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I know you know this -- but it was pier 7 that moved... '-)
When I first saw the tilt of Pier pair #7 -- and the (intact) condition of its "shoes" -- and the position of the deck leaning on Pier 7, I had the exact, same concern you did.
However, I took the time and effort to do a lot of analysis -- to prove my own first impression to be wrong...
The collapse of the deck landward of pier 7 was very sudden and very rapid. That appeared to me to be the force that shoved the top(s) of pier 7 toward the river. I don't have the video on this machine, but, IIRC, that event occurred some fifteen seconds after the bridge started collapsing.
You are probably correct that the proximity of the piers' base to the river allowed the pier-base displacement to occur -- and that scour may have played some role in that weakness.
However, I see no evidence that said weakness contributed at all to the initiation of the failure.
Let’s say you have a truss that has rusted over the years,
and shows a stain.
Other trusses may be rusted as well.
Now add extra weight (more lanes)
Now add extra weight (construction equipment)
Now....add.... JACKHAMMERS.
I have seen SEVERE VIBRATORY OSCILLATIONS do incredible damage to structures.
Why not here? (they were using jackhammers, but that didn’t seem to be mentioned)
Thank you for your very civil reply, and for your analysis.
Having done some legal forensic work, I tend to question assumptions only in the hope of arriving at the truth.
Many times the obvious conclusion is not the correct conclusion. Questioning sometimes leads to a different path that would not be pursued initially.
And then sometimes, the obvious is just that, the obvious.
Thank you for advancing the discussion, and for correcting my reversing of the pier numbers.