NTSB's preliminary report on the collapse of the Fern Hollow Bridge in Pittsburgh

President Biden got press to die for when he visited Pittsburgh last week to tout his administration's infrastructure push: one of the city's rotting bridges suddenly collapsed while he was in town, tipping several cars and a commuter bus into its own crumpled remains. No-one really died, fortunately, and the National Transport Safery Board is investigating what happened.

The bridge was an uncoated weathering steel, three-span, continuous rigid "K" frame structure with two welded steel girders, welded steel floor beams, and rolled steel stringers. [1] The ends of the structure rested on reinforced concrete caps on stone masonry abutments. Each girder was additionally supported by two inclined, welded steel frame legs, also made of uncoated weathering steel, which rested atop reinforced concrete thrust blocks. Although certain areas of the welded steel girders were identified as being fracture critical, no primary fractures were found in these areas.

I think this refers to the alarming viral photo if the bridge (below) and means that this particular rusted-out spar wasn't the point of failure. So there's no obvious cause, yet, beyond the plain facts of the bridge's age and general poor condition. The investigation continues.