Seismic Response of Single-Degree-of-Freedom Systems Representing Low-Ductility Steel Concentrically Braced Frames with Reserve Capacity

Abstract

Steel concentrically braced frames (CBFs) are used widely as a seismic lateral force–resisting system. Although modern CBFs designed for high seismic regions have considerable ductility, CBFs in moderate seismic regions are expected to have limited ductility, even when designed using modern provisions. In addition, older CBFs in high seismic regions also are expected to have limited ductility. In these low-ductility systems, reserve capacity (i.e., secondary strength and stiffness) plays an important role in seismic collapse prevention. Thus, quantifying the impact of reserve capacity on earthquake response for low-ductility systems is critical. This paper presents research that used single-degree-of-freedom systems to represent low-ductility CBFs, where brace fracture causes a sudden loss of strength and stiffness. Postbrace fracture stability was studied by considering variations in reserve system strength and stiffness parameters. Performance was evaluated by considering local ductility demands on the reserve system and global drift demands. For the cases considered, the ductility capacity of the reserve system was typically a more critical constraint than global drift capacity. Reserve capacity is demonstrated to appreciably influence seismic collapse behavior, whereas primary system strength has a small influence.