Very nice job. I had read the online report earlier but your analysis was great.
A question: They are talking gusset plates now. The ones on the bridge were 1/2 in thick. Thick enough and big enough in your opinion?
They were undoubtedly thick enough when new, but the bolted gussset plates (also called splice joints) were badly corroded on this bridge. They say in the report that there was pack rust up to 1/2” think on one of them. That does not leave much material left.
I did not see it in these photos, but I have seen badly rusted splice joints that actually looked like an old fashioned quilt. In other words, the steel plate was actually bowed out between the bolts (or rivets). That has to put a lot of force on the bolts without any additional loads. Actually the splice joints could easily have been a source of the failure. Especially since one was in the high tensile stress area in the bottom chord at mid-span.