If you are really careful, you might be able to get it to 15%. My 10-10-10 brew turned out at around 13%. Check out the 10-10-10 thread
to see how we all approached brewing such a high alcohol beer while getting it dry enough.
One thing I will add is that you will want a good bit of sugar in there. If you read through the 10-10-10 thread, most of us added about 3lbs of sugar. I did not add the sugar to the boil. Rather, I waited until the beer was at 1.030 ish, then incrementally fed the fermenter with 1lb of sugar boiled in about a cup of water plus a little yeast nutrient. I did this 3 days in a row to equal 3lbs of sugar. The end result is a higher alcohol beer that has a finishing gravity of 1.002. Also, make sure you pitch a metric crap ton of yeast for this.
It originally tasted like bananna flavored rocket fuel A year later, it has lost the alcohol punch and really mellowed into a great tasting beer.
Most of the Belgian yeast strains should be able to handle somewhere around that limit. Avoid turbo yeast though. I suggest looking on the yeast manufacturers website and finding the Belgian strain that has the highest tolerance. That may or may not be the "high gravity" strain.