Output-Only Identification of a Footbridge Equipped with Isolators

System identification aims to characterise the behaviour of an unknown system using experimental data. This process is widely used in the field of civil structures, where its application can be useful for vibration-based damage detection strategies and for finite element models updating. This work focuses on the dynamic tests and the output-only identification of a footbridge located in Rieti, Italy. The structure is a steel through arch bridge supported on abutments by high damping rubber isolators and, like many other similar structures, it suffers from excessive vibration caused by human-induced excitation. Dynamic tests for structural identification were carried out in order to investigate the influence of the isolators on the dynamic behaviour of the structure, in particular on the frequencies and damping ratios, and to identify the causes of the serviceability problems. The modal parameters were identified using an output-only subspace technique and compared with those obtained from a preliminary finite element model. This comparison highlights the need to update the finite element model on the basis of experimental results.