[Robert A Cook PE]

I find it to be significant that prior to catastrophic failure, there had already been a condition that induced displacement/deflection/distortion of the critical gusset plates in the area of failure.

That type of steel will yield (bend) before ripping through: And the yielding will occur at point of highest stress.

Your observation (plus those above) sounds right.

[TXnMA]

Here is a rude, crude little sketch based on what I see on page 4 of the NTSB PDF.

The deflections appear to me to be all in the same direction -- as if the deck and upper span beam had shifted somewhat to the east relative to the verticals.

OTOH, after re-examining the overhead view, I now think that the roadway curve was too far south to have imparted any live lateral thrust to the L10/U10 section...

At any rate, jeffers, your suggestion that downward vertical deflection in mid-river could have caused these gusset plate "bends" doesn't seem to fit: That would have put these particular edges in tension.

So far, the deflection looks lateral to me -- but don't ask me how it could have happened... ;-)