AISC LRFD 1.pdf

94App. EAPPENDIX ECOLUMNS AND OTHER COMPRESSION MEMBERSThis Appendix applies to the strength of doubly symmetric columns with thin plate elements,and singly symmetric and unsymmetric columns for the limit states offlexural-torsional and torsional buckling.E3. DESIGN COMPRESSIVE STRENGTH FOR FLEXURAL-TORSIONALBUCKLINGThe design strength of compression members determined by the limit states oftorsional and flexural-torsional buckling is c P n ,where c = 0.85P n = nominal resistance in compression, kips (N)= A g F cr (A-E3-1)A g = gross area of cross section, in. 2 (mm 2 )The nominal critical stress F cr is determined as follows:(a) Forle Q £ 1.5 :2QleFcr= Q(0.658 ) Fy(A-E3-2)(b) ForwhereleQ > 1.5 :Fcré0.877ù= ê F2 ú yë leû(A-E3-3)l =F / Fe y e(A-E3-4)Q=1.0 for elements meeting the width-thickness ratios r of Section B5.1= Q s Q a for elements not meeting the width-thickness ratios r of Section B5.1and determined in accordance with the provisions of Appendix B5.3The critical torsional or flexural-torsional elastic buckling stress F e is determined asfollows:(a) For doubly symmetric shapes:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION