Evaluation of state-of-the-art capacity design approach to seismic design of steel concentrically braced frames

Abstract

Ductile concentrically braced frames (CBFs) are commonly used seismic force-resisting systems in seismic regions. The capacity design approach described in the state-of-the-art seismic codes of the US, Canada, and Turkey defines a design procedure for columns and girders via two structural analyses that intend to capture the nonlinear behavior of CBFs while assuming braces in different stories undergo yielding or buckling, simultaneously. It would be, therefore, befitting to question the validity of the anticipated mechanism controlling CBF design considering limited studies in the literature. In addition to chevron and two-story X-braced frame configurations, an alternative design approach is proposed without changing the current habits in the industry. The present study aims to investigate whether or not the capacity design approach adopted by the leading seismic codes is consistent with the nonlinear behavior of CBFs via pushover analyses and time-history analysis. For the proto-type CBFs, peak and residual inter-story drift ratios were compared as well as ductility demands of braces. In conclusion, it is found that mechanism scenarios stimulated in current seismic codes are unlikely to occur under an earthquake event and the proposed balanced design approach provided significant reductions on seismic demands, only with an 8% increase in weight.