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IS : 875 ( Part 3 ) - 1987 average value. The average value depends on of obstructions which constitute the ground sur ... the time employed in analysing the face roughness. The terrain category used in the meteorolcgical data and this averaging time desJgn of a structure may vary depending on the varies from a re'N seconds to several minutes. The direction of wind under consideration. Wherever magnitude of fluctuating component of the wind sufficient meteorological information is available "spc(;d which is called gust, depends on the aver... about the nature of wind direction, the orientation aging time. III building or structure may be suitably interval, greater is the magnitude of the gust planned. speed. 5.2 Basic Wind Speed _ Figure I give. basic wind speed map of India, as applicable to 10 m height above mean ground level for different zone. of the country. Basic wind .peed Is based on peak gust vcJo!=ity averaged over a short time interval of about 3 seconds and corresponds to mean heights above ground level in an open terr"in ( Category 2). Basic wind speeds presented in Fig. I /lave been worked out for a 50 year return period! Basic wind speed for some important cities/towns i. also given in Appendix A. 5.3 Design Wind Speed ( V. ) - The basic wind .peed ( Vb) for any site shall be obtained from Fig. I and shall be modified to include the following effects to get de.ign wind velocity at any height ( V.) for the chosen structure: a) Risk level; b) Terrain roughness, height and size of struc- ture; and c) Local topography. It can be mathematically expressed as follows: where , V. = Vb *1 KI *, V. = design wind speed at any height .c in mjs; k, - probability factor l risk coeffi· cient ) ( see 5.3.1 ); Jr. _ terrain, height and structure size factor ( see 5.3.2 ); and k. = topography factor ( see 5.3.3 ). NOTE - Design wirid .peep up to 10 III height from mean ground leveJ shall be considered constant. 5.3.1 Risk Coiflicient (*, FactoT) - Figure I give. basic wind speeds for terrain Category 2 as applicable at 10m above ground level based on 50 years mean return period. The suggested life period. to be assumed in design and the corres-- ponding k, factors for different class of structures for the purpose of design is given in Table I. In the design of all buildings and structures, a regional basjc wind speed having a mean return period of 50' years .hall be used except as specifi- ed in the note of Table I. 5.3.2 Terrain, Height and Structure Si;c. FactoT (k l FacloT ) 5.3.2.1 Tt"ain - Selection of terrain cate .. gori .. .shall be made with due regard to the effect 8 Terrain in which a specific structure stands shall be assessed as being one of the following terrain categories: a) Category I - Exposed open terrain with few or no obstructions and in which the ..... average height of any object surrounding the structure is less than 1'5 m. NOT. - This category includes open sea-coast. _ad Bat treeless plains. b) Caugory 2 - Open terrain with well scatt- ered obstructions baving heights generally between 1'5 to 10 m. NOTE - This is the criterion (or me83ure a ment of regional basic wind speeds and includes airfields, open parkland.! and undeveloped spar a sely built-up outskirts of towns and suburbs. Open land adjacent to sea coast may also be c1as.sified as Category 2 due to roughmss of large .ea waves at high windt. e) Category 3 - Tenain with numerous closely obstructions having the size of building-structures up to 10 m in height with or without a few isolated taU struc- tures. NOTB 1 - This category includes well wooded areas, .nd sbrubs. town. and industrial areas full or partially developed . "NoTJI 2 - It is likely that the next higher category than this will not exist in moat design situations and that selection of • more severe category will be deliberate. NOD 3 - Particular attention must be given to performance of obstructions in areas affected by fully developed tropical cyclones. Vegetation which is likely to be blown down or defoliated cannot be relied upon to maintain Category 3 conditions. Where luch .ituation may exist. either an inter- mediate category with velocity multipliers midway between the value. for Category 2 and 3 given in Table 2, .or Category 2 .hould be .elected having due regard to local conditioJ: •• d) Category 4 - Terrain with numerous large high closely spaced obstructions. NOTE - This category includes large city cen" tre., generaJly with obstructions above 25 m and well developed industrial complexe,. 5.3.2.2 Variation qf wind speed with heighl for different si.tes of structures in dijferml terrains ("'. factor) - Table 2 gives multiplying factors ( "I ) by which the basic wind speed given in Fig. I shall be multiplied to obtain the wind speed at different heights, in each terrain category for different sizes of buildings/structures.

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IS:875(Part 3)- 1987 averagevalue.Theaveragevaluedependsonofobstructionswhichconstitutethegroundsur... the timeemployedinanalysingthefaceroughness.Theterraincategoryusedinthe meteorolcgicaldataandthisaveragingtimedesJgnof astructuremay vary dependingonthe varies from are'Nseconds to several minutes.Thedirectionof windunderconsideration.Wherever magnitude of fluctuating componentofthewindsufficientmeteorologicalinformationisavailable " spc(;d which iscalledgust,depends onthe aver...aboutthe nature of wind direction, theorientation agingtime.III buildingorstructuremaybesuitably interval,greateristhemagnitudeofthegustplanned. speed. 5.2BasicWindSpeed _FigureIgive.basic windspeedmapofIndia, asapplicableto10m height above mean ground level fordifferent zone. of the country.Basicwind .peed Isbased onpeak gust vcJo!=ityaveragedover ashort timeinterval ofabout3secondsandcorrespondstomean heightsabovegroundlevelinanopenterr"in (Category2). Basicwindspeedspresentedin Fig.I/lavebeenworked out fora50year return period!Basicwindspeedforsomeimportant cities/towns i. alsogiveninAppendix A. 5.3DesignWindSpeed(V. )- Thebasic wind .peed(Vb)foranysiteshallbeobtained fromFig.Iand shallbemodifiedtoinclude the followingeffectstogetde.ignwindvelocityat any height (V.)for thechosen structure: a)Risk level; b)Terrain roughness,height and size of struc-ture;and c)Localtopography. It canbe mathematically expressed as follows: where , V.=Vb*1KI*, V.=designwind speedatany height .cinmjs; k,- probabilityfactorlriskcoeffi cient )(see5.3.1); Jr._terrain,heightandstructure size factor( see5.3.2 );and k.=topography factor(see5.3.3). NOTE- Design wirid .peep up to 10IIIheight from mean ground leveJ shall be considered constant. 5.3.1RiskCoiflicient(*,FactoT)- FigureI give.basicwindspeeds for terrainCategory2 as applicableat10m aboveground levelbased on 50 yearsmeanreturnperiod.Thesuggestedlife period.tobeassumedindesignandthecorres--pondingk,factorsfordifferentclass of structures forthe purpose of design isgiveninTableI.In thedesignofallbuildingsandstructures,a regionalbasjcwindspeedhaving ameanreturn period of 50' years.hall be usedexceptasspecifi-ed inthenote of TableI. 5.3.2Terrain,HeightandStructureSi;c.FactoT (kl FacloT) 5.3.2.1Tt"ain- Selectionofterraincate.. gori.. .shall be madewith due regardto the effect 8 Terraininwhichaspecificstructurestands shallbeassessedasbeing oneofthefollowing terraincategories: a)CategoryI- Exposedopenterrainwith fewornoobstructionsandinwhichthe .....averageheightof anyobjectsurrounding the structure islessthan1'5 m. NOT. - This categoryincludesopen sea-coast. _adBat treelessplains. b)Caugory2- Openterrain withwellscatt-eredobstructionsbavingheights generally between 1'5to10m. NOTE- Thisisthecriterion(orme83ureamentof regionalbasicwindspeedsandincludes airfields,openparkland.!andundeveloped sparaselybuilt-up outskirts of towns andsuburbs.Open land adjacentto sea coast may also be c1as.sifiedas Category 2 dueto roughmssof large .ea wavesat highwindt. e)Category3- Tenain with numerous closely obstructionshavingthesizeof building-structuresupto10minheight withorwithoutafewisolatedtaUstruc-tures. NOTB1- Thiscategory includeswellwooded areas,.ndsbrubs.town. and industrial areasfull or partially developed . "NoTJI2 - Itislikelythatthenexthigher categorythanthiswillnotexistinmoatdesign situationsandthatselectionofmoresevere category willbe deliberate. NOD 3- Particularattentionmustbegiven to performance ofobstructions in areas affected by fully developed tropical cyclones. Vegetation which is likelyto be blowndownordefoliated cannot be reliedupontomaintainCategory3conditions. Whereluch.ituationmayexist.eitheraninter-mediate category with velocitymultipliers midway between the value. forCategory 2and 3givenin Table 2, .or Category 2.hould be.electedhaving dueregard to local conditioJ: d)Category4- Terrain withnumerous large high closely spaced obstructions. NOTE- Thiscategory includes largecity cen" tre.,generaJly withobstructionsabove25mand well developed industrialcomplexe,. 5.3.2.2Variationqf windspeedwithheighl for differentsi.tesofstructuresindijfermlterrains("'. factor)- Table 2givesmultiplyingfactors("I) bywhichthebasicwindspeedgiveninFig.I shallbemultipliedtoobtainthewind speedat differentheights,ineachterraincategoryfor different sizes of buildings/structures.


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