AISC LRFD 1.pdf

270 WEB-TAPERED MEMBERS [Comm. App. F3.l, is assigned the stiffness I o /l, which is then divided by the stiffness of the restrainingmembers at the end of the tapered column under consideration.4. Design Flexural StrengthThe development of the design bending stress for tapered beams follows closelywith that for prismatic beams. The basic concept is to replace a tapered beam by anequivalent prismatic beam with a different length, but with a cross section identicalto that of the smaller end of the tapered beam (Lee et al., 1972). This has led to themodified length factors h s and h w in Equations A-F3-6 and A-F3-7.Equations A-F3-6 and A-F3-7 are based on total resistance to lateral buckling,using both St. Venant and warping resistance. The factor B modifies the basic F b tomembers which are continuous past lateral supports. Categories a, b, and c ofAppendix F3.4 usually apply; however, it is to be noted that they apply only whenthe axial force is small and adjacent unbraced segments are approximately equal inlength. For a single member, or segments which do not fall into category a, b, c, or d,the recommended value of B is unity. The value of B should also be taken as unitywhen computing the value of F b to obtain M n to be used in Equations H1-1 andC1-1, since the effect of moment gradient is provided for by the factor C m . Thebackground material is given in WRC Bulletin No. 192 (Morrell and Lee, 1974).LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION