Yes, you understand correctly.
The force which bent the gussets was compressive, because the angle between top chord and diagonal lessened. If U10 had returned to original position, it would have pulled the gussets back straight, or at least straighter. But it didn’t.
Whatever moved U10 (downward) in the first place, causing the bent gussets, whether a single event or prolonged abuse, also transmitted significant tension up the U10-L11 diagonal, and thence along the U10-U9 top chord back to...well eventually back to concrete on shore.
That may or may not be the cause of the fatal fracture later on. The triggering failure, in my opinion, was the vertical fracture in both U10E gussets, just south of the U10 vertical strut.
Those plates look like half inch to me. Three quarters on some boxes and H beams, perhaps on the pier gussets. I’ve cut both and the pics look right to me. Those rivet and bolts are pretty big, bigger than your fist is not uncommon at all. Grapefruit sized hex-nuts aren’t rare either.
Of course, most of us aren't prepared to see half-inch steel torn like toilet paper -- even along "perforations" (rows of rivet-holes)...
Thanks again!
BTW, did you follow my reasoning re my "lateral displacement" sketch? With all four gusset plates bent in the same direction, I perceived that as evidence of a lateral bias -- even if most of the distorting force was vertical (compressive). Am I wrong?