The bridges seismic vulnerability is a relevant subject, since a significant part of existing bridges has been designed without considering any seismic action or using technical references that suggested seismic actions generally lower than those currently defined. This generally leads to unsuitable structural performance of existing bridges against earthquakes. Moreover, it must be taken into account that, in the response spectrum analysis method, the seismic stresses are significantly dependent on the structure vibration periods, which is also a consequence of modelling structure base constraint.

This work aims to study the effect of soil-structure interaction on the seismic vulnerability of the recurring type of reinforced concrete viaduct, taken into account by modelling the deformable base constraint. Starting from the real case study of a viaduct with prestressed concrete beams and reinforced concrete piles, the analyses carried out show the soil-structure interaction effect in terms of the ratio between stress and resistance of the piles, also varying the soil type and the piles slenderness.

It is also possible to draw some interesting observations in terms of the influence of the soil-structure interaction on the possible seismic retrofitting interventions.