Field-testing and serviceability assessment of a lively footbridge

Abstract Following a previous work, the paper describes the serviceability assessment of a footbridge over the Lambro River near Milano (Italy), based on the comparative analysis of on-field tests and of the outcome of the Hyvoss guidelines. 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. A reinforced concrete (RC) deck is supported by a steel structure: two longitudinal welded I-profiles, braced in their lower part, support the transverse beams that, in turn, support the RC deck. 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, a series of forced vibration tests was subsequently performed, involving single pedestrians or groups of up to 12 people. Sensors, located as in AVTs, recorded footbridge acceleration, both horizontal and vertical. In all the tests, pedestrians walked in resonance conditions with the first mode frequency along straight trajectories. To excite further the first mode, their spatial configurations were symmetric about the longitudinal axis of the bridge. Difference among tests concerns not only the number of test subjects (TSs), but also their spatial configuration and/or the TSs involved in each test. The comparison among tests sharing similar spatial configurations, with TSs either in a row or in a column, allows a serviceability assessment directly from the experimental footbridge response. The numerical analyses are based on a FE model previously developed, showing an excellent reproduction of the first bending mode, but a poor simulation of the second torsional mode. The outcome of the analyses performed according to HiVoSS guideline is in good agreement with the experimental results, in spite of the very low crowd density of the on-field tests and the unsatisfactory reproduction of the torsional mode within the FE model.