Impact of unique practice in concentrically braced frames on their seismic performance

The seismic provisions for special concentrically braced frame (SCBF) expect the braces capable of providing deformation of 10-20 ductility without fracture in order to withstand design earthquake ground motions. Seismic evaluation studies of the design provisions for SCBF, however, are substantially different from those for any other seismic-force-resisting system (SFRS) in three aspects: (a) the original design provision for a SCBF set a b/t ratio limit on braces to prevent fracture, but such b/t ratio limit is proven incapable of preventing brace fracture; (b) SCBF is the only SFRS in which the brace fracture termination of energy-dissipation mechanism is considered normal instead of the limit state of seismic design; and (c) the collapse probability evaluation and development of design parameters (R, omega 0, Cd) are formulated explicitly including potential but uncertain overstrength factors inherently existing in typical steel buildings. This paper describes a comprehensive inquiry into the origin and evolution of the unique practice in SCBF over a span of 30 years (1992-2022) and its impact on seismic performance and potential hazard of steel buildings with SCBF as a designated SFRS in steel buildings. The important conclusions include (a) seismic braces have fracture ductility range of 5-10; (b) the seismic design provisions and performance evaluation on SCBF have been based on post brace-fracture analysis of buildings with SCBF that is extremely unreliable and often overly optimistic; and (c) The unique seismic design practice of SCBF has a significant impact on seismic performance of steel buildings with SCBF.