Assessment of the Seismic Response of Concentrically-Braced Steel Frames

Abstract

The BRACED project investigated the ultimate behaviour of concentrically braced frames (CBFs). The research programme was designed to validate empirical models for the ductility capacity of hollow section bracing members and recent proposals for the improved detailing of gusset plate connections, to identify active yield mechanisms and failure modes in different brace member/connection configurations, and to provide essential data on the earthquake response of European CBFs. The central element of the integrated experimental and numerical research programme is a series of shake table experiments on full-scale model single-storey CBFs designed to Eurocode 8 (CEN EN 1998-1:2004, Eurocode 8: Design of structures for earthquake resistance—Part 1: General rules, seismic actions and rules for buildings. European committee for standardization, 2004). Twelve separate experiments were performed on the Azalee seismic testing facility at CEA Saclay. The properties of the brace members and gusset plate connections were varied between experiments to examine a range of feasible properties and to investigate the influence of conventional and improved design details on frame response. Each experiment examined the response of the test frame and brace-gusset plate specimens to table excitations scaled to produce elastic response, brace buckling/yielding and brace fracture. These experiments were supported by complementary quasi-static cyclic tests and correlative numerical simulations using pushover and time-history analysis using the OpenSees seismic analysis software. The outputs of the research programme represent a unique set of data on the ultimate earthquake response of CBFs with realistic brace members and connections. The principal experimental outcomes include measurements of elastic frame stiffness and its evolution with brace damage, measurements of the displacement ductility capacity of the brace specimens; an evaluation of the influence of brace connection configuration and gusset plate detailing on frame stiffness, damping and ductility; and observations on the contributions of brace and connection yielding to overall inelastic deformation of CBFs.