AISC LRFD 1.pdf

104 WEB-TAPERED MEMBERS [App. F3.dance with Appendix E3, using an effective slenderness parameter eff computedas follows:leffS=pQFyE(A-F3-2)whereS = KL/r oy for weak axis buckling and KL/r ox for strong axis bucklingK = effective length factor for a prismatic memberK = effective length factor for a tapered member as determined by a rational analysisr ox = strong axis radius of gyration at the smaller end of a tapered member, in.(mm)r oy = weak axis radius of gyration at the smaller end of a tapered member, in. (mm)F y = specified minimum yield stress, ksi (MPa)Q = reduction factor= 1.0 if all elements meet the limiting width-thickness ratios r of Section B5.1= Q s Q a , determined in accordance with Appendix B5.3, if any stiffened and/orunstiffened elements exceed the ratios r of Section B5.1E = modulus of elasticity for steel, ksi (MPa)The smallest area of the tapered member shall be used for A g in Equation E2-1.4. Design Flexural StrengthThe design flexural strength of tapered flexural members for the limit state of lateral-torsionalbuckling is b M n , where b = 0.90 and the nominal strength isM= (5/ 3) S¢Fgn x b(A-F3-3)whereS x = the section modulus of the critical section of the unbraced beam lengthunder consideration2é F ùF ê 1.0 úF 0.60 Fybg= -y£y3 ê2 26B Fsg+ F úwgëû(A-F3-4)unless Fbg £Fy/3, in which caseF = B F + F2 2bg sg wg(A-F3-5)In the preceding equations,Fsg =0.41EhLd / As o f(A-F3-6)LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION