Global and local dynamic behaviour of the Sesia viaduct, a steel-concrete composite railway bridge on the HS line Turin-Milan,

Steel-concrete composite bridges have become an important alternative to concrete structures in the development of new high speed railway lines throughout Europe, due to considerable advantages concerning design, construction time, durability and costs. Nevertheless design process highlighted many still open problems related to the estimation of dynamic and fatigue effects due to the HS train passages lacking reliable numerical models and suitable assessment methodologies. In the framework of the recently launched research project "DETAILS" (funded by European Commission), that aims at analyzing dynamic interaction phenomena and fatigue behaviour of these types of bridges, the Sesia Viaduct, a steel-concrete composite box girder bridge, has been deeply investigated by means of a wide experimental modal analysis campaign in operational conditions. In the experimental campaign, three spans of the bridge were instrumented by accelerometers covering 103 sensor locations and by optic fibers, to obtain detailed mode shape information for correlation and updating of Finite Element models. Furthermore the accelerometer locations have been designed in order to identify the global dynamic behaviour and the ones of particular substructure and elements (i. e. the bottom steel plate, the diagonal of the cross girder, the bottom flange stiffener etc.). Fiber optic sensors have been used to measure the strain field at two stations of the bridge in order to analyze the distortions of the cross section under the train passages. The paper present the acquired vibration data under ambient and train excitation, showing Operational Modal Analysis results and providing a comprehensive description of the dynamic behaviour of the whole structure and relevant structural elements.