Well you do not want to freeze your beer (Haze issue)
Actually, freezing your beer, at least to the slush stage, can be good for it. That's what ice beer is, and it's more than a marketing gimmick. It can drop out some of the tannins and polyphenols, making a smoother tasting beer. Chill haze will develop at temperatures above freezing, and tannins and polyphenols are part of chill haze.
With commercial ice beer (has that stuff disappeared?), a small amount of ice is filtered out, resulting in about 1/2% higher alcohol, but unless you are making an eisbock, you can just let it thaw.
The late George Fix spoke about this at the MCAB II technical conference in St. Louis about ten years ago (although it might have been at the previous conference in Houston). I am currently lagering a Oktoberfest that I kept on the back deck for a few days while the temp hovered in the low 20s F until it I could hear slush in the cornies, then brought it in to the fridge, where it is currently lagering at about freezing (32F).
All that said, altbier is traditionally lagered at higher temperatures than lagers, at about 40F. Lower won't hurt, though.