OMA-Based FE Model Validation of a Lively Footbridge

The paper describes selected results from a comprehensive experimental and numerical study on the serviceability assessment of a footbridge over the Lambro River near Milano (Italy). The 3-span footbridge, for a bicycle-pedestrian mixed use, is 107 m long, 4.4 m wide and roughly symmetric about both mid-span and the longitudinal axis. Ambient vibration tests identified the footbridge modal properties, detecting the fundamental bending mode, with the maximum amplitude recorded at mid-span, at 1.75 Hz, well within the critical range of excitation from walking pedestrians. Hence, the footbridge can represent an useful benchmark for serviceability assessment and development of modelling and response computation tools. In this light, the numerical and experimental studies here presented focused on the development of a validated finite element (FE) model, able to reproduce the experimental frequency of the second torsional mode at 2.91 Hz, not caught in previous works. The paper, that systematically addresses all the possible causes for discrepancy, shows the importance of the correct representation of the two end spans. The assessment of the serviceability conditions, was previously performed according to HiVoSS guideline and through a series of forced vibration tests involving single pedestrians or groups of up to 12 people, walking in resonance conditions with the first mode along straight trajectories. The availability of the validated FE model and of a series of experimental tests makes it possible to establish, based on this case study, a benchmark for future studies on human-induced vibration and human-structure interaction.