Seismic performance of Steel MRFs retrofitted with BRBs: Influence of the design decisions for the devices

Buckling Restrained Braces (BRBs) represent an effective strategy for the seismic retrofit of existing steel Moment Resisting Frames (MRFs), as they contribute to increase the strength, stiffness and energy dissipation capacity of the frame. Nonetheless, the design choices made during the retrofit process have a significant impact on the performance of the structure. For example, the inclusion of ‘large’ BRBs (i.e., high yielding strength and stiffness) may contribute to limit the deformation demands in the MRF; nonetheless, it may also induce large forces in the beams and columns of the existing structure. On the other hand, the inclusion of ‘smaller’ BRBs (i.e., low yielding force and stiffness), while allowing reaching the required safety requirements, may not be able to protect the MRF from damage. Additionally, the sizing of the BRB elements has an influence on the seismic demand parameters affecting the global performance of structural and non-structural components (i.e., peak and residual drifts, as well as storey accelerations). The present study investigates the impact of the design choices in the seismic performance of a retrofitted three-storey case-study frame by considering three retrofit options. The case-study MRF for the bare frame and the three retrofit configurations are modelled and numerically investigated in Opensees by monitoring local damage states (e.g., damage in BRBs, beams, columns, panel zones). First, a comparison is made in terms of non-linear static analyses to identify the deficiencies of the structures. Then, a fragility analysis is carried out through Incremental Dynamic Analyses (IDAs) accounting for the influence of the recordto-record variability. Finally, a comparison is made in terms of local and global Engineering Demand Parameters, by developing fragility curves for the components, for storey drifts and accelerations