Influence of the design objectives on the seismic performance of steel moment resisting frames retrofitted with buckling restrained braces

Buckling restrained braces (BRBs) represent an effective strategy for the seismic retrofit of existing steel moment resisting frames (MRFs), as they contribute to increasing the strength and ductility capacity of the structure. However, current design strategies do not provide recommendations on how the performance increase is achieved. Prioritising either the increase of strength or ductility capacity has an impact on the damage evolution and affects the overall performance of the structure. A low increase of strength typically requires larger exploitation of the ductility capacity (i.e., damage) of the existing structure, while a high increase of strength produces a significant increase of stiffness, which is often accompanied by an increase of the seismic demands that may limit the effectiveness of the retrofitting solution. The present study assesses the impact of these decisions on the overall performance of steel MRFs retrofitted with BRBs. For this purpose, two MRFs with several BRB retrofitting configurations are used as case study structures. Finite Element Models are built in OpenSees and assessed through Incremental Dynamic Analyses to account for the record-to-record variability. Fragility relationships are derived based on local Engineering Demand Parameters (EDPs) to describe structural and non-structural damage, as well as path-dependent damage indicators (i.e., residual drifts and cumulative ductility in BRBs). A comparison of the overall performance of the structures is carried out in terms of risk estimates for a high seismicity location.