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Scour -- again... :-|
Pier(s) 6 -- on the south side where the failure initiated -- are on dry land, well back from the river, and are, AFAIK, unaffected by scour.
Pier(s) 7, on the north side of the river, may have been affected by scour -- but I don't see how that could have affected gusset plates on the opposite side of the river. Pier(s) 7 did tilt toward the river during the collapse -- and I address that displacement in this thread in comment #72, and in comments immediately preceding #72.
My reading was that the visible displacement of Pier(s) 7 -- which may have been weakened by scour occurred fifteen seconds or so after the collapse began -- and after the river span that had been opposing their movement was long since in the river.
U10 gusset plates bent due to scour on the other side of the river? Sorry, I can't envision it happening...
Maybe I shouldn't have used the term 'scour'. :^(
If you look at a full area view, you'll notice that both of the piers were set right at the edge of the riverbanks; it is only encumbered around the base of Pier 6 because of the dam/weir and lock. Which came first? I am betting the bridge was placed after the dam; if you look the water outlet for the lock, it appears it runs behind the pier, so that pier is subject to seepage from both sides, as against only the river side on the other bank.
The principle behind the tension/compression forces along the structure could shift the stresses from the settling of Pier 6, the further weakening of Pier 7, and the noted load imbalance evidenced by the strain gauges; to cause the bend/shearing at U10.
I agree that Pier 7 moved as a result of the collapse, not as the main cause. Thank you for your expertise.