wind load introductio

7/26/2019 Wind Load Introductio

http://slidepdf.com/reader/full/wind-load-introductio 1/22

1.0 AN INTRODUCTORY NOTE

1.1 GENERAL

Wind flow generation is on account of atmospheric pressure differentials and

manifests itself into various forms, such as,

• Gales and monosonic winds

• Cyclones/Hurricanes/Typhoons

• Tornados

• Thunderstorms

• Localised storms

Photographs in f igure- depict some of these storms!

"riction from the earth#s surface leads to #$oundary layer# flow, $ut characteristicsof flow vary depending upon the storm type

!



(a) Cyclonic Storms

(b) Thunderstorm

1. Water submergence; 2,,!,". Wa## $amage an$ r%%& b#%'n a'a(; ). T%'er c%##a*se, +.

a-#ure %& a t%'er; . U*r%%te$ tree $amages nearb( bu-#$-ng

2.0 Structure of Wind

Wind is a randomly varying dynamic phenomenon and a trace of velocityversus time for wind will $e typically as shown in figure %! The wind velocity & can $eseen as a mean plus a fluctuating component responsi$le for creating #gustiness#!Within the earth#s $oundary layer, $oth components not only vary with height, $ut alsodepend upon the approach terrain and topography, as seen from figure '! Whiledealing with rigid structures, the consideration of the #e(uivalent static# wind is



ade(uate! However, in dealing with wind-sensitive fle)i$le structures, theconsideration of the wind-energy spectrum, integral length scale, averaging timeand the fre(uencies of the structure $ecome important! The determination of windvelocity for a certain geographical location is essentially a matter of statisticalreduction of a given measured data! *n this depend the various wind +ones! nother important decision involved is the averaging time! n as far as averaging time isconcerned, it may $e anywhere from .- seconds to 0 minutes to an hour! Theinfluence of averaging time on velocity is seen in figure 1!

3.0 INDIAN STANDARD CODE FOR WIND ANALYSIS – IS 875-PART.3-1987

3.1 Genera

Wind is air in motion relative to the surface of the earth! The primary cause of wind istraced to earth2s rotation and differences in terrestrial radiation! The radiation effects aremainly responsi$le for convection current either upwards or downwards! The windgenerally $lows hori+ontal to the ground at high speeds! 3ince vertical components of atmospheric motion are relatively small, the term 4wind2 denotes almost e)clusively thehori+ontal wind while 4vertical winds2 are always identified as such! The wind speeds areassessed with the aid of anemometers or anemographs, which are installed atmeteorological o$servatories at heights generally varying from 0 to 0 meters a$oveground!

&ery strong winds are generally associated with cyclonic storms, thunderstorms, duststorms or vigorous monsoons! feature of the cyclonic storms over the ndian region isthat they rapidly wea5en after crossing the coasts and move as depressions/ lowsinland! The influence of a severe storm after stri5ing the coast does not, in generale)ceed a$out '0 5ilometers, though sometimes, it may e)tend even up to .05ilometers! &ery short duration hurricanes of very high wind speeds called Kal Baisakior Norwesters occur fairly fre(uently during summer months over 6orth 7ast ndia!

The response of a $uilding to high wind pressures depends not only upon thegeographical location and pro)imity of other o$structions to airflow $ut also upon thecharacteristics of the structure itself!

The effect of wind on the structure as a whole is determined $y the com$ined action ofe)ternal and internal pressures acting upon it! n all cases, the calculated wind loads actnormal to the surface to which they apply!

The sta$ility calculations as a whole shall $e done with and without the wind loads onvertical surfaces, roofs and other parts of the $uilding a$ove average roof level!

