Suhaib Salawdeh

Recommendations for numerical modelling of concentrically braced steel frames with gusset plate connections subjected to earthquake ground motion

Abstract Concentrically Braced Steel Frames (CBFs) are commonly used as an economic and effective means of resisting the lateral loading induced during earthquakes and limiting the associated displacements. Here, an integrated experimental and numerical approach is taken to investigate the performance of CBFs subjected to seismic action of varying intensity. As part of the BRACED transnational research project, shake table experiments on full-scale single-storey CBFs recorded the response of test frames employing various combinations of bracing member sizes and gusset plate connection details to simulated ground motions scaled to produce elastic response, brace buckling/yielding and ultimately brace fracture. This recorded experimental data is used to validate a numerical model developed using the OpenSees seismic response analysis software. Key experimental and numerical model responses are compared. The sensitivity of the model to variations in modelling parameters is assessed and recommendations for future numerical modelling are presented. Results indicate model performance is sensitive to the initial camber applied to the brace members, with a value of 0.8% of overall brace length observed to achieve a more accurate representation of global frame stiffness and drift response than the lower values previously recommended in literature, but an underestimate of the compression resistance of the brace.

Assessment of the Seismic Response of Concentrically-Braced Steel Frames

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.

Direct displacement-based seismic design of multi-storey concentrically braced frame structures

A Direct Displacement Based Design (DDBD) methodology is validated for ‎concentrically braced frame (CBF) systems. CBFs are widely used as the lateral ‎resistance systems because they overcome many limitations in other resistance ‎systems. The validity of the method is assessed by designing four and twelve-‎storey CBF buildings, in which the performance is checked using nonlinear time ‎history analysis (NLTHA) employing eight different accelerograms with ‎displacement response spectra matching the design displacement spectrum. The ‎results show that the displacements and drifts obtained from NLTHA are falling ‎within the design displacement limits used in the DDBD procedure. However, the ‎results are found to be conservative when assuming no contribution to the lateral ‎resistance by compression members.‎