IS: 875(Part3): Wind Loads On Buildings And Structures .

Code & Commentary IS 875 (Part 3)CODECOMMENTARY1. – Scope1.1 -C1.1 –This Standard gives wind forces and theireffects (static and dynamic) that should betaken into account while designing buildings,structures and components thereof.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 (onlystatic forces) is given in IRS and IRCSpecifications. For aerodynamics of bridges,specialist literature may be consulted. Withsubstantial work being done worldwide in the areaof wind engineering, there is growing body ofnew information. The user of this Code is advisedto consult specialist literature for the design oflarge or important projects involving varioustypes of structures.1.1.1–C1.1.1 –Wind causes a random time-dependent load,which can be seen as a mean plus afluctuating component. Strictly speaking allstructureswillexperiencedynamicoscillations due to the fluctuating component(gustiness) of wind. In short rigid structuresthese 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.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 meancomponent 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.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.1.1.2 –C1.1.2 –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 onstructural members, cables and ropes shallbe calculated by assuming an ice coveringThe construction period of a structure is muchsmaller than its expected life. Therefore, a smallerreturn period of 5 to 10 years or longer may beconsidered for arriving at the design factor (factork1) for construction stages/period of a structureIITK-GSDMA-Wind02-V5.0depending on its importance. In snowfall areaswhere icing occurs, wind loads have to be9IITK-GSDMA-Wind04-V3.0

Code & Commentary IS 875 (Part 3)CODECOMMENTARYbased on climatic and local experience.assessed accordingly. Elements such as cables andropes can undergo a dynamic response in suchcases and have to be examined accordingly.1.1.3–In 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:C1.1.3 – See C1.1IS: 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 chimneysIS: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 DesignIS:802 (Part 1)-1995 Code of practice for useof structural steel in overheadtransmission line towers: Part 1 Loadsand permissible stresses (secondrevision)IS:11504-1985 Criteria for structural designof reinforced concrete natural draughtcooling towersNOTE: 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 nd04-V3.0

Code & Commentary IS 875 (Part 3)CODECOMMENTARY2. – Notations2.1–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 Ae Az b Bs CD CL Cf Cfn Cft BC′f Cdyn Cp Cpe Cpi Cfs d D E f0 F Fn Ft F′ Surface area of a structure orpart of a structureEffective frontal areaFrontal contributory area atheight zBreadth of a structure orstructural member normal to thewind stream in the horizontalplaneBackground factorDrag coefficientLift coefficientForce coefficientNormal force coefficientTransverse force coefficientFrictional drag coefficientDynamic response factorPressure coefficientExternal pressure coefficientInternal pressure coefficientCross-wind force spectrumcoefficientDepth of a structure orstructural member parallel towind stream in the horizontalplaneDiameter of cylinder or sphere;Depth of structureWind energy factorFirst mode natural frequency ofvibrationForce on a surfaceNormal forceTransverse forceFrictional 0

Code & Commentary IS 875 (Part 3)CODEgR gv h hx hp Hs Ih IFk k1k2k3k4K Ka Kc Kd Km l L Le Lh N pd pz pe pi Re S Sr T Vb Vh Vz COMMENTARYPeak factor for resonantresponsePeak factor for upwind velocityfluctuationsHeight of structure above meanground levelHeight of development of aspeed profile at distance xdownwind from a change interrain categoryHeight of parapetHeight factor for resonantresponseTurbulence intensityInterference factorMode shape power exponentWindspeedfactorsNotations have been defined also in the text attheir first appearance. A few of the notations havemore than one definition, having been used fordenoting different parameters.multiplicationForce coefficient multiplicationfactor for members of finitelengthArea averaging factorCombination factorWind directionality factorMode shape correction factorLength of a member or greaterhorizontal dimension of abuildingActual length of upwind slopeEffective length of upwind slopeIntegral turbulence length scaleReduced frequencyDesign wind pressureWind pressure at height zExternal wind pressureInternal wind pressureReynolds numberSize reduction factorStrouhal numberFundamental time period ofvibrationRegional basic wind speedDesign wind speed at height hDesign wind speed at height zIITK-GSDMA-Wind02-V5.012IITK-GSDMA-Wind04-V3.0

Code & Commentary IS 875 (Part 3)CODEVz W W′ We X Z α β ε φ η θ COMMENTARYHourly mean wind speed atheight zLesser horizontal dimension ofa building in plan, or in thecross-sectionastructuralmember;Bay width in a multi-baybuilding;Equivalent cross-wind staticforceDistance downwind from achange in terrain category;fetch lengthHeight above average groundlevelInclination of roof to thehorizontal planeEffective solidity ratio; DampingratioAverage height of surfaceroughnessSolidity ratioShieldingfactororeddyshedding frequencyWind direction in plan from agiven axis; upwind ground / -V3.0

Code & Commentary IS 875 (Part 3)CODECOMMENTARY3. – TerminologyFor the purpose of this Code, the followingdefinitions shall apply.Angle of Attack / Incidence (α) - 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 inthe 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 ofthe 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 nondimensional 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 0

Code & Commentary IS 875 (Part 3)CODECOMMENTARYGround Roughness – The nature of theearth’s 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 themaximum 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 astructure 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, wherethe static and design wind pressures aredetermined at the height of the pointconsidered after taking into account theIITK-GSDMA-Wind02-V5.015IITK-GSDMA-Wind04-V3.0

Code & Commentary IS 875 (Part 3)CODECOMMENTARYgeographical 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 thestructure or structural element underconsideration. A factor called shieldingfactor is 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. Thecategories are numbered in increasingorder of roughness.Topography – The nature of the earth’ssurface as influenced by the hill andvalley configurations in the vicinity of theexisting / proposed 4-V3.0

Code & Commentary IS 875 (Part 3)CODECOMMENTARY4. – GENERALC4.1 -4.1 -For the purpose of this Code wind speed has beenconsidered as that occurring at 10 m height abovethe general ground level. Several new recordingstations 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.Wind is air in motion relative to the surfaceof the earth. The primary cause of wind istraced to earth’s 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’ arealways 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 –C4.2 -Very strong winds are generally associatedwith cyclonic storms, thunderstorms, duststorms or vigorous monsoons. A feature ofthe 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.Several atmospheric phenomena are responsiblefor wind storms. Cyclonic storms, that hit someof the coastal regions of India, are the mostdevastating 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 farexceed the wind speeds for design given in theCode at present, and addresses the problem vis-àvis 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 thesummer, mostly in Northern parts of India.These, however, have extremely high windspeeds, often higher than in the severest cyclones.C4.3 -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. Thesegusts cause increase in air pressure but theireffect on stability of the building may not beIITK-GSDMA-Wind02-V5.017Higher the intensity of a gust, lower is itsduration. The Code specifies the basic windspeed as that of a gust of 3 second duration; or inother words, the wind speed averaged over a 3second period. The effect of reduction in theaverage wind pressure with increase in the areaover which the pressure is considered (theIITK-GSDMA-Wind04-V3.0

Code & Commentary IS 875 (Part 3)CODECOMMENTARYso important; often, gusts affect only part ofthe building and the increased localpressures 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. Amaximum reduction of 20% in wind pressures isspecified for tributary area beyond 100 m2.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. Thearea of influence being small, there is bettercorrelation within these areas. These local areaeffects are treated elsewhere in the Code.C4.4 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.The dynamic characteristics of a flexible structuredefined by its time period of vibration anddamping 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 beenadded for limited cases for the first time in theCode to approximate these so called interferenceeffects in Section 7.C4.5 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.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 inmagnitude 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 4.6 –The stability calculations as a whole shall bedone with and without the wind loads onvertical surfaces, roofs and other parts of thebuilding above average roof

Code & Commentary IS 875 (Part 3) CODE COMMENTARY Foreword 0.1 This Indian Standard IS:875 (Part 3) (Third Revision) was adopted by the Bureau of Indian Standards on _(Date), aft