Yesterday, an overpass in Quebec collapsed. More than an hour before it happened, a man called the police to report that a one metre block of concrete had fallen from the overpass onto the highway below. A spokesman for Transport Quebec said:
Is there any truth to this statement? Small pieces of concrete I could see, maybe, but if the debris was large enough to require a maintenance crew to clean it up, surely there had to be significant pieces of concrete coming off of the bridge? Any structural engineers want to chime in?
Notice the spokesmand did say what size piece is normal to have fall off. :dubious:
A 1 meter chunk? :eek: Close the damn road and investigate. Now.
The only think I can think of that would prevent such a more right now is that they did not understand or believe the caller. Even so they dispatched a crew before the colapse. The crew should have shut it down ASAP.
The answer to the question as posed is: it depends. I haven’t seen photos or diagrams yet to really know what happened or even what type of bridge it was. Concrete can and does fall from the underside of bridge decks with no significance other than to the poor guy below whose car gets the brunt of a fall of a piece of it. From the descriptions, it rather sounds like the chunks came off the underside of a girder. Again, depending on what caused the spalling and where it was, it could either be insignificant or catastrophic.
Again I don’t know what kind of bridge it was. If I had to bet, it would be that this was a bridge with prestressed beams and the concrete was not properly compacted or vibrated in the forms. This allowed brine to eventually find its way to the bottom of the beam and start working on the prestressing strands. The rusting strands expand and blow away the concrete under them. At some section of the beam, the rusting strands snap one by one, and the resulting deflections blow off the chunks noted. At some point, the beam has lost enough strands that the remaining strands cannot pick up the slack and the beam fails suddenly.
Did the inspector miss something? To be honest, I doubt it. It’s possible that all this was going on inside a beam that from the outside looked normal up until just before the incident. And if the inspection was not of the arms length up close variety, then it could very easily have been missed.
These are my best guesses based on 28 years of design/load rating/management of bridges. The guys in Quebec with the plans and the photos are going to have the authoritative answers when they complete their investigation.
It wasn’t actually an inspector who first came to check the structure, but a patrolman from the ministry of Transportation, who doesn’t have any special knowledge about bridges and other structures. He also didn’t have any power to order road closures. He picked up the debris and called his supervisors to recommend that the overpass be inspected soon by an engineer. Some time later, there was a second call to police to report falling debris. Apparently the patrolman was sent on the scene again – and a structural engineer as well – but the structure collapsed before they got there.
This is what I’ve pieced together from articles in La Presse and Le Devoir. We’ll have more information after the commission submits its report.
Out here in the Earthquake State, Caltrans jumps all over any sign of structural damage whatsoever. It’s a pain in the ass when they take a decade to reopen a freeway, but at least there’s some assurance it won’t fall on your head.