Substructure-based design method for vertically irregular steel buckling-restrained braced frame structures

Abstract

Buckling-restrained braces (BRBs) can mitigate the uneven inter-story drifts of vertically irregular steel frame structures during earthquakes, preventing unfavorable seismic forces and failure patterns. However, the structural lateral displacement profile of vertically irregular steel buckling-restrained braced frame structures (VSBFSs) can hardly be predicted; hence, parameter tuning and iterative designing are commonly required in conventional direct displacement-based design (DDBD) methods, which strongly rely on the experience of designers and consumes a considerable amount of time. This study proposes a substructure-based design (SBD) method developed explicitly for VSBFSs. In this method, the substructures of every story of a VSBFS are separated from the entire structure and individually analyzed in the subsequent procedures. The boundary conditions of the substructures are initially simplified as hinges, and an explicit equation of BRB stiffness and inter-story drift for the substructures is established. An iterative calculation procedure is consequently applied to correct the equation by considering the influence of the constraints of adjacent stories. In this manner, the inter-story drifts of all the stories of the entire structure uniformly converge to the predetermined targets, in which the demand of BRBs for each story is also determined. Compared with conventional DDBD methods, the proposed method does not assume the structural lateral displacement profile and is not based on experience, yielding relatively economical and safe design outcomes in a timely manner. The proposed SBD method was first validated by comparing it with existing elastic and elastoplastic design methods through a 15-story VSBFS example. Then, the applicability of the SBD method was further explored through four 6-story VSBFSs with different plan layout reductions and two VSBFSs with heights of 20 stories and 30 stories. Finally, the SBD method was applied to redesign the BRBs of a vertically irregular thermal power plant model (an example of a VSBFS) in a shaking table test.