Probabilistic assessment of seismic behaviour of an existing RC building retrofitted with BRBs

The paper presents the seismic performance assessment of a four-storey reinforced concrete (RC) existing framed structure designed for gravity loads only. The sample structure exhibits high seismic vulnerability, i.e. low lateral resistance and limited translation ductility. The structural assessment and retrofitting was deemed necessary due to a variation of the building occupancy. Natural frequencies and damping ratios are experimentally investigated by operational modal analysis; the estimated modal properties were employed to calibrate the numerical finite element structural model. The retrofitting strategy encompasses the use of buckling restrained braces (BRBs). Such braces can be conveniently installed along the perimeter frames of the multi-storey buildings to lower the seismic demand on the existing structure and regularize its dynamic re-sponse. Local strengthening can, however, be necessary. Comprehensive nonlinear static (pushover) and dy-namic (response history) analyses were carried out for both the as-built and retrofitted structures to investigate the efficiency of the adopted intervention strategy. A suite of code-compliant natural strong motion records was selected and employed to perform inelastic response history analyses. A probabilistic approach has been employed to investigate the performance of the structure under critical events and to ensure its acceptable per-formance during its entire lifetime. The seismic fragility is calculated by implementing an incremental dy-namic analysis (IDA) using the method of multiple-stripe analysis. The outcomes of the inelastic analyses show that the existing structure exhibits extremely poor strength capacity and energy dissipation. It is also demonstrated that, under moderate and high magnitude earthquakes, the damage experienced by the retrofitted structural system is located in the added dampers; the response of the existing RC structure is chiefly elastic