Crowd-Induced Vibrations in a Timber Arch Bridge

The typical slenderness and lightness of modern footbridges make them very sensitive to human-induced vibrations, compromising their serviceability. Hence, the vibration serviceability assessment is a key aspect in the design and verification of modern footbridges. A sound prediction of the induced vibrations relies on the reliability of both the structure dynamic model and the load model. In this context, the present paper investigates the dynamic behavior of a timber arch bridge subject to pedestrian crowds. The bridge presents six natural modes (identified from ambient vibration test) with frequencies below 4 Hz, highlighting its meaningful sensitivity to human-induced vibrations. The behavior of different-sized pedestrian crowds is simulated through the Social Force Model, which represents each pedestrian in the crowd as guided by the so-called ‘social forces’, namely motivations and environmental influences. The Social Force Model enables the evaluation of the instant position and velocity of each interacting pedestrian in the crowd. Based on these outcomes, a walking force is defined for each pedestrian adopting a single-step load model, and the footbridge response to different crowd densities is evaluated. Results focus on the footbridge vibrations in vertical direction, obtained accounting for the contribution of both vertical and torsional modes. The estimation of torsional mode contributions is made possible thanks to the pedestrian positions on the bridge deck, simulated through the Social Force Model.