Seismic multi-objective optimization of vertically irregular steel frames with setbacks upgraded by buckling-restrained braces

Abstract

To prevent the vertically irregular structure with setbacks from unfavorable weak-story failure mode in major earthquakes, an upgrade optimization method for installing the buckling-restrained braces (BRBs) is presented. An index based on modal properties is proposed to quantify the vertical irregularity and introduced as constraints to consider the impact of stiffness from additional BRBs. In addition, constraints of deformation and damage are included based on the performance-based seismic design (PBSD) requirements. While minimizing the upgrading cost, the hysteretic energy is transmitted to the secondary BRBs by maximizing their hysteretic energy ratio (HER) at the Collapse Prevention (CP) performance level. The multi-objective non-dominant sorting genetic algorithm II (NSGA-II) is implemented to solve the proposed optimization formula. The upgrade optimization for a set of vertically irregular steel frames with various geometrical setbacks shows that the vertical regularity constraints are more paramount to original vertically irregular structures, and their energy dissipation mechanism can be significantly improved by the story HER objective.