An evaluation of safety and durability of masonry arch bridges with the aim of their conservation and restoration is here presented. The purpose is to design interventions of strengthening in the respect of architectural and cultural values. In case of historical masonry structures there are many uncertainties and the risk is to do not properly understand the structural behaviour. This could bring to unnecessary or over dimensioned interventions. Therefore it is necessary to define a methodology of analysis able to consider all the aspects. The work proposes a procedure for structural analysis based on a multi-scale approach. Models with different levels of detail are analyze in order to evaluate their applicability and reliability. The target is to define guidelines for the structural analysis of masonry arch bridges. The methodology is applied on a case study: the Venice trans-lagoon rail bridge. The subject is topical. The Italian railroad network includes thousands of masonry arch bridges, mainly built during the XIX century, that are still in exercise. This is also common in other European countries. The present rail traffic is heavier than in the past. Weight and velocity of trains had been consistently increased. For these reasons technical regulations about rail bridge increased the overloads that must be considered in the design of new rail bridges. These overloads have to be applied to all the bridges belonging to the Italian railroad network, thus they have to be taken into account even for the evaluation of existing bridges. The capacity of historical masonry arch rail-bridges to carry the actual traffic must be verified. Considering the great number of those bridges, the aim is to define a methodology of analysis that must be reliable and fast. Venice Trans-Lagoon Bridge is an historical masonry arch rail-bridge that was built in 1840’s to connect Venice with its mainland. It consists of 222 arches, 10 metres span, organized in six modules of 37 arches separated by artificial islands, for a total length of more than 4 kilometres. Each module is subdivided in sequences of five or seven arches, separated by big piles. The study of its structural behaviour combines analyses are carried on models representing the bridge in different scales and with different levels of detail. Bridge has been divided in its component: one single arch, used to study the local behaviour of arch, and sequences of 5 and 7 arches, used to study the global behaviour of the bridge. Static and natural frequency analyses utilize 2D and 3D finite element. Different combinations of load are applied to define the worst situation. Limit analysis is carried on to define the critical load factor and mechanisms of collapse. The procedure here presented is the first step of a wider research about restoration and conservation of historical masonry arch bridges.
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