Judging from how much steel they'd have around at a single warehouse near to where I used to live, I'd hazard to guess that it would take much less time than a couple of months to get the steel.

But yeah, it's a moot point, because like you say it's going take far more time to design a new bridge and get together the crews to build it. We don't have some idle SCVs around just waiting to start building as soon as you click on the the location.

And I'm no engineer, but I'm guessing the steel from a bridge that collapsed would be too warped to be useful and the pieces that looked ok would have to undergo testing to make sure the stress they went under didn't compromise the material in some way. I'd guess it would be more expensive to use that steel than to just use new steel.