AISC LRFD 1.pdf

200 STABILITY BRACING [Comm. C3.given by Equations C3-8 and C3-10. Equation C-C3-3 can be used in lieu of EquationsC3-8 and C3-10.The brace strength requirement for relative bracing isP br = 0.004 M u C t C d / h o(C-C3-4a)and for nodal bracingP br = 0.01M u C t C d / h o(C-C3-4b)They are based on an assumed initial lateral displacement of the compressionflange of 0.002L b . The brace strength requirements of Equations C3-7 and C3-9 arederived from Equations C-C3-4a and C-C3-4b assuming top flange loading (C t =2). Equations C-C3-4a and C-C3-4b can be used in lieu of Equations C3-7 and C3-9respectively.4b. Torsional BracingTorsional bracing can either be attached continuously along the length of the beam(for example, metal deck or slabs) or be located at discrete points along the length ofthe member (for example, cross frames). Torsional bracing attached to the tensionflange is just as effective as a brace attached at mid depth or the compression flange.Partially restrained connections can be used if their stiffness is considered in evaluatingthe torsional brace stiffness.The torsional brace requirements are based on the buckling strength of a beam witha continuous torsional brace along its length developed by Taylor and Ojalvo(1966) and modified for cross-section distortion by Yura (1993).CEIb2Mu £ Mcr = ( CbuMo)+2C2b y Ttt(C-C3-5)The term (C bu M o ) is the buckling strength of the beam without torsional bracing. C tt= 1.2 when there is top flange loading and C tt = 1.0 for centroidal loading.β T= nβ T / Lis the continuous torsional brace stiffness per unit length or its equivalentwhen n nodal braces, each with a stiffness T , are used along the span L and the2 accounts for initial out-of-straightness. Neglecting the unbraced beam bucklingterm gives a conservative estimate of the torsional brace stiffness requirement(Equation C3-13). A more accurate estimate of the brace requirements can beobtained by replacing M u with (M u – C bu M o ) in Equations C3-11 and C3-13. The secterm in Equations C3-12, C3-14 and C3-15 accounts for cross-section distortion. Aweb stiffener at the brace point reduces cross-sectional distortion and improves theeffectiveness of a torsional brace. When a cross frame is attached near both flangesor a diaphragm is approximately the same depth as the girder, then web distortionwill be insignificant so sec equals infinity. The required bracing stiffness, Tb ,givenby Equation C3-12 was obtained by solving the following expression that representsthe brace system stiffness including distortion effects:1 1 1= +(C-C3-6)βTβTbβsecThe brace moment requirements are based on an assumed initial twist of 0.002L b / h o .LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION