Measuring the dynamic response of a lively footbridge to ambient and walking excitation

The paper presents selected results of a first experimental campaign on a footbridge over-passing the Lambro River near Milano (Italy). The 3-span footbridge, for a bicycle-pedestrian mixed use, has a reinforced concrete deck supported by a steel structure. The footbridge, 107 m long and with a constant width of 4.4m, is roughly symmetric about both mid-span and the longitudinal axis. As a part of proof tests performed in March 2016, ambient vibration tests identified the footbridge modal properties, detecting at 1.75 Hz the fundamental bending mode with the maximum amplitude recorded at mid-span, a finding confirmed by an ANSYS FE model of the footbridge. A series of forced vibration tests, performed in July 2017, investigated the response of the bridge under different loading conditions. Groups of pedestrians, in number of 1, 2, 3, 4, 6, 8 and 12, crossed the bridge, walking with a step frequency as close as possible to the first fundamental frequency. Different spatial configurations were explored for each number of pedestrians, investigating the spread in data related to different walking people/groups and their formation. Pedestrians followed straight trajectories and their spatial configuration was symmetric about the longitudinal axis of the bridge; single pedestrians walked along the footbridge axis. This paper focuses on a few results related to: (a) single pedestrians; (b) groups of multiple pedestrians in the same configuration, a longitudinal row; (c) 12 pedestrians in different spatial configurations. Experimental results highlight the effect of both intra-subject and inter-subject variability and the influence of spatial configuration on the maximum measured acceleration. The bridge performance is discussed by comparison between experimental results and limit values of the vertical acceleration according to HiVoSS guideline.