Simplified Method for Estimation of Beam Plastic Rotation Demand in Special Moment-Resisting Steel-Frame Structures

It is necessary to predict the plastic deformation demand of members and the story drift ratio for seismic design. In the present study, a simple method of estimating the plastic rotation of beams in special steel moment-resisting frame structures designed by the strong column-weak beam philosophy was developed. The plastic rotation of beams was calculated as the function of the story drift ratio based on the result of elastic analysis. The effects of the design parameters, such as moment redistribution, joint dimension, plastic mechanism, panel zone deformation, and the rocking effect of lateral bracing were addressed. For verification, the proposed method was applied to 6-story special moment frames with and/or without lateral bracing. The plastic rotations of beams in the same floor significantly varied according to the design parameters, although the story drift ratio was the same. The plastic rotations of beams predicted by the proposed method correlated well with those from nonlinear static analysis.