AISC LRFD 1.pdf

32 DESIGN FOR FLEXURE [Sect. F1.See Section B10 for further limitations on M n where there are holes in the tensionflange.2. Lateral-Torsional BucklingThis limit state is only applicable to members subject to major axis bending. Theflexural design strength, determined by the limit state of lateral-torsional buckling,is b M n : b = 0.90M n = nominal flexural strength determined as follows2a. Doubly Symmetric Shapes and Channels with L b L rThe nominal flexural strength is:⎡⎛L⎞⎤b− LpMn = Cb ⎢Mp −( Mp −Mr)⎥ ≤ M(F1-2)⎜ ⎟p⎢⎣⎝Lr− Lp⎠⎥⎦whereL b = distance between points braced against lateral displacement of the compressionflange, or between points braced to prevent twist of the crosssection, in. (mm)L p = limiting laterally unbraced length as defined below, in. (mm)L r = limiting laterally unbraced length as defined below, in. (mm)M r = limiting buckling moment as defined below, kip-in. (N-mm)In the above equation, C b is a modification factor for non-uniform moment diagramswhere, when both ends of the beam segment are braced:12.5MmaxCb=2.5M + 3M + 4M + 3Mmax A B C(F1-3)whereM max = absolute value of maximum moment in the unbraced segment, kip-in.(N-mm)M A = absolute value of moment at quarter point of the unbraced segment,kip-in. (N-mm)M B = absolute value of moment at centerline of the unbraced beam segment,kip-in. (N-mm)M C = absolute value of moment at three-quarter point of the unbraced beamsegment, kip-in. (N-mm)C b is permitted to be conservatively taken as 1.0 for all cases. Equations F1-4 andF1-6 are conservatively based on C b = 1.0. For cantilevers or overhangs where thefree end is unbraced, C b = 1.0.The limiting unbraced length, L p , shall be determined as follows.(a) For I-shaped members including hybrid sections and channels:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION