Yep that's why armies always broke stride when marching across bridges.
One has to wonder when you hear all the conspiracy theorists saying that the WTC was an inside job (steel doesn't just melt). Well... ya gotta think the WTC towers had a LOT more weight riding on their infrastructure when those planes hit. All of those steel units work together and are intended to carry a more balanced load, you remove or damage some or unbalance the load for too long, and all the rest have to work that much harder.
There's a big difference between a third world wooden "peasant, horse, and cart"-grade bridge, and a domestic bridge designed by expert engineers. Try driving an 18-wheeler loaded with three rolls of steel over one of those "break cadence when crossing" bridges in any manner you choose, and see if you can keep it out of the water.