I was invited on Wednesday to go and help wobble the Passerelle Simone de Beauvoir (previous posts here and here). The wobbling was being sollicted in order to conduct ongoing tests on the bridge’s dampers. The tests were being conducted by the CSTB (France’s centre for building science, where I almost ended up doing my projet de fin d’études).
Bridges such as this one, and infamously, London’s Millenium Bridge, are susceptible to wobbling caused by the excitation of one of the bridge’s natural frequencies by the pedestrians who use it. As well as forcing the bridge deck up and down with their footsteps, pedestrians also exert a sideways force as they alternatively plant their left and right feet on the deck. This sideways movement is of a similar frequency to the transverse vibrational mode of lightweight bridges such as this one and the Millenium Bridge. When a bridge does start to shake noticeably, there is a tendency to ‘lock-in’ whereby pedestrians synchronise their steps with the vibration in order to stabilise themselves, but in doing so, give more energy to the vibration. The first time that this lock-in phenomenon was observed was at the opening of the Millenium Bridge.
This sort of vibration is unlikely to cause any damage to the bridge itself but it does make the people onboard feel quite uncomfortable. It is therefore an issue of serviceability. In order to reduce its effects, such bridges are installed with tuned dampers designed specifically to damp out these effects. And in order to check if these dampers are working or not, it takes a group of fifty or so enthusiasts (usually engineers) to jump up and down to see just how much they can get the thing to wobble. I tell you, we got some funny looks from passers by…