Cambering Steel Beams - AISC

1 September 2004 Modern Steel ConstructionThe following highlights can be examinedin greater detail by reading the full paper,available at of Camber:The author distinguishesbetween natural camber (the out-of-straightness remaining after the initialrolling, cooling, and straightening of themember at the mill) and induced camber(the curvature that is applied subsequentto the initial rolling and straighteningprocess, usually in the fabricating shop).The Camber Curve: The deflection curve fora horizontal, uniformly loaded memberof constant cross section theoreticallyapproximates a parabola.

2 However, sincethe sag ratio for Beams , that is, the ratio ofmid-ordinate to chord, is so small, it gen-erally is accepted that adequate accuracyresults if the camber curve is consideredto be a segment of a circular curve. Somecamber calculations are based on circularcurves. Camber usually is expressed interms of the maximum ordinate at mid-span. The full paper gives a method todetermine the approximate camber atother points along this Advice, with Humor to Boot: Once thedeflections are determined, the desiredamount of camber can be selected.

3 Theselection of camber is often arbitrary. Themethods of Cambering are relativelycrude, and the results are less than pre-cise. There is little need nor justification inmeticulous mathematical manipulationor methodical multifarious meditationwhen it comes to determining Cambering Using Force: The types ofsteel used for structural purposes are duc-tile, that is, they have the property ofdeforming extensively under substan-tially constant we cold-camber a beam , the extreme fibers reachstresses and deformations that are on thehorizontal portion of the stress-straincurve.

4 With most cold- Cambering devices,it is impossible or difficult to maintain aconstant stress. The rams which deliverthe force advance when the button ispushed and halt when the button isreleased. When the ram is halted, thedeformation and stresses come to balancein a brief moment, and equilibrium isreached. When the ram is retracted, thebeam relaxes and some residual deforma-tion is evident by the fact that the beam isno longer straight. This essentially is thecold- Cambering all know that bending a wire backand forth enough times will weaken it tothe point where it will fail with very littleeffort.

5 What about the cold-camberedbeam, which we bent in one direction tomake the camber curve and are now load-ing in the opposite direction with its serv-ice loads? Another of the seemingly end-less wonderful properties of structuralsteel is that, if allowed to rest for a fewhours at room temperature, Steel has thetendency to recover its elastic application of mild heat, about 225 Ffor a few minutes, will accelerate theperiod of should be made here of theterm strain hardened.

6 This consists ofan alteration of the elastic properties ofcold-worked Steel and a raising of theproportional-limit stress, as a result of theaforementioned aging or application ofmild heat. Two facts emerge from thisbrief discussion of cold same allowable stresses (or loadfactors) can be applied to cold-cam-bered Beams as to uncambered Beams ,provided that they are allowed to age for a few hours. 2. Never attempt to reduce the camber inan overcambered beam by immedi-ately applying force in the oppositedirection.

7 If this caution is ignored,strain weakening will result and theelastic-limit will be lowered. If normal,allowable stresses subsequently areassigned to the member, the factor ofsafety will be a beam is to be cold-camberedby force, it is usually mounted in sometype of rigid frame which holds the beamsecure while the force is delivered. Theends of the beam must be laterally sup-ported in addition to the compressionflange. The fulcrums must be devised sothat the ends are free to rotate because ofthe severe arc to which the beam is forcedin order to produce the residual are usually two points at which theClassic Papers from AISC sEngineering JournalThe AISC Engineering Journal is a practical journal designed to present papers with infor-mation that is useful and relevant.

8 Many of the questions received by the AISC Steel Solu-tions Center can be answered by referencing one of the papers published in the journal. EJis available to all by subscription and is available to members through free electronic access usingAISC s ePubs feature at all papers in the AISC Engineering Journalmake a contribution to the state of knowl-edge of Steel design and construction, some papers rise above the rest and stand as seminal intheir importance. This regular feature in Modern Steel Constructionmagazine will highlight thosemost notable of works in the AISC Engineering Steel BeamsBy David T.

9 RickerFrom the 4th Quarter 1989 issuereferencespopularload is delivered to the beam . Thesepoints might be about 6 or 8 apart,while the resisting supports can be 22 orso apart. This will produce a curve that isvery close to duplicating a the operator activates the cam-bering device, the rams advance and thebeam deflects, often as much as two orthree times the amount of desired cam-ber. The rams are halted and kept in placefor a few seconds, during which the steelmicrostructure undergoes rearrangementand the stresses relax somewhat.

10 Whenthe rams are retracted, the beam springsback, stresses drop to zero, and a perma-nent set is left in the beam . If the defor-mation is insufficient, the rams can beimmediately reactivated to add morecamber. Contrary to popular belief, coldcambering does not result in significantresidual stresses, at least in the flangeareas. A bar shape, if bent cold to a tightradius, will contain residual stresses but,when a beam is bent, the radius of thebend is so large that the tension flangeundergoes nearly pure tension while thecompression flange is subject to nearlypure compression.