Seismic performance of dual systems coupling moment-resisting and buckling-restrained braced frames

Buckling-restrained braces (BRBs) have proven to be very effective in improving the seismic performance of existing and new structures. They provide strength, stiffness and added energy dissipation to the structure. However, being BRBs characterized by a low post-elastic stiffness, their use may lead to residual deformations hindering the building’s reparability and to excessive cumulative ductility demand possibly compromising the residual capacity of BRBs. To overcome these drawbacks, BRB frames (BRBFs) can be coupled with moment-resisting frames (MRFs) to form dual systems. If properly designed, MRFs acting as back-up frames, allow the control of the residual drifts and the optimization of the performance of the BRBs. The contribution of this study is to provide insights into the performance and residual capacity of BRBFs-MRFs dual systems and to shed light on the influence of the main BRB’s design parameters. To this end, a non-dimensional formulation of the equation of motion is introduced for a single-degree-of-freedom system, and an extensive parametric study is performed for a set of natural ground motion records with different characteristics and scaled to various intensity levels. This allows the investigation of a wide range of configurations, considering different levels of the relative strength and ductility demand of BRBFs and MRFs, and to obtain useful information for their design. Finally, two case study frames, modeled as two-dimensional nonlinear multi-degrees-of-freedom systems, are analyzed and the results compared to those obtained from the non-dimensional formulation to show the capabilities and the limitations of the adopted methodology and of the SDOF approximation.