!.0 WIND SPEED AND PRESS"RE

!.1 - Na#$re %& '(n) (n A#*%+,ere

n general, wind speed in the atmospheric $oundary layer increases with height from+ero at ground level to a ma)imum at a height called the gradient height! There isusually a slight change in direction 8Ekman effect 9 $ut this is ignored in the Code! Thevariation with height depends primarily on the terrain conditions! However, the windspeed at any height never remains constant and it has $een found convenient toresolve its instantaneous magnitude into an average or mean value and a fluctuatingcomponent around this average value! The average value depends on the averagingtime employed in analy+ing the meteorological data and this averaging time can $eta5en to $e from a few seconds to several minutes! The magnitude of fluctuatingcomponent of the wind speed, which represents the gustiness of wind, depends on the



averaging time! 3maller the averaging interval, greater is the magnitude of the windspeed!

!. – /a+( W(n) S,ee) (V b )

"igure gives $asic wind speed map of ndia, as applica$le at 0 m height a$ove meanground level for different +ones of the country! :asic wind speed is $ased on pea5 gust

speed averaged over a short time interval of a$out seconds and corresponds to 0mheight a$ove the mean ground level in an open terrain 8Category .9! :asic wind speedspresented in "ig! have $een wor5ed out for a %0-year return period! The $asic windspeed for some important cities/towns is also given in ppendi) ! of the code!



F($re 12 /a+( '(n) +,ee) (n *+ 4a+e) %n 50 6ear re#$rn ,er(%)

!.3 –De+(n W(n) S,ee) (V z )The $asic wind speed for any site shall $e o$tained from "ig! and shall $e modified to include

the following effects to get design wind speed, &+ at any height, ; for the chosen structure<



8a9 =is5 level,8$9 Terrain roughness and height of structure,8c9 Local topography, and8d9 mportance factor for the cyclonic region!t can $e mathematically e)pressed as follows<

V z = V b k 1 k 2 k 3 k 4,whereV z > design wind speed at any height + in m/s,k 1 > pro$a$ility factor 8ris5 coefficient9 8see %!!9,

k 2 > terrain roughness and height factor 8see %!!.9,k 3 > topography factor 8see %!!9, andk 4 > importance factor for the cyclonic region 8see %!!?9!6*T7< The wind speed may $e ta5en as constant upto a height of 0 m! However, pressures for $uildings less than 0mhigh may $e reduced $y .0@ for sta$ility and design of the framing!

!.3.1 – R(+ C%e&&((en# 41"ig! gives $asic wind speeds for terrain category . as applica$le at 0 m height a$ovemean ground level $ased on %0 years mean return period! The suggested life span to$e assumed in design and the corresponding k 1 factors for different class of structuresfor the purpose of design are given in Ta$le ! n the design of all $uildings andstructures, a regional $asic wind speed having a mean return period of %0 years shall$e used e)cept as specified in the note of Ta$le !



!.3. – Terra(n an) e(# Fa#%r 4Terrain – 3election of terrain categories shall $e made with due regard to the effect of o$structions which constitute the ground surface roughness! The terrain category usedin the design of a structure may vary depending on the direction of wind under consideration! Wherever sufficient meteorological information is availa$le a$out thewind direction, the orientation of any $uilding or structure may $e suita$ly planned!

Terrain in which a specific structure stands shall $e assessed as $eing one of thefollowing terrain categories<a9 Categor 1 A 7)posed open terrain with a few or no o$structions and in which the

average height of any o$Bect surrounding the structure is less than !% m!

6*T7 A This category includes open sea coasts and flat treeless plains!

$9 Categor 2 A *pen terrain with well-scattered o$structions having height generally$etween !% and 0 m!

6*T7 A This is the criterion for measurement of regional $asic wind speeds andincludes airfields, open par5lands and undeveloped sparsely $uilt-up outs5irts of townsand su$ur$s! *pen land adBacent to seacoast may also $e classified as Category . dueto roughness of large sea waves at high winds!

c9 Categor 3 A Terrain with numerous closely spaced o$structions having the si+e of $uilding-structures up to 0 m in height with or without a few isolated tall structures!

6*T7 A This category includes well-wooded areas, and shru$s, towns and industrialareas fully or partially developed!

6*T7 . A t is li5ely that the ne)t higher category than this will not e)ist in most designsituations and that selection of a more severe category will $e deli$erate!

6*T7 A Particular attention must $e given to performance of o$structions in areasaffected $y fully developed tropical cyclones! &egetation, which is li5ely to $e $lowndown or defoliated, cannot $e relied upon to maintain Category conditions! Wheresuch a situation e)ists, either an intermediate category with speed multipliers midway$etween the values for Category . and given in Ta$le . may $e used, or Category .$e selected having due regard to local conditions!



d9 Categor 4 – Terrain with numerous large high closely spaced o$structions!

6*T7 A This category includes large city centers, generally with o$structions taller than .% m and well-developed industrial comple)es!

?!!.!. A &ariation of wind speed with height for different terrains 85. factor9 A Ta$le .gives multiplying factor 85.9 $y which the $asic wind speed given in "ig! shall $emultiplied to o$tain the wind speed at different heights, in each terrain category!

?!!.! A Terrain categories in relation to the direction of wind A s also mentioned in

?!!.!, the terrain category used in the design of a structure may vary depending onthe direction of wind under consideration! Where sufficient meteorological information isavaila$le, the $asic wind speed may $e varied for specific wind directions

?!!.!? - Changes in terrain categories A The speed profile for a given terrain categorydoes not develop to full height immediately with the commencement of that terraincategory $ut develops gradually to height 8h)9 which increases with the fetch or upwinddistance 8)9!

a9 "etch and developed height relationship A The relation $etween the developedheight 8h)9 and the fetch length 8)9 for wind-flow over each of the four terrain categories

may $e ta5en as given in Ta$le !

$9 "or structures of heights greater than the developed height 8h)9 in Ta$le , the speedprofile may $e determined in accordance with the following<8i9 The less or least rough terrain, or

8ii9 The method descri$ed in ppendi) :


http://slidepdf.com/reader/full/wind-load-introductio 10/22!.3.3 – Topography (k 3 factor) –



The $asic wind speed V b given in "ig! ta5es account of the general level of site a$ovesea level! This does not allow for local topographic features such as hills, valleys, cliffs,escarpments, or ridges, which can significantly affect the wind speed in their vicinity!The effect of topography is to accelerate wind near the summits of hills or crests of cliffs, escarpments or ridges and decelerate the wind in valleys or near the foot of cliffs,steep escarpments, or ridges!

?!!! A The effect of topography will $e significant at a site when the upwind slope 89

is greater than a$out o

, and $elow that, the value of k 3 may $e ta5en to $e e(ual to !0!The value of k 3 is confined in the range of !0 to !' for slopes greater than o! method of evaluating the value of k 3 for values greater than !0 is given in ppendi) C! tmay $e noted that the value of k 3 varies with height a$ove ground level, at a ma)imumnear the ground, and reducing to !0 at higher levels, for hill slope in e)cess of 1o!

!.3.! – Importance Factor for Cyclonic Region (k 4 )

Cyclonic storms usually occur on the east coast of the country in addition to the GuBaratcoast on the west! 3tudies of wind speed and damage to $uildings and structures pointto the fact that the speeds given in the $asic wind speed map are often e)ceededduring the cyclones! The effect of cyclonic storms is largely felt in a $elt of appro)imately '0 5m width at the coast! n order to ensure greater safety of structuresin this region 8'0 5m wide on the east coast as well as on the GuBarat coast9, thefollowing values of 5? are stipulated, as applica$le according to the importance of thestructure<

3tructures of postAcyclone importance !0

ndustrial structures !%

ll other structures !00

!.! – De+(n W(n) Pre++$reThe wind pressure at any height a$ove mean ground level shall $e o$tained $y the followingrelationship $etween wind pressure and wind speed<

p+ > 0!' &+ .

where

!z > wind pressure in 6/m. at height +, and

&+ > design wind speed in m/s at height +!The design wind pressure pd can $e o$tained as,

!" = K " # K a# K c # !z

where

K " > Wind directionality factorK a > rea averaging factorK c > Com$ination factor 83ee '!.!!96*T7 A The coefficient 0!' 8in 3 units9 in the a$ove formula depends on a num$er of factors and mainlyon the atmospheric pressure and air temperature! The value chosen corresponds to the average ndianatmospheric conditions!

6*T7 . A K a should $e ta5en as !0 when considering local pressure coefficients!

!.!.1 – W(n) D(re#(%na(#6 Fa#%r: K d

Considering the randomness in the directionality of wind and recogni+ing the fact thatpressure or force coefficients are determined for specific wind directions, it is specified

that for $uildings, solid signs, open signs, lattice framewor5s, and trussed towers



8triangular, s(uare, rectangular9 a factor of 0!D0 may $e used on the design windpressure! "or circular or near A circular forms this factor may $e ta5en as !0!

"or the cyclone affected regions also, the factor K " shall $e ta5en as !0!

5.0 – W(n) L%a) %n In)(;()$a <e*er+

When calculating the wind load on individual structural elements such as roofs and walls, andindividual cladding units and their fittings, it is essential to ta5e account of the pressuredifference $etween opposite faces of such elements or units! "or clad structures, it is, therefore,

necessary to 5now the internal pressure as well as the e)ternal pressure! Then the wind load, ",acting in a direction normal to the individual structural element or cladding unit is<

$ = %C !e – C !i & ' !"

whereC !e > e)ternal pressure coefficient,C !i > internal pressure coefficient

' > surface area of structural element or cladding unit, and

pd / $es-gn '-n$ *ressure

6*T7 - f the surface design pressure varieswith height, the surface areas of the

structural element may $e su$-divided so that the specifiedpressures are ta5en over appropriateareas!

NOTE 2 %s-t-e '-n$ #%a$ -n$-cates

t3e &%rce act-ng t%'ar$s t3e structura#

e#ement 4*ressure5 an$ negat-e a'a(&r%m -t 4suct-%n5.

5. 1 In#erna Pre++$re C%e&&((en#+ C pi

nternal air pressure in a $uilding depends upon the degree of permea$ility of claddingto the flow of air! The internal air pressure may $e positive or negative depending on thedirection of flow of air in relation to openings in the $uilding

n case of $uildings where the claddings permit the flow of air with openings not more

than a$out % percent of the wall area $ut where there are no large openings, it is



necessary to consider the possi$ility of the internal pressure $eing positive or negative!Two design conditions shall $e e)amined, one with an internal pressure coefficient of E0!. and other with an internal pressure coefficient of A0!.!

The internal pressure coefficient is alge$raically added to the e)ternal pressurecoefficient and the analysis which indicates greater distress of the mem$er, shall $eadopted! n most situations a simple inspection of the sign of e)ternal pressure will atonce indicate the proper sign of the internal pressure coefficient to $e ta5en for design!

B(il"ings wit) me"i(m an" large o!enings -:uildings with medium and large openingsmay also e)hi$it either positive or negative internal pressure depending upon thedirection of wind! :uildings with medium

:uildings with one open side or openings e)ceeding .0 percent of the wall area may $eassumed to $e su$Bected to internal positive pressure or suction similar to those for $uildings with large openings! few e)amples of $uildings with one-sided openings areshown in "ig!. indicating values of internal pressure coefficients with respect to thedirection of wind!

n $uildings with roofs $ut no walls, the roofs will $e su$Bected to pressure from $othinside and outside !



6.0 PRO!"#S



$.0 CO%C!&S'O%

W-n$ 3as t'% as*ects. T3e &-rst 6 a bene&-c-a# %ne 6 -s t3at -ts energ( can be ut-#-7e$ t% generate *%'er, sa-#

b%ats an$ c%%# $%'n t3e tem*erature %n a 3%t $a(. T3e %t3er 6 a *aras-t-c %ne 6 -s t3at -t #%a$s an( an$ eer(%b8ect t3at c%mes -n -ts 'a(. T3e #atter -s t3e as*ect an eng-neer -s c%ncerne$ '-t3, s-nce t3e #%a$ cause$ 3as

t% be susta-ne$ b( a structure '-t3 t3e s*ec-&-e$ sa&et(. A## c--# an$ -n$ustr-a# structures ab%e gr%un$ 3ae

t3us t% be $es-gne$ t% res-st '-n$ #%a$s. T3-s -ntr%$uct%r( n%te -s c%ncern-ng t3e as*ect %& '-n$ eng-neer-ng

$ea#-ng '-t3 c--# eng-neer-ng structures.

S"!"CT" '!'OR*P+,

1. A3ma$, 93a:ee#, W-n$ *ressures %n #%' r-se 3-* r%%& bu-#$-ngs<. 3.D. T3es-s, C--# Eng-neer-ng De*artment,

Un-ers-t( %& R%%r:ee 4N%' In$-an Inst-tute %& Tec3n%#%g( R%%r:ee5, =a( 2000.2. >a-#e(, .A., an$ ?'%:, ?.C.9., Inter&erence E@c-tat-%n %& T'-n Ta## >u-#$-ngs<. %urna# %& W-n$ Eng-neer-ng an$

In$ustr-a# Aer%$(nam-cs, B%#. 21, 1", **. 2.

. >a(ar, D.C., Drag c%e&&-c-ents %& #att-ce$ t%'ers<. %urna# %& 9tructura# Eng-neer-ng, A9CE, B%#. 112, 1), **.

!1+!0.!. >%'en, A.., T3e *re$-ct-%n %& mean '-n$ s*ee$s ab%e s-m*#e 2D 3-## s3a*e<. %urna# %& W-n$ Eng-neer-ng an$

In$ustr-a# Aer%$(nam-cs, B%#. 1", 1, ** 2"220.

". C%%:, N.., T3e $es-gners gu-$e t% '-n$ #%a$-ng %& bu-#$-ng structures art 1<. >utter'%rt3s, %n$%n, 1".). Daen*%rt, A.F., Fust #%a$-ng &act%rs<. %urna# %& t3e 9tructura# D--s-%n, A9CE, B%#. , 1)+, **. 11!.+. Fum#e(, 9.., A *arametr-c stu$( %& e@treme *ressures &%r t3e stat-c $es-gn %& can%*( structures<. %urna# %& W-n$

Eng-neer-ng an$ In$ustr-a# Aer%$(nam-cs, B%#. 1), 1!, ** !").

. Fu*ta, Ab3a(, W-n$ tunne# stu$-es %n aer%$(nam-c -nter&erence -n ta## rectangu#ar bu-#$-ngs<. 3.D. T3es-s, C--#

Eng-neer-ng De*artment, Un-ers-t( %& R%%r:ee 4N%' In$-an Inst-tute %& Tec3n%#%g( R%%r:ee5, 1).. G%#mes, .D., =ean an$ &#uctuat-ng -nterna# *ressures -n$uce$ b( '-n$<. r%c. " t3 Internat-%na# C%n&erence %n W-n$

Eng-neer-ng, %rt C%##-ns, 1+, **. !"!"0.

10. G%#mes, .D., Recent $ee#%*ment -n t3e c%$-&-cat-%n %& '-n$ #%a$s %n #%'r-se structures<, r%c. As-aac-&-c

9(m*%s-um %n W-n$ Eng-neer-ng, R%%r:ee, In$-a, December 1", **. [email protected]. Gussa-n, =., an$ ee, >.E., A '-n$ tunne# stu$( %& t3e mean *ressure &%rces act-ng %n #arge gr%u*s %& #%' r-se

bu-#$-ngs<. %urna# %& W-n$ Eng-neer-ng an$ In$ustr-a# Aer%$(nam-cs, B%#. ), 10, ** 20+22".

12. I9 +" art661+

wind load introductio

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