AISC LRFD 1.pdf

22 STABILITY BRACING [Sect. C3.compression flange. The connection between a torsional brace and the beam mustbe able to support the required moment given below.(a) Nodal BracingThe required bracing moment is:Mbr.uM LnC LThe required cross-frame or diaphragm bracing stiffness is:bb(C3-11)= 0 024 secwhereβTβTb=⎛ βT ⎞⎜1−⎟⎝ β ⎠(C3-12)βT24=φ2. LMu2nEIyCb(C3-13)3 333 . E ⎛15. hto wtb ⎞s sβ(C3-14)sec= ⎜ + ⎟ho⎝ 12 12 ⎠ = 0.75L = span length, in. (mm)n = number of nodal braced points within the spanE = 29,000 ksi (200 000 MPa)I y = out-of-plane moment of inertia, in. 4 (mm 4 )C b = is a modification factor defined in Chapter Ft w = beam web thickness, in. (mm)t s = web stiffener thickness, in. (mm)b s = stiffener width for one-sided stiffeners (use twice the individual stiffenerwidth for pairs of stiffeners), in. (mm) T = brace stiffness excluding web distortion, kip-in/radian (N-mm/radian) sec = web distortional stiffness, including the effect of web transverse stiffeners,if any, kip-in/radian (N-mm/radian)If sec < T , Equation C3-12 is negative, which indicates that torsional beam bracingwill not be effective due to inadequate web distortional stiffness.When required, the web stiffener shall extend the full depth of the braced memberand shall be attached to the flange if the torsional brace is also attached to theflange. Alternatively, it shall be permissible to stop the stiffener short by a distanceequal to 4t w from any beam flange that is not directly attached to the torsionalbrace. When the actual spacing of braced points is less than L q , then L b inEquation C3-11 shall be permitted to be taken equal to L q .(b) Continuous Torsional BracingFor continuous bracing, use Equations C3-11, C3-12 and C3-13 with L/n taken asLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION