AISC LRFD 1.pdf

196 STABILITY BRACING [Comm. C3.flange of the beams and a rigid support assumes that there is no lateral movement atthe column locations.The brace requirements will enable a member to potentially reach a maximum loadbased on the unbraced length between the brace points and K = 1.0. This is not thesame as the no-sway buckling load as illustrated in Figure C-C3.2 for the bracedcantilever. The critical stiffness is 1.0 P e /L, corresponding to K = 1.0. A brace withfive times this stiffness is necessary to reach 95 percent of the K = 0.7 limit. Theoretically,an infinitely stiff brace is required to reach the no-sway limit. Bracingrequired to reach specified rotation capacities or ductility limits is beyond the scopeof these recommendations. Member inelasticity has no significant effect on thebrace requirements (Yura, 1995).Winter (1958 and 1960) developed the concept of dual criterion for bracing design,strength and stiffness. The brace force is a function of the initial columnout-of-straightness, o , and the brace stiffness, . For a relative brace system, therelationship between column load, brace stiffness and sway displacement is shownin Figure C-C3.3. If = i , the critical brace stiffness for a perfectly plumb member,then P = P e only if the sway deflection gets very large. Unfortunately, such large dis-Fig. C-C3.1. Types of bracing.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION