AISC LRFD 1.pdf

268Comm. A-FAPPENDIX FBEAMS AND OTHER FLEXURAL MEMBERSF1. DESIGN FOR FLEXUREThree limit states must be investigated to determine the moment capacity of flexuralmembers: lateral-torsional buckling (LTB), local buckling of the compressionflange (FLB), and local buckling of the web (WLB). These limit states depend,respectively, on the beam slenderness ratio L b /r y , the width-thickness ratio b/tofthe compression flange and the width-thickness ratio h/t w of the web. For convenience,all three measures of slenderness are denoted by .Variations in M n with L b are shown in Figure C-F1.1. The discussion of plastic,inelastic, and elastic buckling in Commentary Section F1 with reference to lateral-torsionalbuckling applies here except for an important difference in the significanceof p for lateral-torsional buckling and local buckling. Values of p for FLBand WLB produce a compact section with a rotation capacity of about three (afterreaching M p ) before the onset of local buckling, and therefore meet the requirementsfor plastic analysis of load effects (Commentary Section B5). On the otherhand, values of p for LTB do not allow plastic analysis because they do not providerotation capacity beyond that needed to develop M p . Instead L b L pd (Section F1.3)must be satisfied.Analyses to include restraint effects of adjoining elements are discussed inGalambos (1998). Analysis of the lateral stability of members with shapes not coveredin this appendix must be performed according to the available literature(Galambos, 1998).See the Commentary for Section B5 for the discussion of the equation regarding thebending capacity of circular sections.F3. WEB-TAPERED MEMBERS1. General RequirementsThe provision contained in Appendix F3 covers only those aspects of the design oftapered members that are unique to tapered members. For other criteria of designnot specifically covered in Appendix F3, see the appropriate portions of this Specificationand Commentary.The design of wide-flange columns with a single web taper and constant flangesfollows the same procedure as for uniform columns according to Section E2, exceptthe column slenderness parameter c for major axis buckling is determined for aslenderness ratio K L/r ox , and for minor axis buckling for KL/r oy , where K is aneffective length factor for tapered members, K is the effective length factor for pris-LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION