This has nothing to do with wind chill...but it gives you pretty much the magic number for a pipe to burst. From "The Weather Channel"!

When should homeowners be alert to the danger of freezing pipes? That depends, but in southern states and other areas where freezing weather is the exception rather than the rule (and where houses often do not provide adequate built-in protection), the temperature alert threshold is 20°F.

This threshold is based upon research conducted by the Building Research Council at the University of Illinois. Field tests of residential water systems subjected to winter temperatures demonstrated that, for un-insulated pipes installed in an unconditioned attic, the onset of freezing occurred when the outside temperature fell to 20°F or below.

This finding was supported by a survey of 71 plumbers practicing in southern states, in which the consensus was that burst-pipe problems began to appear when temperatures fell into the teens.
(Note: Please disregard any wind chill! *J*)