Numerical modelling and vibration serviceability assessment of a steel footbridge with a significant 3D dynamic behaviour

This paper focuses on the vibration serviceability assessment and numerical modelling of an existing steel truss footbridge located in the outskirts of Castellon, Spain. The footbridge is rather slender and composed by a main span and two access ramps supported on three 4-arm piers of different heights. Due to the connection between the main span and the ramps at the top of the tall piers, longitudinal and lateral bending/torsion natural coupled modes of vibration coexist at low frequencies, with a relevant contribution of the piers and access ramps deformation. This leads to a significant three-dimensional and rather complex dynamic response under service conditions. With the aim of characterising the structural dynamic properties and assessing the level of vibrations induced by crossing pedestrians, two in-situ experimental test programmes are conducted. On the one hand, the structural response is measured during several hammer tests and the modal properties are identified and used to update a detailed 3D numerical model by means of a Genetic Algorithm. Due to the lack of information regarding the detailing of the piers foundations, two alternative models are analysed. The relevance of the pier-foundation system rotational stiffness is highlighted for the particular configuration. On the other hand, the footbridge main span response is recorded under different pedestrian activities: walking, running and vandal simulated actions. Finally, the vibration serviceability of the structure is assessed based on current codes and regulations.