you could untick the "Is Building" in the object editor, then simply rorate the buildings, but cleaner would be to simply create them via a trigger, because you can set the angle upon creation.
If you need to do that with all barracks in game, there are work arounds. You could detect, whenever one of these barracks enters the game and replace it by a similar barracks, that you created via a trigger (to set the degrees). You could extract the barracks from the mpq and change the rotation using a model editor (magos for example), and import it to your map and use it. You could read the tutorial on turnable towers here on the hive and use a custom root ability to set the facing of the barracks...