The paper investigates the reliability of retrofitted multi-span simply supported bridges using seismic isolation strategy. For bridges, the seismic isolation aim is to protect the support piers by limiting the seismic shear transferred by the deck. The effectiveness of this retrofit strategy depends on the mass ratios and overall dynamic interaction between the decks and the piers. To this end, a portfolio of Italian bridges, built in the 60s with box girder and different retrofit isolation strategies, have been considered. The effectiveness of the examined seismic protection strategies has been investigated through fragility curves within the PBEE method. In particular, performance indexes, evaluated by nonlinear dynamic analyses, have been considered to calculate fragility functions using MSA method approach. The achieved results lead to assess the effectiveness of the seismic isolation retrofitting of simply supported span bridge, highlighting the influence of the piers on dynamic behaviour.
