SEISMIC DEMANDS ON STEEL BRBF CONSIDERING VARIOUS BRB HYSTERETIC MODELS

Steel frames equipped with buckling restrained braces (BRBF) are widely adopted as lateral load resisting system in recent decades. The buckling restrained braces (BRB) can provide superior load-carrying capacity, as well as excellent energy dissipating capability than the conventional steel braces. For simplicity, the seismic behaviour of BRB has often been modelled as bilinear or trilinear models. Many important research results on seismic demands of BRBF, such as maximum inter-story drift, cumulative plastic ductility, and residual inter-story drift, are based on these simplified models. However, experimental studies have identified significant isotropic hardening as well as tension-compression asymmetry characteristics of BRB under cyclic loading. The isotropic hardening effect of BRB is due to the strain hardening of steel core plates, while the capacity asymmetry in tension and compression is mainly due to friction between core and sleeve in compression. Specifically, the capacity of BRB in compression may as higher as 15% than that in tension. In recent decade, some researchers have developed various novel models to represent realistic BRB hysteretic behaviour in numerical studies, such as trilinear force-deformation analytical model, Bouc-Wen model, modified Ramberg-Osgood combined hardening model and Menegotto-Pinto combined hardening model. Zona et al. reviewed the benefits and limitations of those models and developed a new refined hysteretic model of BRB, (hereinafter referred to as Zona model), which accurately considered kinematic and isotropic hardening as well as the tension-compression asymmetry behaviour of BRB. This refined BRB model has been developed into OpenSees 2.5.0, called SteelBRB uniaxialMaterial. How much deviation will be brought by using a simplified model of BRB to quantify seismic demands on BRBF is still a research gap. Since the refined hysteretic model of BRB is available currently, it is time to systematically quantify the seismic demands on BRBF considering various BRB hysteretic models, thus providing an insight into the performance-based seismic design of BRBF. In this study, three and six-story BRBF are adopted as the prototype structures. Three hysteretic models of BRB, including Zona model and two bilinear models, are considered. The maximum inter-story drifts, cumulative plastic ductility and residual inter-story drifts response are compared under both design-based earthquakes and maximum considered earthquakes. It indicates that the simplified hysteretic models of BBR can predict the maximum inter-story drift response of BRBF with acceptable accuracy. Meanwhile, the simplified models have natural deficiencies in predicting the structural residual drift and cumulative plastic ductility of BRB. This limitation should be fully considered if simplified models are adopted in seismic performance evaluation of the BRBF.