ASMEfag here: What was the design strength of the 'foreign' (or otherwise) reinforcing steel used with the concrete? Design Strength of steel can be higher than 60,000 psi if used in a 'static' application; BUT should never be greater than ~ 17,000 [+ safety factor of 4; i.e. 17,000 /4] in a 'dynamic' application, i.e. where the stress is 'cycling' [work hardening the steel]. In a concrete bridge span the concrete(+reinforcing steel) will be constantly vibrating, causing stress cycling. I can imagine the amount of stress cycling that occurred in these concrete spans (top & bottom) just in constructing the bridge. A construction inspector even reported seeing cracks in the concrete before the contractor swung the bridge into place, the Friday before. If what I have outlined herein happened, and I have seen it happen, and have been fortunate to catch it many times in my career - then NOT ENOUGH STEEL was used to reinforce the concrete. << Goodman's Stress, i.e. reduction of design strength of steel in 'dynamic' applications…. [I am not Goodman]