Nonlinear Finite Element Modeling of the Load-Carrying and Energy Dissipation Capacities for Buckling-Restrained Braces

Abstract

Conventional concentric braces buckle under compressive loading. To overcome this limitation, the Buckling-Restrained Braces (BRBs) were introduced in the mid 70's which typically consist of a low-yield steel core and a restraining mechanism, which in this study is Concrete-Filled Steel Tube (CFST). This thesis identifies the most influential BRB design parameters, and examines their effect on the load-carrying and energy dissipation capacities under cyclic loading. This investigation is carried out by developing a nonlinear finite element (FE) model using the commercial software ABAQUS. The FE model is verified against experimental tests reported by the University of California at San Diego and a good correlation has been observed. An extensive parametric study is conducted in this research using the verified nonlinear FE model in order to achieve enhanced understanding for the BRB system behavior. The parameters of interest are related mainly to the steel core and the CFST encasement. The core parameters consist of the material yield stress, the core thickness, and the yield-to-total length ratio, whereas, the encasement parameters include the concrete thickness, the HSS thickness, and the concrete compressive strength. The parametric study program comprises of several FE simulations considering wide ranges of numerical values for the core and encasement parameters. The research concluded that core yield stress and thickness are the main contributing factors towards affecting the load-carrying and energy dissipating capacities of the BRB. It is also observed that using an excessively high yield-to-total length ratio increases the chance of global buckling for the BRB system. Conversely, using low yield-to-total length ratio triggers excessive stretch in the steel core (necking), especially when higher steel grades are used. The study also concludes that the investigated concrete compressive strengths, ranging from 14 to 41 MPa, did not have a considerable effect on the encasement confinement. Lastly, the HSS and concrete thicknesses are found to be the major factors that decide the effectiveness on the encasement in restraining the steel core against buckling.