Design of concentrically braced steel frames for earthquakes
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AbstractThe west coast and eastern zones of Canada are susceptible to earthquakes such as those that have happened in California. Braced frame construction is advantageous in reducing earthquake-induced displacements, but is susceptible to loss of ductility, currently attributed to brace behaviour. A numerical model has been developed that is capable of accurately simulating the hysteresis behaviour of bracing members. The factors that affect that behaviour can therefore be studied without conducting expensive experimental tests. The three-dimensional, nonlinear, elasto-plastic finite element model has been used to simulate the behaviour of hollow structural steel (HSS) tubes connected to gusset plates at their ends when subjected to reversed axial displacements. A combined orthotropic kinematic hardening material model was used. Four-node, finite membrane strain quadrilateral shell elements were used to model both the HSS and the gusset plate. Fixed boundary conditions were applied at the ends of the specimens with axial displacements imposed for each cycle. The model was able to simulate the hysteresis behaviour of previously tested bracing members (Shaback 2001) in terms of the peak loads and displacements, energy dissipation, fracture life and the degradation of both strength and stiffness. Based on the results of the parametric study carried out using the finite element model, a total of 10 concentric braces were tested in the structural engineering laboratory at the University of Calgary. The braces were designed according to the weak brace/strong gusset concept. The objectives of the experimental study were to quantify the effects of displacement history, gusset plate thickness, and effective slenderness ratio, on the energy dissipation capacity and fracture life of these braces. The results showed good agreement with findings from previous studies at this university. The finite element model was refined to include a damage accumulation model for low-cycle fatigue. The model provided new insight for analysis of load-displacement hysteresis curves. New fracture life and energy life equations are thus proposed, based on a better understanding of the important parameters influencing brace behaviour.
Bibliography: p. 297-301.