The multispan stone-masonry bridge was built in 1906 in the Ribellasca valley, along the border between Italy and Switzerland. The bridge is 100 m long and its five arches are supported by stone-masonry pillars which reach the maximum height of 25 m. After a preliminary historical analysis of the construction and restoration phases, a wide experimental investigation, including in situ and laboratory tests, was carried out in order to analyse the static and dynamic behaviour of the bridge. Coring and video-camera surveys were used to analyse the structural characteristics of the masonries and flat-jack tests to measure the state of stress and the deformability characteristics. The vibration modal shapes of the bridge were than determined by in situ dynamic test. All the experimental investigations as well as the installation of the accelerometers for the dynamic test were carried out with the aid of specialized climbers without any scaffolding. Owing to the difficulties of calculation of an historical bridge, it seemed advisable to compare the results of two different types of structural analysis: "rigid block analysis" proposed by Gilbert and "non-linear analysis with finite element model" which was carried out by using the mechanical parameters obtained by the experimental investigations. The two types of calculation which were used for the structural analysis of the bridge lead to convergent results and clearly show that the structure does not respect the safety standards required by the regulations in force. For this reason a strengthening intervention has been proposed and the design of the intervention has been developed in order to induce negligible modifications of the actual structural behaviour of the bridge and to preserve at the same time the visual impact of the structure.
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