Bridge over troubled water

I had a very frigthening experience yesterday evening. I was with my parents in their car, on the road from their flat to the hospital where my gran'mother has been operated this morning. We stopped on a bridge over the A4 highway (the highway between Paris and Germany) and the bridge was moving. Yes moving. It's not a bridge like the Golden Gate or the bridge over the Tage in Lisbon. No, just an old plain short bridge in reinforced concrete. And it was not moving because of the wind, but its was resonating to the vibrations caused by the trucks on the highway !!!

I have learned during my studies that concrete bridges should not resonate, and that dozens of soldiers died long ago in Bayonne because a bridge resonated to their synchronized steps and broke. The amplitude of this short bridge - not more than 50 meters long - was about 10-15 centimeters. Not something I want to experience again.