AISC LRFD 1.pdf

Comm. K1.] FLANGES AND WEBS WITH CONCENTRATED FORCES 251For flanges not restrained against rotation:h/tw>17 .(C-K1-2)l/bfwhere l is as shown in Figure C-K1.2.Web sidesway buckling can also be prevented by the proper design of lateral bracingor stiffeners at the load point. It is suggested that local bracing at both flanges bedesigned for one percent of the concentrated force applied at that point. Stiffenersmust extend from the load point through at least one-half the beam or girder depth.In addition, the pair of stiffeners should be designed to carry the full load. If flangerotation is permitted at the loaded flange, neither stiffeners nor doubler plates willbe effective.In the 1st Edition LRFD Manual, the web sidesway buckling equations were basedon the assumption that h/t f = 40, a convenient assumption which is generally truefor economy beams. This assumption has been removed so that the equations willbe applicable to all sections.This criterion was developed only for bearing connections and does not apply tomoment connections.6. Web Compression BucklingWhen compressive forces are applied to both flanges of a member at the same loca-Fig. C-K1.2. Unbraced flange length.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION