Design of BRBs for and Collapse Performance of Strongback Braced Frames with Varying BRB Configurations

Strongback braced frames (SBFs) are a relatively new structural system with potential benefits to seismic performance, resilience, and design flexibility. Design of SBFs involves selection of energy-dissipating resistance—typically buckling-restrained braces (BRBs)—and sizing the elastic spine, here termed the strongback. The current design methods for the BRBs do not support the potential design flexibility of SBFs because they do not support both the redistribution of BRB forces and configurations without BRBs at every level. Furthermore, it is unclear if proposed design methods for the strongback adequately support configurations without BRBs at every level. In this work, an overturning-based method for designing the energy-dissipating resistance is investigated. The method is simple, supports a broad range of configurations, and can be formulated to produce a single BRB size. To evaluate the performance of the method, a simplified model with a rigid strongback was used to evaluate a broad set of input configurations. A more detailed model with actual member sizes was used to design and evaluate the performance of a subset of the input configurations to evaluate the ability of proposed strongback design methods to support configurations without BRBs at every level. Nonlinear time history analysis and incremental dynamic analysis were used to evaluate structural responses and probability of collapse. The results of the analyses show that the overturning method provides BRB sizes that can provide consistent collapse performance for a broad range of SBF configurations. However, results from the more detailed model show that current strongback design methods may not be appropriate for designing SBFs without BRBs at every level. These results imply that design of the strongback is the more challenging task, and that a simple and flexible method for selection of the BRBs is both appropriate and desirable. This work provides a candidate for such a design method, and identifies necessary developments to strongback design so that the potential benefits of SBFs can be realized.