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.
I have not had much time to watch the videos or see the news but heard some concrete guys here speculate that the train might play a part in the final report, if the bridge failed at that point. The live load of all the stopped traffic, the steep slope cut into the bank next to the tracks, combine with shaking from the train, and all that silt on the bottom, no bedrock there, could have made for ‘a whole lot of shaking going on’.