Modal testing through forced sine vibrations of a timber footbridge

The work aims at performing the dynamic modal identification of the long-span laminated timber footbridge built on the Marecchia River near Rimini, Italy. A first sine vibration test has been performed adopting a mechanical shaker just after the footbridge construction in 2000 in order to check the structural behavior that has been assumed in the project. A second test has been replicated in 2005 using the same excitation and almost the same test set-up adopted in the first one. The dynamic modal extraction is performed on the FRFs through both the peak picking method, applied together with the half power bandwidth technique, and the circle-fit method. The orthogonal properties of the identified mode shapes are verified through the Modal Assurance Criterion (autoMAC). Finally, the results obtained for the two tests and through the two techniques are mutually compared. It is worth noting that the FRFs, evaluated for different intensity levels of the exciting force, reveal an inherent non-linear behavior of the footbridge. The analyses also show that the first natural frequencies of the structure are included in the frequency range of the human step and this could lead to unpleasant feelings for the pedestrians.