Seismic stability of buckling-restrained braced frames

Buckling Restrained Braced Frames (BRBFs) are widely used as a seismic force-resisting system due to their advantageous properties for ductility and energy dissipation. However, because of the modest overstrength and relatively low post-yielding stiffness, BRBFs subjected to seismic loading may be susceptible to concentrations of story drift and global instability triggered by P-Δ effects. Due to the use of simplistic methods that are based on elastic stability, current code design provisions do not address seismic stability rigorously and do not consider the particular inelastic response of a system. As can occur in multistory structures, even for ductile systems, BRBFs tend to develop drift concentration that is intensified by P-Δ effects and may lead to dynamic instability through the formation of story mechanisms. Furthermore, large residual drifts have been observed during numerical and experimental studies of BRBFs. Beyond code provisions, several alternatives that aid in preventing these undesirable response characteristics of BRBFs have been studied before. This study used the FEMA P-695 Methodology to evaluate the response of current U.S. code-based BRBF designs and to study the effect on seismic stability of additional alternatives. In accordance with the Methodology, the collapse performance was evaluated through nonlinear static and dynamic analyses that were used to investigate the inelastic behavior and determine the collapse fragility of each considered prototype. Several design prototypes, with different number of stories, were developed to study code-based stability provisions, and three alternatives of improvement: strong-axis orientation for BRBF columns, gravity column continuity, and BRBFSMRF dual systems. Furthermore, two design procedures were studied for the BRBF-SMRF dual systems. In this thesis, results from the collapse performance evaluation process are presented and discussed for the different alternatives to address seismic stability of BRBFs. Results from nonlinear static (pushover) analyses and nonlinear dynamic (response history) analyses allowed assessment of seismic behavior through critical response quantities, such as overstrength, ductility, story drift and BRB demands. Finally, results from collapse performance evaluation permitted quantifying the improvement that is achieved with each alternative and provided a means of comparison. The well-established negative impact of P-Δ on the seismic stability of BRBFs was demonstrated and the improvement achieved by the use of current code provisions for global stability through the B2 multiplier was shown to be minimal. Since code provisions for global stability are based on elastic stability considerations, essentially the same inelastic behavior was observed whether or not code provisions related to stability were used. In contrast, the increased flexural capacity provided by the use of strong-axis orientation for BRBF columns significantly improved the seismic stability performance of the system. Similarly, the flexural strength contribution provided by continuous gravity columns resulted in considerably improved performance. These two alternatives helped preventing the formation of story mechanisms and distributing inelastic demands more evenly. Finally, BRBF-SMRF dual systems demonstrated superior seismic stability performance compared to all other alternatives. The improvement achieved by the use of these systems is related to the contribution of the SMRF that remains elastic after the BRBs have yielded and later provides restoring forces and additional energy dissipation capacity. Overall, it was observed that the most important condition for seismic stability is reliable positive stiffness at large inelastic drifts, and this is not addressed by current code provisions. The small increase in primary system strength that arises from current code provisions based on elastic stability considerations may in some cases provide a small benefit with respect to seismic stability, but these provisions do not fundamentally address inelastic seismic stability behavior