AISC LRFD 1.pdf

266Comm. A-BAPPENDIX BDESIGN REQUIREMENTSB5. LOCAL BUCKLING1. Classification of Steel SectionsThe limiting width-thickness p and r ratios for webs in pure flexure (P u / b P y =0)and with axial compression have been revised in terms of (h/t) rather than (h c /t).The simplified formulation in Table B5.1 for r based on double symmetry withequal flanges (h/h c = 1) is unconservative when the compression flange is smallerthan the tension flange, and conservative if the reverse is true. The more accuratelimit is given in Appendix B5.1 as a function of h c . Figure C-A-B5.1 illustrates the r variation for axial compression and flange asymmetry effects.The 3 4 minimum and 3 2 maximum restrictions on h/h c in Equations A-B5-1 andA-B5-2 approximately correspond to the 0.1 and 0.9 range of I yc /I y for a member tobe considered a singly symmetric I shape. Otherwise, when the flange areas differby more than a factor of two, the member should be conservatively designed as a teesection./cc/ //cccFig. C-A-B5.1. Web local buckling for I-shaped members.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION