A Hybrid Seismic Design Method for Steel Irregular Space Moment Resisting Frames

A performance-based hybrid force/displacement (HFD) seismic design method for space steel moment resisting frames irregular in plan view and in elevation is developed. Irregularity in elevation is either due to non-uniform distribution of mass or due to the presence of setbacks along the height of the frame. More specifically, 30 different frames irregular in plan view for the first case (plan-irregularities), 40 frames with setbacks (vertical stiffness irregularities) for the second case, and 18 frames with mass discontinuities (vertical mass irregularities) at the first, intermediate and top storey for the third case are considered. All these frames are designed according to Eurocodes 3 and 8 and subjected to 42 pairs of ordinary ground motions. Through nonlinear seismic analyses, seismic response databanks for these three types of irregular frames are generated corresponding to four performance levels. These databanks are then utilized for the development of simple expressions that determine the behavior (or strength reduction) factors of the frames. These are functions of frames geometrical/dynamic characteristics including measures of their irregularities as well as the target maximum interstorey drift ratio and member local ductility. The proposed design method, even though it is mainly a force-based design method, controls deformation and therefore damage through the proposed deformation-controlled behavior factors. Design examples are presented to validate the effectiveness of the method to account for the irregularity effects on the preliminary design stage while time-history analysis results demonstrate its advantages to control better the inelastic response of the frames over the conventional force-based seismic design method of Eurocode 8.