Probabilistic performance assessment of low ductility r.c. frames retrofitted by elasto-plastic dissipative braces

The paper illustrates a probabilistic methodology for assessing the vulnerability of existing r.c. buildings with limited ductility capacity and retrofitted by means of dissipative braces. The methodology is based on the development of single component and system fragility curves before and after the retrofit. The proposed approach allows to highlight the possible changes in the most significant collapse modalities before and after the retrofit and to evaluate the effectiveness of the retrofit by taking into account the probabilistic properties of the seismic behaviour of the considered systems. A benchmark 2-dimensional reinforced concrete frame with low ductility capacity is considered as case study. The frame is designed for gravity-loads only and does not comply with modern anti-seismic code requirements. It is retrofitted by introducing elasto-plastic dissipative braces designed for different levels of their target base-shear capacity, following a design method involving the pushover analysis of the system before and after retrofit. The obtained results show that the use of braces yields a significant increase in the seismic capacity, though an increased dispersion of the behaviour is observed in the retrofitted system