Seismic Performance Assessment of Low-Ductility Concentrically Braced Frames

Abstract

Current US seismic design provisions allow structures in areas of lower seismicity (e.g., the Midwest and eastern North America) to be designed with modest seismic requirements as compared with the requirements that must be satisfied in areas of higher seismicity (e.g., western North America). Because of the reduced seismic detailing and proportioning requirements in these areas of moderate seismicity, seismic force–resisting systems (SFRSs) within these regions are classified as low-ductility, in contrast to the high-ductility systems common in areas of higher seismicity. Considering the prevalence of low-ductility concentrically braced frames (CBFs) in moderate seismic regions of the US, a thorough performance assessment of these low-ductility CBFs was conducted using detailed OpenSees building models and dynamic numerical simulations. A matrix of 12 buildings was assessed, which varied by number of stories (3, 6, and 9), system configuration (chevron and split-x), and system type [R=3 CBF and ordinary CBF (OCBF)]. An additional set of six buildings was designed, using R=4 with modified seismic detailing and proportioning, to investigate an alternate prototype low-ductility SFRS with improved seismic performance. Using the established seismic performance evaluation framework, R=3 CBF systems did not pass, whereas all but one of the OCBF systems passed. All but one of the prototype R=4 CBF systems passed the seismic performance evaluation, and for the majority of cases, the weight-normalized performance was better than the corresponding OCBF systems.