I-beams are selfish. They used U-beams, made of balsa which identified as steel.
Someone got too much partial credit on their exams in engineering school.
I’m sort of serious... I just looked at videos on FOX of the bridge, both before and after collapse. The main deck of the bridge looks very thin. And, where the bridge has broken, it has just snapped. I’d expect to see some twisted girders protruding from the concrete, but I don’t see them. I realize it’s a pedestrian bridge, not a vehicular interstate bridge, but I’d expect to see evidence of significant steel understructure amidst the debris of such a total collapse.