Experimental modal properties of a steel arch bridge during the static load test

Base isolated steel arch bridges are very complex engineering systems whose behaviour may depend not only on an accurate design but also on its correct realization during all construction phases. One of the most effective method, fast and cheap, to check the global behaviour of the structure and the correctness of the design structural models is based on the comparison between modal parameters, in terms of frequencies and mode shapes, experimentally identified by means vibration measurements with those theoretically predicted by the structural models. This paper describes the dynamic test carried out during the standard static load test of a half-through steel arch bridge. The bridge is built over the Potenza river, in central Italy, and its structural design is characterized by some notable aspects: the river crossing is obtained by two coupled steel arches, having a span length around 115 m, and a steel-concrete composite deck sustained by thirty couples of steel hangers; the arches are supported on rubber bearing seismic isolators. An accelerometer array was installed over the bridge deck to study the evolution of the modal parameters of the structure at specific phases of the load test: first on the unloaded configuration, then on two different loaded configurations, made by placing trucks over the deck, and finally after the bridge unloading. Measurements of ambient vibrations were carried out and Operational Modal Analysis was performed to estimate modal properties of the deck. Experimental values of the resonance frequencies and related mode shapes are compared with those obtained with the predictive finite element model developed for design purposes and opportunely modified to account for the real conditions of the bridge during the tests. In particular, the dynamic interaction between bridge and trucks was investigated by considering trucks not only in terms of added mass but as dynamic systems characterized by mass, stiffness and damping properties. The obtained results show the effects of the trucks on the dynamic parameters of the bridge and allow the reliability assessment of the numerical model