posted on Aug, 2 2007 @ 06:48 AM
Its a steel truss frame bridge.
It works on the same basic principle as a stack of cards, insofar as each structural member relies on the other to distribute the weight evenly to the
If one of those trusses fails, either through overload, shearing forces or metal fatigue its entirely possible that the whole thing can collapse,
because the weight distribution would suddenly become uneven, and the forces on the surrounding members will go up. They may have been within safety
tolerances before the affected member broke, but after that any microfractures, fatigues etc would be amplified and its probable that they would start
to fail in a cascade effect.
The "plumes" seen are, I would wager, concrete being blown off the reinforcement bars in the bridge deck itself as they came under tension. I'm not
sure from the photo's if there were any pre-stressed elements on the bridge deck but I can tell you from personal experience that a pre-stressed
element suffering a failure produces a hugely loud "crack/bang" noise as the tension forces let go, this, and the systematic popping of the rivets
and bolted joints would explain the "roar" heard if a number of members failed in unison, as would be consistent with the support structure failing
As for causes - well I'm speculating here but its out of 20 years experience in Civil Engineering - its possible that during the maintenance works
and lane restrictions that a harmonic built up on part of the bridge the traffic was passing over - it wouldn't be enough to cause a failure in the
steelwork normally but the loading on the deck would be more concentrated over one spot rather than distributed. This would induce extra stresses in
fatigued pieces as they dealt with more lateral loading.
Too much induced stress on one spot, and theres potential for failure.
I'm not sure of the construction/traffic standards in the US, but I would have thought it prudent to contraflow traffic over a bridge like this to
keep as even a load spread as possible.
Terrible terrible accident if its gone down that way