Experts dissect FIU bridge collapse, but can't identify cause

LRFD puts a 1.2-1.6 multiplier on loads (and at time less than 1 for cases where you're relying on it for balance), and resistances are in the .8-.9 range.

I never did a whole lot with the Factor of Safety era codes, but they were generally in the same range once you multiply out all the factors. (As you'd expect, the newer codes tended to hit about the same design point, with some deviations.)

There are two main things to keep in mind here:

* A factor of safety is for normal variability in loads and normal variability in material/connection strengths. It doesn't cover you for blunders.

* The loads that are calculated are the extreme loads on the system, not the normal loads. 1.4 DL + 1.6 LL + 1.2 WL is often the design envelope, and that's... a lot. Add a huge factor of safety on that and you're into physically impossible cases. Like, people are being crushed to death if you get more than 300psf of human load over a significant area, but the typical Live Load is in the range of 100psf. (Now books on a reference library moving shelf system? That's 300psf)

From what I've seen of this bridge, there was no live load at the time, so it's unlikely that it was overloaded as designed. If I was doing failure analysis, I'd be looking at a combination of factors, and the ones I'd start with are the shop drawings vs. the engineer's plans and the quality of the concrete materials, placement, and curing.