Seminar by Paul Petry

Vibratory response of a down-scaled pedestrian bridge: modal analysis and tuning of dynamic absorbers

During the construction of a footbridge, its global vibratory damping is assumed to be the same as its main material. However, this hypothesis does not correspond to the real behaviour of a bridge leading to an increased amount of vibration felt by pedestrians. Given the differences between the model and reality, an experimental study of a bridge is good approach for a better understanding of its behaviour and to find solutions to minimize them.
We built a down-scaled model of a footbridge at the LVA. This allowed us to perform practical measurements usually too complicated to apply on a real structure: the modal vibratory damping analysis and the exploration of the possible implementations of dynamics absorbers. Firstly, we supposed that the dry friction from bolts and the beam bending were the main sources of damping. Afterward, the impact of the dynamic absorbers was then evaluated with the structure quadratic displacements generated by two vibratory sources, a sine wave and a white noise.
We showed that the tiny size of the structure biased our damping ratios. Thus, the hypothetical damping sources of could not be verified. Nevertheless, the absorbers were effective when their anti-resonance frequency bandwidth matched the inputs.
To conclude, the vibratory response of bridges could be improved by using dynamic absorbers tuned to match the pedestrian walk frequency.