Brace response and assessment: computation, experiments and design (BRACED)

ReportThe BRACED project investigated the ultimate behaviour of concentrically braced frames(CBFs). The research programme was designed to validate empirical models for theductility capacity of hollow section bracing members and recent proposals for theimproved detailing of gusset plate connections, to identify active yield mechanisms andfailure modes in different brace member/connection configurations, and to provideessential data on the earthquake response of European CBFs. The central element of anintegrated experimental and numerical research programme is a series of shake tableexperiments on full-scale model single-storey CBFs designed to Eurocode 8. Twelveseparate experiments were performed on the Azalee seismic testing facility at CEA Saclayin the March-May 2013. The properties of the brace members and gusset plate connectionswere varied between experiments to examine a range of feasible properties and toinvestigate the influence of conventional and improved design details on frame response.Each experiment examined the response of the test frame and brace-gusset platespecimens to table excitations scaled to produce elastic response, brace buckling/yieldingand brace fracture. These experiments were supported by complementary quasi-staticcyclic tests on a simplified version of the shaking table test frame that retained all of theimportant characteristics of that structure, including overall dimensions, beam-to-columnconnections, brace properties and gusset-plate connection design. Correlative pre-testpredictions and post-test simulations using pushover and time-history analysis were alsoperformed using the OpenSees seismic analysis software. The outputs of the researchprogramme represent a unique set of data on the ultimate earthquake response of CBFswith realistic brace members and connections. The principal experimental outcomesinclude measurements of elastic frame stiffness and its evolution with brace damage,measurements of the displacement ductility capacity of the brace specimens; anevaluation of the influence of brace connection configuration and gusset plate detailing onframe stiffness, damping and ductility; and observations on the contributions of brace andconnection yielding to overall inelastic deformation of CBFs. The numerical modelling hasvalidated a methodology of modelling this class of structure in OpenSees, while theproject as a whole supports an assessment of Eurocode 8 design guidance for CBFs.non-peer-reviewe