Improvement of the dynamic behaviour of the varesine-garibaldi footbridge in Milan, Italy by using Tuned Mass Dampers

This paper presents the results of two dynamic identification research programmes, carried out before and after the installation of a tuned mass damper on the Varesine-Garibaldi footbridge, with the aim of assessing the effectiveness of the intervention. This footbridge is a two-span simply supported steel footbridge sited in close proximity to the Milano Garibaldi railway station. The equipment includes fifteen uni-axial accelerometers, with seventeen channels data acquisition system. Response measurements made on ambient excited-structure were firstly analysed; then, harmonic forced vibration tests with vertical, transversal and longitudinal direction of the excitation were carried out. In addition, free vibrations of the footbridge when subject to different pedestrian loads (pedestrians marching in time or crowd random walking) were processed. The identification procedure is based on modal analysis. System identification was performed using three different techniques in the frequency domain: frequency domain decomposition, enhanced frequency domain decomposition and poly-reference least square complex frequency domain. The results show that the proposed control system mitigates vibrations, sensibly reducing the response amplitude during the forced phase. Concerning the comfort and safety of the footbridge, the tuned mass damper system increases the in-service lifetime of the analysed structure and improves the dynamic behaviour of the whole footbridge.