Stability Problems With Steel-Concrete Composite Construction_tcm45-341089 | Beam (Structure) | Concrete

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  B Y R AMON J. C OOK C HIEF E NGINEER , C USTOM F ORMS T HE B URKE C OMPANY C ONVERSE , T EXAS R ecent changes in the steel con-s t ruction and design codes al-l ow much lighter steel frames BE-FORE CONCRETE than pre v i o u s l y c o n s i d e red practical. This is doneby using thinner, narrower steelsections and by allowing highers t resses in them. I believe we areseeing more of these compositeframes in bridges and buildings be-cause designers are under incre a s-ing pre s s u re to create more eco-nomical stru c t u res; because theirstaffs are encouraged, in schoolsand elsewhere, to use the natura la d vantages of composite action;and because contractors continueto bid by the pound and yard in-stead of by the work required. Buildings and bridge in trouble In a recent 18-month period, I w o rked with contractors in the Ba l-t i m o re area on four buildings andone bridge where serious instability of unbraced steel was found duri n g e rection and concreting. All we re, inmy opinion, seriously life-thre a t e n-ing. In each case, no contractor per-sonnel on the job or in the officehad the slightest idea there was ap roblem. All jobs had been bid and we re being built the “same old way  we always did it.” In two cases thes t ru c t u re was actually coming d own, albeit slow l y, but coming d own just the same—during con-c reting. One was saved by a superi n-tendent halting work, getting at ruckload of screw-jack post shore s,and raising the falling frame back toa p p roximate gra d e. Another wasnot shored, but work was stopped,cold-jointed, and finished withg reatest care some days later. Bo t hbuildings have locally buckled floorb e a m s. A third job, a small tra p e-zoidal-section steel and concre t eg i rder was shored with towers andb raced internally until it wouldstand alone. Two other buildings we re shored before forming andc o n c reted without damage—exc e p tto the contra c t o r’s budget becausehe never guessed there would beany need to shore the steel when hebid the work . Bracing and shoring needed forconstruction load In each case the steel was loadedf i ve to seven times its unbraced andu n c o n c reted load rating. In no casedid the designer say so. In no casedid the designer tell the contra c t o rto bra c e, to shore, or even to havesomeone check out the frame be-f o re construction for the constru c-tion loads that would be applied. Ap p a rently each designer took refuge in the AISC (American In s t i-tute of Steel Co n s t ruction) “brace itif you have to” general note (Se c t i o n7.9.1 of the Code of St a n d a rd Pra c-t i c e ), and hoped the “other consid-e ra t i o n s” para g raph would take careof him (page 2-96 of the Manual of Steel Construction  ) .Neither of these AISC prov i s i o n s was sufficient to meet the pro b l e m son these five jobs. Co n c rete people, watch out! Steel people, check with your lawye r s. Telling contractors notto shore this work is the same astelling them not to brace it. To themit means the same. Also, re l e a s i n g steel design drawings without ex-t e n s i ve notes to the builders re g a rd-ing necessary bra c i n g — -when you k n ow the beams or columns cannot safely stand without being bra c e d —  is the same as loading and cocking a gun, then handing it to someone who doesn’t know what it is.I may well be starting the old ar-gument again over what constitutesa bra c e. But after more than 30 ye a r sin the construction business, I feelm o re strongly than ever that any en-gineer who cannot tell a work m a nin simple and stra i g h t f o rw a rd lan-guage what to bra c e, where to do it,and what to do it with, isn’t much of an engineer. Especially when he de-pends on a jobsite concrete super tof i g u re it out for him instead. Other construction found safe In addition to the five near- d i s a s-ters re p o rted, we have worked onbuildings with deep, stiff, we l d e d -d own corrugated steel decking  which was installed on light steelbeams before concreting, and foundthese eminently safe at all stages. References for more information 1. “Code of Standard Practice forSteel Buildings and Bridges,” Ameri-can Institute of Steel Construction,400 North Michigan Avenue, Chicago,Illinois 60611; 1976, 32 pages.2. Manual of Steel Construction,American Institute of Steel Construc-tion, Chicago, eighth edition, 1980.3. “Recommended Practice for Con-crete Formwork, ACI 347-78,” Ameri-can Concrete Institute, P.O. Box19150, Detroit, Michigan 48219.4. “Standard Specification for HighwayBridges,” American Association ofState Highway and Transportation Of-ficials, Washington, DC, eleventh edi-tion, 1973.5. Handbook of Composite Construc-tion Engineering, edited by GajananM. Sabnis, Van Nostrand Reinhold,New York, 1979, 380 pages. Guest editorial  Stability problems with steel-concretecomposite construction  UNDERSTANDINGCOMPOSITE CONSTRUCTION Composite construction is pro-duced by combining different ma-t e rials or stru c t u ral members insuch a way that the combinedp a rts act together as a singlem e m b e r. Composite constru c t i o ncan be a laye red wood system, acombination of cast-in-place withp recast concre t e, or wood andc o n c rete beams. But the mostcommon combination in build-ings and bridges is the steel-con-c rete composite in which a steelbeam and a re i n f o rced concre t eslab (either cast in place or pre-cast) are joined with shear con-nectors so that they act togetheras a unit. The steel beam may befully encased in concre t e, part i a l-ly encased, or placed below thes l a b. Sometimes the beam has ac o n c rete haunch above it. Theshear connectors may be studs,steel bars, or rolled shapes we l d e dto the top of the steel beam andembedded in the concrete slab.Composite construction be-came generally accepted by engi-neers for bridges during the 1950sand for buildings during the1 9 6 0 s. Du ring