Serviceability assessment of a suspension foorbridge before and after the implementation of tuned mass dampers

The paper summarizes the main results of the dynamic tests performed to assess the vibration serviceability of a lively pedestrian bridge. Firstly, the dynamic characteristics of the suspension footbridge were extensively investigated before its opening. Operational modal testing (with the excitation being mainly provided by micro-tremors and wind) allowed to identify 10 vibration modes in the frequency range 0-5 Hz: the frequency of the fundamental mode was close to 1.0 Hz and five modes turned out to fall in the frequency range 2-3 Hz. Subsequently, groups of volunteers simulated normal walking and running at different step rates along the deck. The human-induced vibrations were measured and the results clearly indicated that the maximum vertical acceleration significantly exceeds the comfort criteria threshold suggested in guidelines and codes of practice, so that a control system with 5 tuned mass dampers (TMDs) was designed. After the installation of the TMDs, the dynamic tests were repeated at different air temperature and clearly revealed only a partial success of the vibration mitigation due to the high sensitivity of the structure (and its natural frequencies) to the environmental effects.