AISC LRFD 1.pdf

Sect. B10.] PROPORTIONS OF BEAMS AND GIRDERS 132. Design by Plastic AnalysisDesign by plastic analysis is permitted, as limited in Section A5.1, when flangessubject to compression involving hinge rotation and all webs have awidth-thickness ratio less than or equal to the limiting p from Table B5.1. For circularhollow sections see Footnote d of Table B5.1.3. Slender-Element Compression SectionsFor the flexural design of I-shaped sections, channels and rectangular or circular sectionswith slender flange elements, see Appendix F1. For other shapes in flexure ormembers in axial compression that have slender compression elements, see AppendixB5.3. For plate girders with slender web elements, see Appendix G.B6. BRACING AT SUPPORTSAt points of support for beams, girders and trusses, restraint against rotation abouttheir longitudinal axis shall be provided.B7. LIMITING SLENDERNESS RATIOSFor members in which the design is based on compression, the slenderness ratioKl/r preferably should not exceed 200.For members in which the design is based on tension, the slenderness ratio l/rpreferablyshould not exceed 300. The above limitation does not apply to rods in tension.Members in which the design is dictated by tension loading, but which may besubject to some compression under other load conditions, need not satisfy the compressionslenderness limit.B8. SIMPLE SPANSBeams, girders and trusses designed on the basis of simple spans shall have aneffective length equal to the distance between centers of gravity of the members towhich they deliver their end reactions.B9. END RESTRAINTBeams, girders, and trusses designed on the assumptions of full or partial endrestraint, as well as the sections of the members to which they connect, shall havedesign strengths, as prescribed in Chapters D through K, equal to or exceeding theeffect of factored forces and moments except that some inelastic but self-limitingdeformation of a part of the connection is permitted.B10. PROPORTIONS OF BEAMS AND GIRDERSWhen rolled or welded shapes, plate girders and cover-plated beams are proportionedon the basis of flexural strength of the gross section:(a) If0.75F u A fn 0.9F y A fg (B10-1)no deduction shall be made for bolt or rivet holes in either flange,whereLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION