AISC LRFD 1.pdf

Comm. H2.] UNSYMMETRIC MEMBERS AND MEMBERS UNDER TORSION 213ond equation). Therefore, the requirement that the nominal compressive strength P nbe based on the effective length KL in the general equation is continued in theLRFD Specification as it has been in the AISC ASD Specification since 1961. It isnot intended that these provisions be applicable to limit nonlinear secondary flexurethat might be encountered in large amplitude earthquake stability design (ATC,1978).The defined term M u is the maximum moment in a member. In the calculation of thismoment, inclusion of beneficial second order effects of tension is optional. Butconsideration of detrimental second order effects of axial compression and translationof gravity loads is required. Provisions for calculation of these effects are givenin Chapter C.The interaction equations in Appendix H3 have been recommended for biaxiallyloaded H and wide flange shapes in Galambos (1998) and Springfield (1975).These equations which can be used only in braced frames represent a considerableliberalization over the provisions given in Section H1; it is, therefore, also necessaryto check yielding under service loads, using the appropriate load and resistancefactors for the serviceability limit state in Equation H1-1a or H1-1b with M ux = S x F yand M uy = S y F y . Appendix H3 also provides interaction equations for rectangularbox-shaped beam-columns. These equations are taken from Zhou and Chen (1985).H2. UNSYMMETRIC MEMBERS AND MEMBERS UNDER TORSION ANDCOMBINED TORSION, FLEXURE, SHEAR, AND/OR AXIALFORCEThis section deals with types of cross sections and loadings not covered in SectionH1, especially where torsion is a consideration. For such cases it is recommended toperform an elastic analysis based on the theoretical numerical methods availablefrom the literature for the determination of the maximum normal and shear stresses,or for the elastic buckling stresses. In the buckling calculations an equivalent slendernessparameter is determined for use in Equation E2-2 or E2-3, as follows:λ e = F y / F ewhere F e is the elastic buckling stress determined from a stability analysis. Thisprocedure is similar to that of Appendix E3.For the analysis of members with open sections under torsion refer to AISC (1997).LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION