consideration. Where

sufficient meteorological information isavailable, the basic wind speed may bevaried for specific wind directions.

5.3.2.4

Changes in terrain categories The speedprofile for a given terrain category does notdevelop to full height immediately with thecommencement of that terrain category butdevelops gradually to height (hx) whichincreases with the fetch or upwind distance(x).

a) Fetch and developed height relationship The relation between the developedheight (hx) and the fetch length (x) forwind-flow over each of the four terraincategories may be taken as given inTable 3.

b) For structures of heights greater than thedeveloped height (hx) in Table 3, thespeed profile may be determined inaccordance with the following:

(i) The less or least rough terrain, or

(ii) The method described in Appendix

B.

C5.3.2.4-

Self explanatory.

Table 3: Fetch and developed height relationship [Clause 5.3.2.4]

Developed Height hx(m)Fetch (x)

(km) TerrainCategory 1

TerrainCategory 2

TerrainCategory 3

TerrainCategory 4

(1) (2) (3) (4) (5)

0.2 12 20 35 60

0.5 20 30 55 95

1 25 45 80 130

2 35 65 110 190

5 60 100 170 300

10 80 140 250 450

20 120 200 350 500

50 180 300 400 500



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CP1 Photograph Indicative of Terrain Category 1 Features




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CODE COMMENTARY

5.3.3

Topography (k3 factor) The basic windspeed Vbgiven in Fig. 1 takes account of thegeneral level of site above sea level. Thisdoes not allow for local topographic featuressuch as hills, valleys, cliffs, escarpments, orridges, which can significantly affect the windspeed in their vicinity. The effect oftopography is to accelerate wind near thesummits of hills or crests of cliffs,escarpments or ridges and decelerate thewind in valleys or near the foot of cliffs, steepescarpments, or ridges.

5.3.3.1

The effect of topography will be significant at

a site when the upwind slope () is greaterthan about 3

o, and below that, the value of k3

may be taken to be equal to 1.0. The value ofk3 is confined in the range of 1.0 to 1.36 forslopes greater than 3

o. A method of

evaluating the value of k3 for values greaterthan 1.0 is given in Appendix C. It may benoted that the value of k3varies with heightabove ground level, at a maximum near theground, and reducing to 1.0 at higher levels,for hill slope in excess of 17

o.

5.3.4 Importance Factor for Cyclonic Region (k4)

Cyclonic storms usually occur on the eastcoast of the country in addition to the Gujaratcoast on the west. Studies of wind speed anddamage to buildings and structures point tothe fact that the speeds given in the basicwind speed map are often exceeded duringthe cyclones. The effect of cyclonic storms is

largely felt in a belt of approximately 60 kmwidth at the coast. In order to ensure greatersafety of structures in this region (60 km wideon the east coast as well as on the Gujaratcoast), the following values of k4 arestipulated, as applicable according to theimportance of the structure:

Structures of postcyclone importance 1.30Industrial structures 1.15All other structures 1.00

C5.3.3 -

The factor k3 is a measure of the enhancementthat occurs in wind speeds over hills, cliffs andescarpments.

C5.3.3.1No increase in wind speed is indicated for upwindground slopes upto 3o, while a maximum increase

of 36% is specified for slopes beyond 17o.

Maximum effect is seen to occur at the crest of acliff or escarpment and reduces gradually withdistance from the crest. Also, locally k3 reducesfrom the base of a structure to its top.

C5.3.4A belt of approximately 60 km width near seacoast in certain parts of the country is identified

to be affected by cyclonic storms. The peak windspeeds in these regions may exceed 70 m/s.Therefore, factor k4 has been introduced with amaximum value of 1.30. However, the highestvalue may be used only for structures of post-cyclone importance such as cyclone shelters,

hospitals, school and community buildings,communication towers, power-plant structures,and water tanks, while a lower value of 1.15 maybe used for industrial structures, damage to whichcan cause serious economic losses. For reasons ofeconomy, other structures may be designed for a

k4value of unity, that is, without considering theeffect of the possible higher wind speeds in

cyclonic storms.For non-cyclonic regions, the factor k4 shallobviously be taken as 1.0.



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CODE COMMENTARY

5.4 Design Wind Pressure

The wind pressure at any height above meanground level shall be obtained by thefollowing relationship between wind pressureand wind speed:

2zV0.6zp =

wherepz= wind pressure in N/m

2at height z, and

Vz= design wind speed in m/s at height z.

The design wind pressure pdcan be obtained

as,

pd= Kd. Ka. Kc. pz

where

Kd = Wind directionality factorKa = Area averaging factorKc = Combination factor (See 6.2.3.13)

NOTE 1 The coefficient 0.6 (in SI units) in theabove formula depends on a number offactors and mainly on the atmosphericpressure and air temperature. The valuechosen corresponds to the average Indianatmospheric conditions.

NOTE 2 Ka should be taken as 1.0 whenconsidering local pressure coefficients.

5.4.1 Wind Directionality Factor, KdConsidering the randomness in thedirectionality of wind and recognizing the factthat pressure or force coefficients aredetermined for specific wind directions, it isspecified that for buildings, solid signs, open

signs, lattice frameworks, and trussed towers(triangular, square, rectangular) a factor of0.90 may be used on the design windpressure. For circular or near circular formsthis factor may be taken as 1.0.

For the cyclone affected regions also, thefactor Kdshall be taken as 1.0.

C5.4

The relationship between design wind speed Vzand the pressure produced by it assumes the massdensity of air as 1.20 kg/m

3, which changes

somewhat with the atmospheric temperature andpressure.

To obtain the design wind pressure, variousmodifications through factors Kd, Kaand Kcare to

be applied. These factors are explained infollowing Sections.

C5.4.1 -The factor recognizes the fact of (i) reducedprobability of maximum winds coming from anygiven direction (ii) reduced probability of the

maximum pressure coefficient occurring for anygiven wind direction.

This factor has not been included in the 1987version of the Code. Some of the other Codes(ASCE/Australian) give varying values of thefactor for different situations based on a more

detailed study of wind directionality. A flat valueof 0.9 has been used in the present revision exceptfor circular, near circular and axisymmetricsections which offer a uniform resistance,irrespective of the direction of wind. These havebeen assigned a value of 1.0 for the factor Kd.



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CODE COMMENTARY

5.4.2

Area Averaging Factor , KaPressure coefficients given in Section 6.2 area result of averaging the measured pressurevalues over a given area. As the areabecomes larger, the correlation of measuredvalues decrease and vice-versa. Thedecrease in pressures due to larger areasmay be taken into account as given in Table4.

Table 4: Area averaging factor (Ka)[Clause 5.4.2]

TributaryArea (A) (m

2)

AreaAveragingFactor (Ka)

10 1.0

25 0.9

100 0.8

5.5 Offshore Wind Speed

Cyclonic storms form far away from the seacoast and gradually reduce in speed as theyapproach the sea coast. Cyclonic stormsgenerally extend up to about 60 kilometersinland after striking the coast. Their effect onland is already reflected in basic wind speedsspecified in Fig.1. The influence of windspeed off the coast up to a distance of about200 kilometers may be taken as 1.15 timesthe value on the nearest coast in theabsence of any definite wind data.

C5.4.2 -

It is well recognized that the incoming windbecomes increasingly un-correlated as the areaconsidered increases. This would naturally lead toa lack of correlation amongst pressures inducedby the wind impinging on a surface, pressuresbeing directly proportional to the square of the

wind speed. In fact, the lack of correlationamongst the pressures gets modified because ofthe generation of local eddies and the distortion ofthose contained in the incoming wind, as the windflows past a surface. The reduced correlation isdeemed to be accounted for by introducing thearea reduction factor, to be used as a multiplier

for the pressures/forces occurring on the structure.The area to be considered for any part of thebuilding for computing the area reduction factor,Ka, shall be the surface area from which the windpressures/forces get transferred to theelement/part of the structure being designed. Thisarea is defined as the tributary area for theelement/part of the structure. Thus, as anexample, the tributary area will be smaller for a

purlin as compared to that for a roof truss or aframework.

Conversely, near the edges and corners of astructure, there are local area effects. Because of

separation of the flow at the edges and thecorners, suctions are experienced at these

locations, which can be quite high, though thearea of influence of such suction peaks isexpected to be small. The magnitude of thesesuctions can be greatly influenced by thegeometry of the structure and the angle of windincidence. Local area effects are already being

taken into account in the 1987 version of theCode for the design of the cladding and itsconnections to the supporting framework. Theseshould not be used for calculating the forces on

the roof or the framework as a whole.

C5.5 -

The cyclonic storms are formed away from thecoasts and have wind speeds much higher thanrecorded on the coasts. At least 15% higher windspeed than at the coast may be considered for

distances upto about 200 kilometers into the seain the affected regions.



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CODE COMMENTARY

6. WIND PRESSURESAND FORCES ONBUILDINGS/STRUCT-URES

6.1 GeneralWind load on a building shall be calculatedfor:

a) The building as a whole,

b) Individual structural elements such asroofs and walls, and

c) Individual cladding units includingglazing and their fixtures.

C6.1 -A major purpose of the Code is to determineforces and pressures on components of a building

or a structure as required for design purposes. Forclad buildings, pressures on the cladding arerequired in order to design the cladding and itssupporting elements, from which the forces get

transferred to the framework. Thus the buildingframe experiences the cumulative effect of

pressures produces forces on different parts of thecladding both on the walls as well as the roof asthe case may be. These forces are used indesigning the framework. The Code providesvalues of pressure coefficients for a variety ofcases covered. Besides, force coefficients are

given for (i) clad buildings and (ii) uncladstructures and (iii) elements. These coefficientscan be used to determine forces on an element, oran assembly of members or a framework. Localpressure coefficients given in Section 6.2.3.3 can

be used for individual cladding units includingtheir fixtures.

Both pressure and force coefficients are derived

on the basis of models tested in wind tunnels.

6.2- Pressure Coeffic ients

The pressure coefficients are always givenfor a particular surface or part of the surfaceof a building. The wind load acting normal toa surface is obtained by multiplying area ofthe surface or its appropriate portion by the

pressure coefficient (Cp) and thecorresponding design wind pressure. Theaverage values of these pressure coefficientsfor some building shapes are given inSections 6.2.2 and 6.2.3.

Areas of high local pressure orsuction frequently occur near the edges ofwalls and roofs. In addition, higher valuesmay also be experienced on small (local)areas of walls. Coefficients for these aregiven separately for the design of cladding inSection 6.2.3.3. Coefficients for the localeffects should only be used for calculation offorces on these local areas affecting roof

C6.2/6.2.1Pressure Coefficients

Wind causes pressure or suction normal to the

surface of a building or structure. The nature andmagnitude of these pressures/suctions is

dependent upon a large number of variables,namely, the geometry, the nature of the incidentwind, direction of wind incidence, point of

separation etc., which determine the nature ofwind flow over or around a building/structure. Asmentioned in C 6.1.2, separation of the flow at

the edges and corners and formation of vorticesgenerates suctions, often large in magnitude. Thepressures caused are also often quite sensitive tochanges in geometry and the angle of windincidence. The most common approach to thedetermination of pressure distribution on

different building forms is to test geometricallysimilar rigid models in a simulated wind

environment in wind tunnels. This is generallycarried out by making point pressuremeasurements over the model and averaging the



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CODE COMMENTARY

sheeting, glass panels, individual cladding

units including their fixtures. They should notbe used for calculating forces on entirestructural elements such as roof, walls orstructure as a whole.

NOTE 1 The pressure coefficients given indifferent tables have been obtainedmainly from measurements on models inwind tunnels.

NOTE 2 For pressure coefficients for structuresnot covered here, reference may bemade to specialist literature on thesubject or advice may be sought fromspecialists in the subject.

NOTE 3 Influence of local values of suction orpressure may not be of muchconsequence for the overall safety of thestructure but can be a cause of localdamage to cladding or glazing. This inturn may have a chain effect and leadto much economic loss.

pressure values over a specified tributary area.

Early wind tunnel work did not recognize theimportance of simulating the boundary layerflow of wind and its characteristics, primarily theturbulence. However, there has been a realizationof the importance of such simulation over the last3-4 decades. The body of information that has

thus emerged is expected to better represent thewind effects expected in the field. The lack of

adequacy of the database, however, remainsbecause of the large variability involved both,with respect to the wind its structure anddirectionality - as well as the building geometry.

Typically, pressure coefficient contours over agable roof may be as seen in Figure C2.Obviously, it will be ideal to divide the roof intoa large number of zones to specify the pressuresfor each zone. This would increase accuracy butwill create difficulties in practical design work.

Making a coarser grid-work will lead to averagedout values such as in Figure C 3. The approachadopted in practice is to go by the latter and usearea averages which, in an overall analysis, maybe on the conservative side.

Pressure coefficients are commonly based on thequasi steady assumption, whereby the pressure

coefficient is taken to be the ratio of meanpressure measured over a point or pressureaveraged over a small tributary area divided by

the dynamic pressure (21 V2) for the mean speed

of incident wind. Here is the mass density of airand V the wind speed. The approach followed in

the present Indian Code as well as the proposedrevision (and several other Codes) is to take V asthe peak gust value. Some Codes use the meanwind speed averaged over a longer period. Thisapproach implicitly assumes that the fluctuationsin pressure follow directly those in the speed.

This of course may not be true, since the windturbulence gets modified as it approaches thestructure, and eddies form at separation.However, the method has the advantage ofsimplicity, though it may not be suitable for verylarge structures. This is for two reasons (i) theincreasing lack of correlations over an extendedarea, and (ii) the dynamics of a large structural

system.

Pressures are caused both on the exterior as wellas the interior surfaces, the latter being dependenton openings (or permeability) in the structure,mostly in the walls. The following sections,namely 6.2.2 and 6.2.3, respectively give values



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CODE COMMENTARY

of pressure coefficients for the interior and

exterior surfaces.

6.2.1 Wind Load on IndividualMembers

When calculating the wind load on individualstructural elements such as roofs and walls,and individual cladding units and their fittings,it is essential to take account of the pressuredifference between opposite faces of suchelements or units. For clad structures, it is,therefore, necessary to know the internalpressure as well as the external pressure.Then the wind load, F, acting in a directionnormal to the individual structural element orcladding unit is:

F = (Cpe Cpi) A pd

where

Cpe = external pressure coefficient,Cpi = internal pressure coefficientA = surface area of structural element or

cladding unit, andpd = design wind pressure

NOTE 1 - If the surface design pressure varieswith height, the surface areas of thestructural element may be sub-dividedso that the specified pressures are takenover appropriate areas.

NOTE 2 Positive wind load indicates the forceacting towards the structural element(pressure) and negative away from it(suction).



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(a)

(b)

Cpmean

Fig. C2 : Typical Contours of Pressure Coeffic ients over a Pitched Roof (a) Cp min(b) Cp

mean

Fig. C3 : Variation of ressure over a Pitched Roof ( ) and the Average Value (- -)



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CODE COMMENTARY

6.2.2 Internal PressureCoefficients Cpi

Internal air pressure in a building dependsupon the degree of permeability of claddingto the flow of air. The internal air pressuremay be positive or negative depending onthe direction of flow of air in relation toopenings in the building.

6.2.2.1

In case of buildings where the claddingspermit the flow of air with openings not morethan about 5 percent of the wall area butwhere there are no large openings, it isnecessary to consider the possibility of theinternal pressure being positive or negative.Two design conditions shall be examined,one with an internal pressure coefficient of+0.2 and other with an internal pressurecoefficient of 0.2.

The internal pressure coefficient isalgebraically added to the external pressurecoefficient and the analysis which indicatesgreater distress of the member, shall beadopted. In most situations a simpleinspection of the sign of external pressurewill at once indicate the proper sign of theinternal pressure coefficient to be taken fordesign.

Note: The term small permeability relates to theflow of air commonly afforded bycladdings not only through open windowsand doors, but also through the slitsround the closed windows and doors andthrough chimneys, ventilators andthrough the joints between roofcoverings, the total open area being lessthan 5 percent of area of the walls havingthe openings.

C6.2.2

Internal pressures are not influenced much by theexternal shape or geometry of the building but

are primarily a function of the openings in it.These can be positive or negative and have to be

combined algebraically with the external values,Cpe, to obtain the critical design combination.Internal pressures vary with the degree ofpermeability, specified herein as small, mediumand large. Small permeability implies upto 5%openings and may be deemed to occur even with

doors and windows closed, since flow can takeplace through slits and recesses in doors,

windows, etc. Buildings with one large opening

may be treated as per Fig. 2 in the Code.

6.2.2.2

Buildings with medium and large openings -Buildings with medium and large openingsmay also exhibit either positive or negativeinternal pressure depending upon the

direction of wind. Buildings with medium



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CODE COMMENTARY

openings between about 5 to 20 percent of

the wall area shall be examined for aninternal pressure coefficient of + 0.5 andlater with an internal pressure coefficient of 0.5, and the analysis, which produces greaterdistress in the members, shall be adopted.Buildings with large openings, that is,openings larger than 20 percent of the wallarea shall be examined once with an internalpressure coefficient of + 0.7 and again withan internal pressure coefficient of 0.7, andthe analysis, which produces greater distressin the members, shall be adopted.

Buildings with one open side or openingsexceeding 20 percent of the wall area maybe assumed to be subjected to internalpositive pressure or suction similar to thosefor buildings with large openings. A fewexamples of buildings with one-sidedopenings are shown in Fig.2 indicatingvalues of internal pressure coefficients withrespect to the direction of wind.

In buildings with roofs but no walls, the roofswill be subjected to pressure from both insideand outside and the recommendations are asgiven in 6.2.3.2.

6.2.3 External Pressure Coeff ic ients C6.2.3-It has been explained in C 6.2/6.2.1 as to howpressure coefficients are obtained. Since thepresent version of I.S. 875 (Part 3)-1987 waswritten, there have been further studies of windeffects on low buildings. These have furtherunderlined the influence of wind incidence angle

particularly on edges and corners. Recent versionsof some international Codes have been revised onthe basis of these studies. However, acomparative analysis has shown that the overalldesign values as obtained by the present IS Code

do not differ by significant enough extent towarrant a revision of these coefficients.Furthermore, though the revised internationalCodes have become more elaborate, and, also alittle more accurate, these have also becomesomewhat more complex to use. Most part of thissection has therefore been retained as it occurs in

the current Code.

6.2.3.1

Walls - The average external pressurecoefficient for the walls of clad buildings ofrectangular plan shall be as given in Table 5.

In addition, local pressure concentrationcoefficients are also given.

C6.2.3.1Table 5 provides mean pressure coefficients forwalls of closed rectangular buildings with

different aspect ratios. Local pressure coefficients

at the edges of the wall, which have relevance tothe design of the cladding and its connections to



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CODE COMMENTARYthe supporting framework, are also given in theTable. Information on force coefficients for freestanding walls is given separately in 6.3.2.2.

6.2.3.2

Pitched, Hipped and Monoslope Roofs ofRectangular Clad Buildings The averageexternal pressure coefficients and pressureconcentration coefficients for pitched roofs ofrectangular clad buildings shall be as given inTable 6. Where no pressure concentrationcoefficients are given, the averagecoefficients shall apply. The pressurecoefficients on the underside of anyoverhanging roof shall be taken in

accordance with 6.2.3.5.NOTE 1 - The pressure concentration shall be

assumed to act outward (suctionpressure) at the ridges, eaves, cornicesand 90 degree corners of roofs.

