[bert]

The author is in way over his head. There are no girders in this bridge. It is a truss.

The gussets also known as connections are certain to have failed. That has been known here since the first hour or so. The question is which of several possible modes led to the failure. Flaw is possible but not likely since the bridge has been operatopnal for a long time.

I referred in earlier posts to bolted connections. A freeper told us they were riveted. I didn’t think they were still using rivets in the 60’s but close ups of the bridge seems to show rivets rather than high strength bolts.

[Paladin2]

As the frogs were saying in 1967 (and are still saying for truck frames today), ribbet.

[concretebob]

Girder is also a term applied to some individual components of a truss.
Something else I noticed was the individual members appear to be light gage square tube, drilled to decrease weight.
Can't really see the wall thickness.