In first place, we should know what the pieces will be used for. The fact that they're galvanized suggests (but doesn't confirm) that they'll be used for roofing. In this case, I don't see any problem due to low temperature, because they won't likely be subjected to stresses other than wind and snow loads. They might also be used as structural steel, which sometimes is galvanized, and in this case I DO see problems, because structural steel IS subjected to stress. Another thing we must consider is that Alaska is a seismic region, so the stresses will be higher. If this is the case, i.e. they are structural steel, then, as a preliminary opinion I'd say that plain carbon steel isn't suitable because chances are (strong chances, by the way) that at the sub-zero temperatures of Alaska, the steel has passed the ductil-fragile transition and has entered into fragile behavior.
In this case, before buying the steel you should run impact tests of the steel you're buying. These tests can be made by the supplier or, alternatively, they might have been done by the steel mill. Check which is your case.
Now, this is a preliminary opinion, because we need to know what kind of steel you're talking about and what are you going to use it for.
Giovanni S. Crisi
Sao Paulo - Brazil (a tropical country, where we don't have such sub-zero temperature problems)

There are a few things you need to find out.
Is the weldment to be kept indoors or out?
What is the minimum design temperature that the weldment is rated for?
Which is the applicable job governing code?
With that information plus, as Mr. Crisi mentioned, the base metal type. you can find what you need.