NOTE 2 - The pressure concentration shall not beincluded with the net external pressurewhen computing overall load.

NOTE 3 For hipped roofs, pressure coefficients(including local values) may be taken onall the four slopes, as appropriate fromTable 6, and be reduced by 20% for thehip slope.

For monoslope roofs of rectangular cladbuildings, the average pressure coefficientand pressure concentration coefficient formonoslope (lean-to) roofs of rectangular cladbuildings shall be as given in Table 7.

C6.2.3.2 This clause provides information on the roofs ofclad buildings, which are perhaps the most

commonly used. Table 6 gives pressurecoefficients for pitched roofs with different aspectratios and varying roof pitch for two directions ofwind incidence - 0o and 90o. The roof surface isdivided into different zones for the purpose ofspecifying the design pressure coefficients. Thevalues on the leeward slope are not affected muchby the variations in geometry, which is not so for

the windward slope where values vary from large

pressures to suctions. Local pressure coefficients(for the design of cladding and its connections) atthe edges and ridge are also given these actupwards, i.e., suction. It is now known that winddirections other than 0o and 90o can give valueshigher than those at 0oand 90o. However, values

are given here for 0oand 90

oonly, for simplicity

in design.

For monoslope and hipped roofs also the pressure

coefficients can be taken from Table 6, for theapplicable roof slope. It has, however, been

shown that hipped roofs experience smaller

suction as compared to pitched roofs ofcorresponding geometry (see Fig. C4 and alsoC5). Thus a relief of 20% is being permitted inthe pressure coefficients for the hipped slopes.

Furthermore, clad buildings with monoslope roofsare covered in detail in Table 7. Pressurecoefficients for different angles of wind incidenceare given therein.



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Fig. C 4: Worst Peak Negative Pressure Coeffic ients all azimuths (Meecham 1992)

Wind

Wind

Wind

(c) Pyramidal roofs gets lowest uplift

(b) Hip roofs gets lower uplift

(a) Gable ended roofs gets high uplift

high uplift

Low upliftHip endPushed

Push

Lowest uplift

Fig. C 5 : Effects of Roof Architecture on Uplifts



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(a) For (b/d) > 1 (b) For (b/d) < 1

(c) For (b/d) = 1, use average values

+0.8

d

bWind

-0.5

-0.4

-0.7

-0.3

+0.8

d

b

Arrows indicate direction of wind.

Figure 2: Large opening in buildings (values of coefficients of internal pressure) withtop closed [Clause 6.2.2.2]



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Table 5 External Pressure Coefficients (Cpe) for Walls of Rectangular Clad Buildings(Clause 6.2.3.1)

6>w

h

W

W

W

W

W

W

W

W

W

W

WW

W



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Table 6 External Pressure Coefficients (Cpe) for Pitched Roofs of Rectangular Clad Buildings

Roof Angle Wind angle 0

oWind angle

90o

BuildingHeightRatio Degrees EF GH EG FH

2

1

w

h

05

1020304560

-0.8-0.9-1.2-0.4

0+0.3+0.7

-0.4-0.4-0.4-0.4-0.4-0.5-0.6

-0.8-0.8-0.8-0.7-0.7-0.7-0.7

-0.4-0.4-0.6-0.6-0.6-0.6-0.6

-2.0-1.4-1.0-0.8

2

3

w

h

2

1 <

05

1020304560

-0.8-0.9

-1.1

W

-0.7-0.2+0.2+0.6

-0.6-0.6

-0.6-0.5-0.5-0.5-0.5

-1.0-0.9

-0.8-0.8-0.8-0.8-0.8

-0.6-0.6

-0.6-0.6-0.8-0.8-0.8

-2.0-2.0

-2.0-1.5-1.0

h

W

6w

h

2

3


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NOTE 1 h is the height to eaves or parapet and w is the lesser plan dimension of a building.NOTE 2 Where no local coefficients are given, the overall coefficients apply.NOTE 3 For hipped roofs the local coefficient for the hip ridge may be conservatively taken as the central ridge value.

