I’m a Chemical Engineer, not a Civil...so, take this for what it costs... I heard a report that parts of the bridge were under construction.... and someone mentioned feeling vibrations being generated by a jackhammer.
My first guess would be: Harmonic vibrations generated by the jackhammer... coupled with the old age of the bridge, and the heavy loading of a full stack of autos....
Just a guess.... but, I know harmonics can do very strange things on a bridge.
Four of the eight lanes were closed to traffic. Thus it would seem today's traffic load was half the normal weight on the bridge.
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.
Wasn’t the collapse of the Tacoma Narrows bridge in 1940 blamed on resonance?