Numerical and Experimental Evaluation of the Dynamic Performance of a Footbridge with Tuned Mass Dampers

Abstract

This article presents an evaluation of the dynamic behavior of a slender steel footbridge before and after the installation of two tuned mass dampers (TMDs). The results of the experimental study show that the damping devices lead to an increase of the effective damping ratio of the critical mode. Additional tests involving the structural vibrations induced by a limited number of persons revealed that the TMD units are effective in reducing the structural response. However, the obtained reduction highly depends on the type of human excitation. To verify the response of the footbridge for large groups and crowds, a comprehensive numerical analysis is performed. The results are compared to the response predicted by the procedures of the Sétra and HiVoSS design guides. For the bridge without TMD units, a significantly higher structural response is predicted by the design guides; the bridge has a short span and is lightly damped, so the steady-state resonant conditions assumed in the design guides are not reached for pedestrians who cross the bridge at a normal walking speed. For the bridge with TMD units, the design guides predict a significantly lower structural response than do the more detailed numerical simulations because the critical mode is highly damped and no longer dominates the structural response. In addition, the effectiveness of the damping devices is considerably less than that expected on the basis of the corresponding increase in effective damping ratio because crowd-induced loading is near-harmonic and transient, not harmonic.