w

WIND

E G

F H

y y

y

L

KEY PLAN

Y = h or 0.15 W,whichever is the smaller



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CODE COMMENTARY

6.2.3.3 Canopy roofs with - The pressurecoefficients are given in Tables 8 and 9separately for mono pitch and double pitchcanopy roofs such as open-air parkinggarages, shelter areas, outdoor areas,railway platforms, stadiums and theatres.The coefficients take account of thecombined effect of the wind exerted on andunder the roof for all wind directions; theresultant is to be taken normal to the canopy.Where the local coefficients overlap, thegreater of the two given values should betaken. However, the effect of partial closuresof one side and / or both sides, such as thosedue to trains, buses and stored materialsshall be foreseen and taken into account.

The solidity ratio is equal to the area of

obstructions under the canopy divided by thegross area under the canopy, both areas

normal to the wind direction. = 0

represents a canopy with no obstructions

underneath. = 1 represents the canopy

fully blocked with contents to the downwindeaves. Values of Cpfor intermediate soliditiesmay be linearly interpolated between thesetwo extremes, and applied upwind of theposition of maximum blockage only.

Downwind of the position of maximumblockage the coefficients for = 0 may be

used.

In addition to the pressure forces normal tothe canopy, there will be horizontal loads onthe canopy due to the wind pressure on anyfascia and due to friction over the surface ofthe canopy. For any wind direction, only thegreater of these two forces need be takeninto account. Fascia loads should becalculated on the area of the surface facingthe wind, using a force coefficient of 1.3.

Frictional drag should be calculated using thecoefficients given in 6.3.1.

C6.2.3.3

Tables 8 and 9 give pressure coefficients for the

limited ratios of h/w (41 to 1) and L/W (1 to 3) for

free standing canopies for the roof slope varyingbetween 0oand 30o. Positive values are not affected

by the blockage under the roof while the suctions

(negative values) are given for two cases, = 0 forno blockage and = 1 for full blockage.



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Table 7 External Pressure Coefficients (Cpe) for Monoslope Roofs for Rectangular

Clad Buildings with 2 0.6

a/l > 0.6


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CODE COMMENTARY

6.2.3.7 Cylindrical structures

For the purpose of calculating the wind

pressure distribution around a cylindricalstructure of circular cross-section, the valueof external pressure coefficients given inTable 13 may be used provided that theReynolds number is greater than 10,000.These may be used for wind blowing normalto the axis of cylinders having axis normal tothe ground plane (that is, chimneys and silos)and cylinders having their axis parallel to theground plane (that is, horizontal tanks)provided that the clearance between the tankand the ground is not less than the diameterD of the cylinder.

his height of a vertical cylinder or length of ahorizontal cylinder. Where there is a free flowof air around both ends, h is to be taken ashalf the length when calculating h/D ratio.

In the calculation of the resulting pressure onthe periphery of the cylinder, the value of Cpishall be taken into account. For open-endedcylinders, Cpishall be taken as follows:

a) -0.8 where h/D is not less than 0.3,and

b) 0.5 where h/D is less than 0.3.

C6.2.3.7

Wind effects on cylindrical structures are influenced

by the Reynolds Number, Re given by VD/, whereV is the velocity of wind, D the diameter, and thekinematic viscosity of air. The values given in theCode are for Re greater than 10,000, a valuecommonly achieved in practice. These are given fordifferent proportions of a cylinder, and values of Cpiare specified for open ended cylinders.

Slender cylinders, such as those with h/D greater

than 5 may experience aerodynamic effects in thealong-wind as well as across-wind direction. Theseare dealt with later in Sections 8 and 9.

6.2.3.8 Roofs and bottoms of cylindricalstructures - The external pressurecoefficients for roofs and bottoms ofcylindrical elevated structures shall be asgiven in Table 14. For details of roof pressuredistribution see Fig. 5.

