AISC LRFD 1.pdf

249Comm. KCHAPTER KCONCENTRATED FORCES, PONDING, AND FATIGUEK1. FLANGES AND WEBS WITH CONCENTRATED FORCES1. Design BasisThe LRFD Specification separates flange and web strength requirements into distinctcategories representing different limit state criteria, i.e., flange local bending(Section K1.2), web local yielding (Section K1.3), web crippling (Section K1.4),web sidesway buckling (Section K1.5), web compression buckling (Section K1.6),and web panel-zone shear (Section K1.7).These criteria are applied to two distinct types of concentrated forces which act onmember flanges. Single concentrated forces may be tensile, such as those deliveredby tension hangers, or compressive, such as those delivered by bearing plates atbeam interior positions, reactions at beam ends, and other bearing connections.Double concentrated forces, one tensile and one compressive, form a couple on thesame side of the loaded member, such as that delivered to column flanges throughwelded and bolted moment connections. See Carter (1999) for guidelines on columnstiffener design.2. Flange Local BendingWhere a tensile force is applied through a plate welded across a flange, that flangemust be sufficiently rigid to prevent deformation of the flange and the correspondinghigh-stress concentration in the weld in line with the web.The effective column flange length for local flange bending is 12t f (Graham, et al.,1959). Thus, it is assumed that yield lines form in the flange at 6t f in each directionfrom the point of the applied concentrated force. To develop the fixed edge consistentwith the assumptions of this model, an additional 4t f and therefore a total of10t f , is required for the full flange-bending strength given by Equation K1-1. In theabsence of applicable research, a 50 percent reduction has been introduced forcases wherein the applied concentrated force is less than 10t f from the member end.This criterion given by Equation K1-1 was originally developed for moment connections,but it also applies to single concentrated forces such as tension hangersconsisting of a plate welded to the bottom flange of a beam and transverse to thebeam web.3. Web Local YieldingThe web strength criteria have been established to limit the stress in the web of amember into which a force is being transmitted. It should matter little whether themember receiving the force is a beam or a column; however, Galambos (1976) andAISC (1978), references upon which the LRFD Specification is based, did makeLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION