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As a college student studying human physiology I had to endure a lot of chemistry, organic chemistry and biochemistry. I would like to answer this question with objective information that should be quite accurate and easy to understand - and pretty fun too!
I have something else for you to consider. I have seen bottled beers reach -8 degrees and still not burst.
This stems from chemistry class where we know that things under pressure can withstand a "lower outside temperature"(because technically it is higher within). PV=NRT is the constant and really the formula is PressurexVolume=n(a constant)xR(another constant)xTemperature. So if you increase Pressure(the pressure within the can), the temperature actually goes up, If you RELEASE that pressure, the temperature goes down.
Think about a co2 fire extinguisher - its all liquid CO2 in the bottle, as soon as the trigger is pressed, the CO2 loses all pressure and hyper cools.
A colleague of mine actually explained this greatly in other words. He and his buddies use MolsonXXX beer(a 7.3 % alcohol beer by volume) when they sled. They keep the beers in the snow banks, which usually t the time are sub 30, somewhere around 20 degrees. When they actually open the beers is when they freeze, expand and explode, not when they are in can because of the pressure within the can. As soon as the pressure is released, the beer RAPIDLY "cools" expands and fizzes. The beer in the can is actually still all liquid.
So the point of this is, take the beer you have inside, and leave 1 can on the step to figure out the temperature for the beer itself.
What is more is that the beer will actually pseudo separate because of the alcohol and water within it. The water will obviously freeze right around 32, alcohol(ethanol), which has a freezing temp of -114.3 will actually stay, while the ice falls out of solution leaving you with a beer that is slightly stronger, but with an even LOWER freezing temp.
Laymans terms on this: your beer may not actually freeze at that temperature and burst, but would actually freeze when the pressure is released when you open the can. To be sure of this, and to figure it out yourself, just leave 1 can outside, take the rest inside so you can see what the tolerable level is. The higher the alcohol the lower the freezing point, but the water will still freeze per say, this expansion of the ice is what causes the burst.
For a scientific explanation on this you can research freezing point depression - http://en.wikipedia.org/wiki/Freezing-point_depression
This is also the same reason why already frozen beers kept very still, when struck or shook, explode - the disruption of the otherwise "low" energy status of the liquid is given more energy(literally) when shaken, this causes an excitation of the molecules, more heat(hardly measurable) and this causes the explosion.
Truth be told you can freeze some beer, keep it very calm and still and not have it explode - add motion to "should-be-frozen" liquid beer and KAPOW!