The total resultant load (P) acting on the roofof the structure is given by the followingformula:

P = 0.785 D2 (pi Cpepd)

where pi is the pressure inside the tankcaused by the stored fluid vapours.

The resultant of Pfor roofs lies at 0.1 Dfromthe center of the roof on the windward side.

C6.2.3.8

The clause specifies forces on roofs over acylindrical structure, placed on ground or elevated.The roof may be flat, sloping or domical. WhileTable 14 gives the overall force coefficients, detailed

pressure distribution over a conical roof is given inFig. 5.

In addition to the external pressures/forces, internalpressure may also occur on the roof of a container.This may be due to the vapour of the liquid stored, ordue to wind where there is a degree of permeability

to allow entry to the wind. Cpi should be taken aszero for an R.C.C. water tank, as the roof is made

monolithic with the walls and the opening in roof isalways kept closed.


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Table 13: External pressure distribut ion coefficients around cylindrical structures

[Clause 6.2.3. 7]

D

hCpe

h/D = 25h/D = 7

h/D = 1

Wind

_

D

Pressure Coefficients, CpePosition of

Periphery (degrees)

h/D= 25 h/D= 7 h/D= 1

0 1.0 1.0 1.0

15 0.8 0.8 0.8

30 0.1 0.1 0.1

45 -0.9 -0.8 -0.7

60 -1.9 -1.7 -1.2

75 -2.5 -2.2 -1.6

90 -2.6 -2.2 -1.7

105 -1.9 -1.7 -1.2

120 -0.9 -0.8 -0.7

135 -0.7 -0.6 -0.5

150 -0.6 -0.5 -0.4

165 -0.6 -0.5 -0.4

180 -0.6 -0.5 -0.4



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Table 14: External pressure coefficients for roofs and bottoms of cylindricalbuildings [Clause 6.2.3. 8]

h H

D

Cpe

Cpe

Cpe

P

e = 0.1 DDirection

of wind

(c) Conical roof

(a) Flat roof

(b) Curved roof

H

D

Cpe

Cpe

P

e = 0.1 D

(d) Elevatedstructure

Z

Coefficient of External Pressure, Cpe

Roof shape / Bottom of Elevated Structure

a, b and c d

H/D Roof (Z/H)-1 Roof Bottom0.5 -0.65 1.00 -0.75 -0.8

1.00 -1.00 1.25 -0.75 -0.7

2.00 -1.00 1.50 -0.75 -0.6



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a

0.5

1.01.5

h0.2D< h< 3.D

tan < 0.2

(< 11.5o)

D

SECTION AA


a = 0.5Dfor 2< (h/D)


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Table 15: External pressure coeffi cients , Cpefor combined roofs. [Clause 6.2.3.9]

0.8

0.6

0.4

0.20.13

- 0.2

- 0.4

- 0.6

- 0.8

- 1.0

Direction Direction 130o

b1b2c d

b

a

b1 b2

h2h1

a

Cpe= 0.4 (h1/h2 )-0.6

0.5 1.0 1.5 1.8 2.0 2.5 3.0 3.5 h1/h2C e

Cpe= (h1/h2)-1.7a

a

Direction 1 Direction 2c d

ba

h2

h1e

1.2

Cpe= 2(h1/h2)-2.9

1 2

Values of Cpe

6.1.1.1 Portion 6.1.1.2 Direction 1 Direction 2

a From the Diagram

bCpe= -0.5, (h1/h2) 1.7

Cpe= +0.7, (h1/h2) > 1.7

-0.4

c and d See 6.2.3.2

e See 6.2.3.5



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Table 16: External pressure coeffi cients , Cpefor roofs w ith a skylight. [Clause 6.2.3.10 ]

Wind-0.6

+0.4 -0.6-0.5

Wind a -0.6

-0.8

-0.5b

h2 h1

b1 b2

b1> b2 b1b2

Portion a b a and b

Cpe -0.6 +0.7 See Table 15



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CODE COMMENTARY

6.2.3.9 Combined roofs - The averageexternal pressure coefficients for combinedroofs are shown in Table 15

C