AISC LRFD 1.pdf

92 LOCAL BUCKLING [App. B5.b = actual width of a stiffened compression element, as defined in SectionB5.1, in. (mm)b e = reduced effective width, in. (mm)t = element thickness, in. (mm)f = computed elastic compressive stress in the stiffened elements, based onthe design properties as specified in Appendix B5.3c, ksi (MPa). If unstiffenedelements are included in the total cross section, f for the stiffenedelement must be such that the maximum compressive stress in theunstiffened element does not exceed c F cr as defined in Appendix B5.3dwith Q = Q s and c = 0.85, or b F y Q s with b = 0.90, as applicable.(c) For axially loaded circular sections with diameter-to-thickness ratio D/tgreater than 0.11E/F y but less than 0.45E / F y0.038E2Q = Qa= +F ( D/ t) 3y(A-B5-13)whereD = outside diameter, in. (mm)t = wall thickness, in. (mm)3c. Design PropertiesProperties of sections shall be determined using the full cross section, except as follows:In computing the moment of inertia and elastic section modulus of flexural members,the effective width of uniformly compressed stiffened elements b e , as determinedin Appendix B5.3b, shall be used in determining effective cross-sectionalproperties.For unstiffened elements of the cross section, Q s is determined from AppendixB5.3a. For stiffened elements of the cross sectioneffective areaQ a=(A-B5-14)actual areawhere the effective area is equal to the summation of the effective areas of the crosssection.3d. Design StrengthFor axially loaded compression members the gross cross-sectional area and theradius of gyration r shall be computed on the basis of the actual cross section. Thecritical stress F cr shall be determined as follows:(a) For l c Q £ 1.5:(b) For l c Q > 1.5:2QlcFcr= Q(0.658 ) Fy(A-B5-15)LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION