AISC LRFD 1.pdf

260Comm. MCHAPTER MFABRICATION, ERECTION, AND QUALITY CONTROLM2. FABRICATION1. Cambering, Curving, and StraighteningThe use of heat for straightening or cambering members is permitted forA514/A514M and A852/A852M steel, as it is for other steels. However, the maximumtemperature permitted is 1,100°F (593C) compared to 1,200°F (649C) forother steels.Cambering of flexural members, when required by the contract documents, may beaccomplished in various ways. In the case of trusses and girders, the desired curvaturecan be built in during assembly of the component parts. Within limits, rolledbeams can be cold-cambered at the producing mills.Local application of heat has come into common use as a means of straightening orcambering beams and girders. The method depends upon an ultimate shortening ofthe heat-affected zones. A number of such zones, on the side of the member thatwould be subject to compression during cold-cambering or “gagging,” are heatedenough to be “upset” by the restraint provided by surrounding unheated areas.Shortening takes place upon cooling.While the final curvature or camber can be controlled by these methods, it must berealized that some deviation, due to workmanship error and permanent change dueto handling, is inevitable.2. Thermal CuttingPreferably thermal cutting shall be done by machine. The requirement for a positivepreheat of 150°F (66C) minimum when thermal cutting beam copes and weldaccess holes in ASTM A6/A6M Group 4 and 5 shapes, and in built-up shapes madeof material more than two in. (50 mm) thick, tends to minimize the hard surfacelayer and the initiation of cracks.5. Bolted ConstructionIn the past, it has been required to tighten all ASTM A325 or A325M and A490 orA490M bolts in both slip-critical and bearing-type connections to a specified tension.The requirement was changed in 1985 to permit most bearing-type connectionsto be tightened to a snug-tight condition.In a snug-tight bearing connection, the bolts cannot be subjected to tension loads,slip can be permitted, and loosening or fatigue due to vibration or load fluctuationsare not design considerations.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION