AISC LRFD 1.pdf

31Chap. FCHAPTER FBEAMS AND OTHER FLEXURAL MEMBERSThis chapter applies to compact and noncompact prismatic members subject to flexureand shear. For members subject to combined flexure and axial force, see Section H1. Formembers subject to fatigue, see Section K3. For members with slender compression elements,see Appendix B5. For web-tapered members, see Appendix F3. For members withslender web elements (plate girders), see Appendix G.F1. DESIGN FOR FLEXUREThe nominal flexural strength M n is the lowest value obtained according to the limitstates of: (a) yielding; (b) lateral-torsional buckling; (c) flange local buckling; and(d) web local buckling. For laterally braced compact beams with L b L p , only thelimit state of yielding is applicable. For unbraced compact beams and noncompacttees and double angles, only the limit states of yielding and lateral-torsional bucklingare applicable. The lateral-torsional buckling limit state is not applicable tomembers subject to bending about the minor axis, or to square or circular shapes.This section applies to homogeneous and hybrid shapes with at least one axis ofsymmetry and which are subject to simple bending about one principal axis. Forsimple bending, the beam is loaded in a plane parallel to a principal axis that passesthrough the shear center or the beam is restrained against twisting at load points andsupports. Only the limit states of yielding and lateral-torsional buckling are consideredin this section. The lateral-torsional buckling provisions are limited to doublysymmetric shapes, channels, double angles, and tees. For lateral-torsional bucklingof other singly symmetric shapes and for the limit states of flange local bucklingand web local buckling of noncompact or slender-element sections, see AppendixF1. For unsymmetric shapes and beams subject to torsion combined with flexure,see Section H2. For biaxial bending, see Section H1.1. YieldingThe flexural design strength of beams, determined by the limit state of yielding, is b M n : b = 0.90M n M p(F1-1)whereM p = plastic moment (= F y Z 1.5M y for homogeneous sections), kip-in. (N-mm)M y = moment corresponding to onset of yielding at the extreme fiber from anelastic stress distribution (= F y S for homogeneous section and F yf S for hybridsections), kip-in. (N-mm)LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION