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One problem is steel reinforced concrete is nearly impossible to detect rusting until it makes its way to the outside of a member. The ends of the bridge were reinforced concrete, so it's possible they just couldn't detect the problem and so didn't deem it unsafe.

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IIRC some one traveled under this bridge last year and took photographs of several of the metal supports which were entirely rusted through and not connected to anything.

I saw those photos as well, and the presence of steel cables with turnbuckles to reinforce the structure lead me to believe that the damage was known and someone decided that the minimal reinforcement was adequate.

I suspect that if that was the primary cause of the collapse, the NTSB would have mentioned it directly already.

Either way, we'll know eventually.

Which may have been fine if other supports were taking the load. Visibly rusted parts are easy to diagnose, thus if that particular part was unsafe it would have been quickly deemed as such. It could be that it contributed to the collapse, but that it alone wasn't enough to justify it as unsafe.

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