AISC LRFD 1.pdf

108 DESIGN FLEXURAL STRENGTH [App. G2.M= S R Fn xt e yt(A-G2-1)(b) For compression-flange buckling:Mn = SxcRPGReFcr(A-G2-2)wherea ærhcE öR PG = 1- - 5.70 £ 1.01,200 + 300a çt F÷è ør w cr(A-G2-3)R e= hybrid girder factor312 + ar(3 m-m)=£ 1.0 (for non - hybrid girders, Re= 1.0)12 + 2ara r = ratio of web area to compression flange area ( 10)m = ratio of web yield stress to flange yield stress or to F crF cr = critical compression flange stress, ksi (MPa)F yt = yield stress of tension flange, ksi (MPa)S xc = section modulus referred to compression flange, in. 3 (mm 3 )S xt = section modulus referred to tension flange, in. 3 (mm 3 )h c = twice the distance from the centroid to the nearest line of fasteners at thecompression flange or the inside of the face of the compression flangewhen welds are used, in. (mm)The critical stress F cr to be used is dependent upon the slenderness parameters , p , r , and C PG as follows:(a) For p :(b) For p r :F cr = F yf(A-G2-4)(c) For r :é 1 æ l - l öùF C F 1 Fêë2 úûpcr=b yf ê - ç ú£yflrl÷è -pø(A-G2-5)CPGFcr=(A-G2-6)2lIn the foregoing, the slenderness parameter shall be determined for both the limitstate of lateral-torsional buckling and the limit state of flange local buckling; theslenderness parameter which results in the lowest value of F cr governs.(a) For the limit state of lateral-torsional buckling:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION