[CurtEgerer]

I highly doubt a standard 60 or 90-pound jackhammer on the pavement could have triggered this (unless the deterioration of the steel framing was incredibly severe). Compare that to the resonance from, say, several fully loaded semi-tankers or cement trucks bouncing across this thing at 70MPH day in and day out. But who knows at this point? If there were, indeed, "out-of-plane girders" all normal assumptions can be thrown out the proverbial window.