Experimental Assessment of Human–Structure Interaction in an Urban Pedestrian Footbridge with Multiaxial Dynamic Sensitivity

Recent advances in structural engineering have enabled lightweight and slender footbridges. However, these structures are often susceptible to excessive vibrations induced by pedestrian activities, primarily impacting urban footbridge serviceability. Most studies evaluate human–structure interaction (HSI) uniaxially based on peak dynamic sensitivity. Nevertheless, this approach may overlook multidirectional mechanisms in footbridges with similar vertical and lateral modal properties. This study presents an extensive experimental assessment of HSI effects on a functional urban pedestrian bridge, known as the Premier-Footbridge, which exhibits clear multiaxial dynamic sensitivity within the frequency range associated with human walking. The structure was characterized by using operational modal analysis (OMA) based on ambient vibration measurements. Subsequently, a comprehensive human gait campaign evaluated different pedestrian density loads (PDL) and gait conditions, including synchronized, non-synchronized, and random walking. The results show that the multiaxial interaction effects were predominantly concentrated around the first lateral vibration mode (1.07 Hz), despite concurrent dynamic sensitivity in both lateral (1.07 Hz) and vertical (1.98 Hz) directions. Increasing PDL levels led to higher vibration amplitudes and measurable changes in HSI-related structural dynamics, including apparent damping and pedestrian step frequency. The experimental results indicated an apparent increase in effective structural damping with increasing pedestrian density, as identified through nonlinear trend fitting of the measured modal response parameters. Despite the multiaxial dynamic sensitivity of the structure, lateral vibrations remained the dominant factor governing the serviceability response, even under moderate PDL conditions.