The second in the series of three photos (with attempts at descriptions) is
You can use their arrows to page backward/forward to the other two photos.
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.
1) What stress/relative movement between components could have bent those plates?
2) Could continuation of that deflection in the same direction have led to structural failure of that assembly? More particularly, could increasing the angles of those "bends" have caused failure of the type that you see in the post-failure components?
Bridge in equilibrium, everything working as advertised.
If you then point load the midspan, or increase the distributed load across the mainspan, then the result is the tendancy for the U10 gusset to move towards midspan and down, basically along the line of the U10-L11 diagonal.
The gusset doesn’t, and wasn’t intended to resist that. The top chord shoreward from there is designed to resist that, and the diagonal shoreward from U10 is designed to resist that.
The deflection of the gusset there is an ancilliary side effect, not in and of itself weakening the bridge, but...
...the fact that the gusset did not return to original shape indicates that dimensions have changed since construction, and possibly that repetitve stresses were involved in those dimensional changes, which possibly exceeded elastictiy limits for the members that failed.
I consider these possible implications just as important as you.
However, even more important still, would be preserving evidence of an official cover-up, should something like that ever happen.
I need to go over those pics.
More later.
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.