Mat Chapter 19

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Mat Chapter 19
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  COLD FORMED STEEL SECTIONS-I COLD FORMED STEEL SECTIONS – I 1.0INTRODUCTION Thin sheet steel products are extensively used in uildin! industry and ran!e #ro$ purlins to roo# sheetin! and #loor dec%in!& 'enerally these are availale #or use as asic uildin! ele$ents #or asse$ly at site or as pre#aricated #ra$es or panels& These thinsteel sections are cold-formed,  i&e& their $anu#acturin! process involves #or$in! steelsections in a cold state (i&e& )ithout application o# heat* #ro$ steel sheets o# uniform thic%ness& These are !iven the !eneric title Cold Formed Steel Sections & So$eti$esthey are also called  Light Gauge Steel Sections  or Cold Rolled Steel Sections & Thethic%ness o# steel sheet used in cold #or$ed construction is usually 1  to 3  $$& Muchthic%er $aterial up to 8 mm  can e #or$ed i# pre-!alvanised $aterial is not re+uired #or the particular application& The $ethod o# $anu#acturin! is i$portant as it di##erentiatesthese products #ro$ hot rolled steel   sections& Nor$ally the yield stren!th o# steel sheetsused in cold-#or$ed sections is at least 280 N/mm 2 althou!h there is a trend to use steelso# hi!her stren!ths and so$eti$es as lo) as 230 N/mm 2 &Manu#acturers o# cold #or$ed steel sections purchase steel coils o# 1.0  to 1.25 m  )idth slit the$ lon!itudinally to the correct )idth appropriate to the section re+uired and then#eed the$ into a series o# roll #or$s& These rolls containin! $ale and #e$ale dies arearran!ed in pairs $ovin! in opposite direction so that as the sheet is #ed throu!h the$ itsshape is !radually altered to the re+uired pro#ile& The nu$er o# pairs o# rolls (called stages * depends on the co$plexity o# the cross sectional shape and varies #ro$ 5  to 15 &,t the end o# the rollin! sta!e a #lyin! shearin! $achine cuts the $e$er into the desiredlen!ths&,n alternative $ethod o# #or$in! is y press - ra%in! )hich is li$ited to short len!thso# around 6 m  and #or relatively si$ple shapes& In this process short len!ths o# strip are pressed et)een a $ale and a #e$ale die to #aricate one #old at a ti$e and otain the#inal re+uired shape o# the section& Cold rollin! is used )hen lar!e volu$e o# lon! products are re+uired and press rea%in! is used )hen s$all volu$e o# short len!th products are produced&'alvaniin! (or inc coatin!* o# the pre#or$ed coil provides very satis#actory protectiona!ainst corrosion in internal environ$ents& , coatin! o# 275 g/m 2  (total #or oth #aces* isthe usual standard #or internal environ$ents& This corresponds to inc coatin! o# .&./ mm & Thic%er coatin!s are essential )hen $oisture is present #or lon! periods o# ti$e&Other than !alvanisin! di##erent $ethods o# pre-rollin! and post-rollin! corrosion protection $easures are also used&0 Copyri!ht reserved Version II   19-1 19  COLD FORMED STEEL SECTIONS-I ,lthou!h the cold rolled products )ere developed durin! the First 1orld 1ar their extensive use )orld)ide has !ro)n only durin! the last 2. years ecause o# their versatility and suitaility #or a ran!e o# li!hter load earin! applications& Thus the )ideran!e o# availale products has extended their use to pri$ary ea$s #loor units roo# trusses and uildin! #ra$es& Indeed it is di##icult to thin% o# any industry in )hich ColdRolled Steel products do not exist in one #or$ or the other& 3esides uildin! industry they are e$ployed in $otor vehicles rail)ays aircra#ts ships a!ricultural $achinery electrical e+uip$ent stora!e rac%s house hold appliances and so on& In recent years )ith the evolution o# attractive coatin!s and the distinctive pro#iles that can e$anu#actured cold #or$ed steel construction has een used #or hi!hly pleasin! desi!nsin practically every sector o# uildin! construction& In this chapter the ac%!round theory !overnin! the desi!n o# cold #or$ed steel ele$entsis presented in a su$$ary #or$& Desi!n o# cold #or$ed steel sections are dealt )ith inIS4 5.6-6789 )hich is currently due under revision& In the asence o# a suitale Li$itState Code in India the Code o# :ractice #or Cold For$ed Sections in use in the ;&<&(3S979. :art 9 * is e$ployed #or illustratin! the concepts )ith suitale $odi#icationsappropriate to Indian conditions& 2.0ADVANTAGES OF COLD FORMED SECTIONS Cold #or$in! has the e##ect o# increasin! the yield stren!th o# steel the increase ein! theconse+uence o# cold )or%in! )ell into the strain-hardenin! ran!e& These increases are predo$inant in ones )here the $etal is ent y #oldin!& The e##ect o# cold )or%in! isthus to enhance the $ean yield stress y 15% - 30%.  For purposes o# desi!n the yieldstress $ay e re!arded as havin! een enhanced y a $ini$u$ o# 15%. So$e o# the $ain advanta!es o# cold rolled sections as co$pared )ith their hot-rolledcounterparts are as #ollo)s4 ã Cross sectional shapes are #or$ed to close tolerances and these can e consistentlyrepeated #or as lon! as re+uired& ã Cold rollin! can e e$ployed to produce al$ost any desired shape to any desiredlen!th& ã :re-!alvanised or pre-coated $etals can e #or$ed so that hi!h resistance tocorrosion esides an attractive sur#ace #inish can e achieved& ã ,ll conventional =ointin! $ethods (i&e& rivetin! oltin! )eldin! and adhesives* can e e$ployed& ã >i!h stren!th to )ei!ht ratio is achieved in cold-rolled products& ã They are usually li!ht $a%in! it easy to transport and erect&It is possile to displace the $aterial #ar a)ay #ro$ the neutral axis in order to enhancethe load carryin! capacity (particularly in ea$s*& Version II   19-2  COLD FORMED STEEL SECTIONS-I There is al$ost no li$it to the type o# cross section that can e #or$ed& So$e typicalcold #or$ed section pro#iles are s%etched in Fi!&6&In Tale 6 hot rolled and cold #or$ed channel section properties havin! the sa$e area o# cross section are sho)n& Fro$ Tale 6 it is ovious that thinner the section )alls thelar!er )ill e the correspondin! $o$ent o# inertia values (  I   xx  and  I   yy * and hence capaleo# resistin! !reater endin! $o$ents& The conse+uent reduction in the )ei!ht o# steel in!eneral applications produces econo$ies oth in steel costs as )ell as in the costs o# handlin! transportation and erection& This indeed is one o# the $ain reasons #or the popularity and the conse+uent !ro)th in the use o# cold rolled steel& ,lso cold #or$ steelis protected a!ainst corrosion y proper !alvanisin! or po)der coatin! in the #actoryitsel#& ;sually a thic%ness li$itation is also i$posed #or co$ponents li%e lippedchannels& Table - 1 Comparison of Hot Rolled and Cold Rolled sections  A1193 mm 2 1193 mm 2 1193 mm 2 1193 mm 2  I   xx 1.9 ×   10 6 mm 4 2.55 ×   10 6   mm 4 6.99 ×   10 6   mm 4 15.53 ×   10 6   mm 4  Z   xx 38   ×   10 3  mm 3 43.4 ×   10 3  mm 3 74.3 ×   10 3  mm 3 112 ×   10 3  mm 3  I   yy 0.299 ×   10 6   mm 4 0.47 ×   10 6   mm 4 1.39 ×   10 6   mm 4 3.16 ×   10 6   mm 4  Z   yy 9.1 ×   10 3  mm 3 11.9 ×   10 3  mm 3 22 ×   10 3  mm 3  33.4 ×   10 3  mm 3 Version II   19-3  Hot rolled c! el  100507.7  5 mm   #old rolled c! el 5 mm117.558.8 11.8 #old rolled c! el 3 mm18894   18.032 mm   #old rolled c! el    277.5138.827.8 Fig. 1 Tpical Cold Formed Steel !rofiles  COLD FORMED STEEL SECTIONS-I 1hile the stren!th to )ei!ht ratios otained y usin! thinner $aterial are si!ni#icantlyhi!her particular care $ust e ta%en to $a%e appropriate desi!n provisions to account #or the inevitale uc%lin! prole$s& 2.1 T!es o S#i ene$ %n$ Uns#i ene$ E&e'en#s ,s pointed out e#ore cold #or$ed steel ele$ents are either stiffened   or unstiffened  & ,nele$ent )hich is supported y )es alon! oth its lon!itudinal ed!es is called a stiffened  ele$ent& ,n unstiffened   ele$ent is one )hich is supported alon! one lon!itudinal ed!eonly )ith the other parallel ed!e ein! #ree to displace& Sti##ened and unsti##enedele$ents are sho)n in Fi!& 2&,n intermittentl stiffened element   is $ade o# a very )ide thin ele$ent )hich has eendivided into t)o or $ore narro) su ele$ents y the introduction o# inter$ediatesti##eners #or$ed durin! rollin!& In order that a #lat co$pression ele$ent e considered as a stiffened element  it should esupported alon! one lon!itudinal ed!e y the )e and alon! the other y a )e or lip or other ed!e sti##ener (e!& a end* )hich has su##icient #lexural ri!idity to $aintainstrai!htness o# the ed!e )hen the ele$ent uc%les on loadin!& , rule o# thu$ is that thedepth o# si$ple ?lips@ or ri!ht an!led ends should e at least one-#i#th o# the ad=acent plate )idth& More exact #or$ulae to assess the ade+uacy o# the sti##eners are provided inCodes o# :ractice& I# the sti##ener is ade+uate then the ed!e sti##ened ele$ent $ay etreated as havin! a local uc%lin! coe##icient (  $  value o# 4.0 & I# the ed!e sti##ener isinade+uate (or only partially ade+uate* its e##ectiveness is disre!arded and the ele$ent)ill e re!arded as unstiffened  #or purposes o# desi!n calculations& Version II   19-( &t'((e ed eleme t ) *t'((e ed eleme t  I termed'!te *t'((e er  I term'tte tly *t'((e ed    eleme t  Fig. Stiffened and #nstiffened elements  +dge *t'((e ed    eleme t  ,'*
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