AISC LRFD 1.pdf

Comm. C3.] STABILITY BRACING 197placements produce large brace forces. For practical design, must be kept small atthe factored load level.The brace stiffness requirements, br , for frames, columns, and beams were chosenas twice the critical stiffness. The = 0.75 specified for all brace stiffness requirementsis consistent with the implied resistance factor for elastic Euler column buckling,i.e. 0.877 c = 0.75. For the relative brace system shown in Figure C-C3.3, br=2 i gives P br = 0.4% P e for o = 0.002L. If the brace stiffness provided, act ,isdifferentfrom the requirement, then the brace force or brace moment can be multipliedby the following factor:1b2 - bbract(C-C3-1)No is specified in the brace strength requirements since is included in the componentdesign strength provisions in other chapters of this Specification.The initial displacement, o , for relative and nodal braces is defined with respect tothe distance between adjacent braces, as shown in Figure C-C3.4. The initial o is adisplacement from the straight position at the brace points caused by sources otherthan brace elongations from gravity loads or compressive forces, such as displacementscaused by wind or other lateral forces, erection tolerances, column shorten-Fig. C-C3.2. Braced cantilever.D = D o3 b i112 b0.8i0.83 b i2 b ib = b i D oPP br0.60.6b = b iPPP e0.4LP e0.4D0.20.2o = 0.002LP0.4% P e0 00 4 8 12 16 20 0.5% 1.0% 1.5% 2.0%(a) DT/ Do(b) P br (%of P)Fig. C-C3.3. Effect of initial out-of-plumbness.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION