Holy cow! Check this out:
http://www.startribune.com/10204/story/1343624.html
“At the site, Hoeppner talked to construction workers who survived the fall. They had been doing repair work but expressed concern to him that the bridge had been wobbling several days before it collapsed. Every layer of concrete the workers removed, the bridge would wobble even more, they told Hoeppner.”
See my post above.
A chair with three good legs and one loose.
Thanks for the link.
That’s an interesting quote, but it’s hard to apply to the collapse progression. The road deck gives the bridge rigidity, but only in the plane of the road deck.
It could help keep the middle of the bridge from swaying side to side, for example, but it wouldn’t help hold the bridge up, and removing road deck concrete would’t help bring the bridge down, unless something else was very, very wrong.
That brings us back to the diagonal bracing between the kingposts above the piers on the south end of the mainspan. If they were already compromised, then the road deck would have helped keep the kingposts from swaying side to side.
If that was the case, the problem should have been readily apparant to anyone under the bridge looking at them.
All structures have some give to them. It is very difficult to accurately judge how much give is occurring when you’re standing on the structure. You can say “a little” or “a lot”, but relative comparisons are risky without a transit and repeatible observations and calculations. A human’s inner ear and perception are subjective at best.
In the past, I’ve experienced what felt like excessive sway in some of my structures and the procedure in all such cases is the same. Slowly and carefully get off the structure, then report the sway up the chain of command and go looking for the cause.
In one case, we had a new guy removing braces that should not have been touched. In another case, the sway was significant, but within design tolerances. Both structures survive to this day.
That is an interesting report, to be sure, but I’d be careful giving it more credibility that it deserves. I’ve never seen a construction stiff stay around when he had real concerns about structural integrity. Sometimes that’s the fastest you see them move all day. If they stayed on the bridge, they either weren’t too concerned about stability, or else they were driven by slave-owning tyrants with guns.