Reserve capacity and implications for seismic collapse prevention for low-ductility braced frames in moderate seismic regions

Although there has been extensive research on achieving ductile behavior and robust performance for steel structures in high seismic regions, little is known about the behavior and performance for low-ductility steel structures that are prevalent in moderate seismic regions. The seismic lateral-force-resisting system recognized in ASCE 7 that is currently widely applied to steel structures in moderate seismic regions is the R=3 system, where no ductile detailing is required. While not explicitly stated, the viability of an R=3 system rests on the notion that there is some degree of reserve capacity within the system that allows design forces to be reduced when there is no ductile detailing. The present research is systematically studying reserve capacity and its effect on the seismic behavior and performance of low-ductility braced frames. Specific aspects of the research are: (1) modeling and full-scale testing of beam-column gravity connections with angles; (2) full-scale testing of nonductile brace connection limit states; (3) large-scale testing of multi-story braced frames with nonductile limit states; (4) nonlinear static and dynamic analysis of buildings with nonductile braced frames. In all cases, quantifying reserve capacity and establishing the effect of reserve capacity on the system behavior are emphasized. These results provide a valuable resource for design of low-ductility steel braced frame systems to provide seismic collapse prevention performance.