A large portion of the existing masonry arch bridges in Italy are still in service in the infrastructure system and are located in a geographical area of high seismic risk. Most of them were built more than 100 years ago taking into account only gravitational loads during the design phase without any seismic analysis. For this reason, a seismic vulnerability assessment has been appointed as mandatory regular activity from the Italian government in order to define the priority of seismic retrofit interventions. In this study, a multi-span slender masonry bridge considered as the most vulnerable typology of masonry bridges to seismic action will be assessed. Then, the results obtained from three different seismic assessment approaches will be discussed and compared. In particular, two approaches based on different FE modeling and the last one built on rigid blocks analysis are considered. Finally, a detailed 3D finite element analysis allowed representing all the collapse mechanisms (global and local) of the bridges are presented. Simplified approaches, even though cannot describe all the collapse mechanisms of the bridges due to seismic action can lead to reliable results.

Historical masonry arch bridges are a fundamental part of the road and railway networks in Europe. Very often, due to factors such as lack of maintenance, increase of traffic loads, etc., these structures need interventions in order to guarantee their adequate structural performance. For this reason, an important research effort has been devoted in previous decades to study the behavior of masonry arches and to identify innovative techniques able to increase their ultimate capacity, such as fiber reinforced polymer (FRP) composites. In this paper, the results of an experimental campaign carried out on masonry arches strengthened with one FRP layer applied at the structure intrados are used to calibrate a numerical analysis model. Then, the model is used to predict the contribution that this type of strengthening would have had on the well-known Prestwood Bridge. The numerical results show that the hypothetical intervention of the Prestwood Bridge would imply an increase in the ultimate load of the structure, although it would be significantly lower than that usually obtained for the case of arches tested in laboratory.