Seismic evaluation of geometrically irregular steel moment resisting frames with setbacks considering their dynamic characteristics

A setback building has a sudden discontinuity in the frame geometry along the height. This kind of irregularity causes an abrupt discontinuity in stiffness, strength and mass of the building frame. In this study, a total of nineteen mid-rise 9-story steel moment resisting frames with setbacks including the broad range of different geometrical configurations were studied. An eigenvalue analysis was performed to evaluate and scrutinize the dynamic characteristics of setback structures. The effect of geometrical configurations on the seismic responses of setback frames was studied by means of nonlinear response history analysis using a set of far-field ground motion records. Moreover, due to the rapidly increasing use of pushover analysis for the seismic evaluation of structures in recent years, enhanced pushover analyses (EPAs) including the modal pushover analysis, the upper bound pushover analysis, the consecutive modal pushover and the extended N2 methods were implemented as a main part of this study. The findings show that two factors including the location of setback and the degree of setback are of key importance and influence the dynamic characteristics and seismic responses of setback structures. The degree of accuracy of the enhanced pushover analysis methods generally depends on the dynamic characteristics (geometrical configuration) of the setback frames. The largest error in the EPAs in predicting the story drifts generally occurs in a setback frame with a larger amount of the ratio between the effective modal participating mass ratio of the higher modes and that of the first mode.