Lateral instability and lateral bracing of steel beams subjected to cyclic loading

This paper presents an analytical study of the lateral-torsional instability and lateral bracing effects of wide-flange steel beams subjected to cyclic loading. Numerical analysis using the large deformation theory was conducted to collect the necessary data. Examined were wide-flange steel beams bent in double curvature and subjected to cyclic loading with increasing amplitudes up to the maximum beam end rotation of 0.045 rad. Cross-sectional properties, slenderness ratios, material strength, loading history, and unbraced length were chosen as analysis variables. The lateral instability effect was found to differ significantly between cyclic and monotonic loading. For slenderness ratios about the weak axis not smaller than 100, the strength that can be sustained under cyclic loading was much smaller than that obtained under monotonic loading due to the accumulation of out-of-plane deformations. Equations are proposed for the beam unbraced length with which no detrimental reduction in strength is present in cyclic loading up to the maximum beam end rotation of 0.045 rad. It was also found that the unbraced length requirements stipulated in the American Institute of Steel Construction Seismic Provisions are a reasonably conservative measure to ensure sufficient beam rotation capacity. Lateral instability of reduced beam section (RBS) beams was also analyzed. It is notable that the RBS beam is not necessarily more susceptible to lateral instability than the corresponding standard beam, primarily because of a smaller yielding region and smaller forces induced in the cross section of the RBS beam. This phenomenon was interpreted using a simple flange buckling analogy. The lateral bracing requirements stipulated for standard beams are applicable to ensure sufficient rotation capacity for RBS beams if local buckling effects would not occur.