the past decade, re-s e a rch on continuous compositebeams and connections has led tog reater economy through the useof composite design in continu-ous stru c t u res to increase stre n g t hand stiffness. Shored or unshored? Composite construction may be either shored or unshore d .No rmally where steel beams areset in position and forms for thec o n c rete deck are supported onthem, the beams carry the deadload without any composite ac-tion. Composite action start s when the live load is applied. Ino rder to make the system com-posite for both dead load and liveload, it is necessary to support thesteel beam until the concrete hasenough strength to become partof the stru c t u re instead of just an-other load. This shoring keeps thesteel beam in a no-load conditionuntil the concrete has set. Thenthe supports are re m oved and thee n t i re composite section acts tos u p p o rt both the dead and livel o a d s.Sh o ring may be re q u i red by thes t ru c t u ral design, even when thes t ru c t u ral steel frame is able tos u p p o rt the weight of forms andf reshly placed concre t e. Sh o ring isf requently installed to control de-flection. This can be important forboth appearance and long-terms e rviceability of the stru c t u re. AISC standard provisions Recommendations of the A m e rican Institute of Steel Co n-s t ruction are directed pri m a rily todesigners and steel ere c t o r s, butthey can have an important bear-ing on concrete work, as the edito-rial points out. Section 7.9.1 of the AISC “ Code of St a n d a rd Pra c t i c e”  s a y s :Te m p o ra ry support s, suchas tempora ry guys, bra c e s,f a l s e w o rk, cribbing or otherelements re q u i red for thee rection operation will bed e t e rmined and furn i s h e dand installed by the ere c t o r.These tempora ry support s will secure the steel fra m i n g ,or any partly assembled steelf raming against loads com-p a rable in intensity to thosefor which the stru c t u re wasdesigned, resulting fro m wind, seismic forces ande rection opera t i o n s, but notthe loads resulting from thep e rf o rmance of work by orthe acts of others. . .Section 7.9.3 applies to steelf rames which re q u i re intera c t i o n with other elements not classifiedas stru c t u ral steel in order to haven e c e s s a ry stability. For this kind of f ra m e, the AISC Code says:Such frames shall be clear-ly identified in the contra c td o c u m e n t s. The contra c tdocuments specify the se-quence and schedule of placement of such elements.The erector determines theneed and furnishes and in-stalls the tempora ry support sin accordance with this infor-mation. The owner is re s p o n-sible for the installation andtimely completion of all ele-ments not classified as St ru c-t u ral Steel that are re q u i re dfor stability of the fra m e.The “other considera t i o n s”mentioned in the editorial aref rom the AISC Manual of St e e lCo n s t ruction. They include thef o l l ow i n g : Adequate lateral supportfor the compression flange of the steel section will be pro-vided by the concrete slab af-ter hardening. Du ring con-s t ruction, howe ve r, latera ls u p p o rt must be provided, or w o rking stresses must be re-d u c e d . . . Steel deck with ade-quate attachment to thec o m p ression flange, or pro p-erly constructed concre t ef o rm s, will usually prov i d ethe necessary lateral supportfor the type of constru c t i o ns h own...[in the accompany-ing design tables]. For con-s t ruction using fully encasedb e a m s, particular attentionshould be given to latera ls u p p o rt during constru c t i o n .Thus the AISC documents pro-vide some warnings about theneed for support s, but may some-times give a false sense of securi t y  with the statement that form w o rk usually provides the necessary s u p p o rt . Related ACI formworkrequirements  ACI 347-78, “Re c o m m e n d e dPractice for Fo rm w o rk ,” states inSection 5.3.5 that any tempora ry s h o res re q u i red for composite  c o n s t ruction should be shown inthe contract drawings or specifica-t i o n s. The warning is repeated inSection 5.4.1—re q u i rements fors h o ring or other deflection contro lshould be clearly presented by thee n g i n e e r- a rchitect in the specifi-c a t i o n s.Since the need for shoring andb racing is determined by the de-sign, the ACI guideline seems re a-s o n a b l e. The amount and kind of s h o ring and lateral bracing to sta-b i l i ze or support the steel fra m ea re best understood by the design-e r. Re q u i rements for camberi n g the construction to compensatefor future deflections should alsobe made clear. Ot h e rw i s e, thec o n t ractor may feel justified ins u p p o rting the form w o rk and itsapplied loads on the alre a d y placed steel fra m e. Howe ve r, asthe editorial points out, whens h o ring and bracing needs for thistype of construction are ove r-looked or misunderstood, objec-tionable deflection or hazard o u sconditions result. An alert con-t ractor under present conditionscan either allow for shoring in hisbid, or obtain positive assura n c ef rom the stru c t u ral designer that itis not needed. AASHTO specifications recog-nize need for lateral support Some bridges on a supere l e va-tion have had all their main beamsroll over downhill under a load of  wet concre t e. These beams, whether they be steel or pre c a s tc o n c re t e, must be laterally sup-p o rted until the concrete hash a rdened and become a part of the stru c t u ral system. Reflecting anumber of disastrous experi e n c e s,the bridge specifications of the A m e rican Association of St a t eHighway and Tra n s p o rtation Offi-cials warn the designer to makec e rtain that the steel beams will bestable during the time when thec o n c rete deck is being placed anduntil the time when the concre t ehas reached its strength and canp rovide anticipated lateral di-a p h ragm support . PUBLICATION #C840815 Co py right © 1984, The Ab e rdeen Gro u p All rights re s e rve d
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