Stability Requirements for Beams in Seismic Steel Moment Frames

Abstract

An analytical study was conducted on the cyclic loading behavior of beams in steel moment frames. Lateral–torsional buckling and local buckling were explicitly considered in the analysis. Beam cross-sectional dimensions were varied to represent a wide range of rolled wide flange shapes. The unbraced length of the beams corresponded to slenderness ratios about the weak axis ranging between 60 and 100. Based on the analyses, flange and web width-thickness limits were established that would permit the beam to achieve various target rotation capacities. These limits are presented in terms of limit curves which plot the flange width-thickness ratio against the web width-thickness ratio. The limit curves clearly show strong flange–web interaction. Different limit curves were developed for a range of target rotation angles, weak axis slenderness ratios, and residual strength levels. Postbuckling behavior and strength degradation mechanism were studied. The results of the analyses are compared against current building code requirements for beam stability in seismic steel moment frames, and the adequacy of the current code requirements are evaluated.