A better question to ask yourself might be: how close am I willing to get to the yield stress? Speaking as one who has embarrassed himself significantly by sitting on (and breaking) a long table in a business meeting, picture your face after the wife of one of the bulwarks of the church innocently sits on this table and, well, you get the picture. I would try to creatively envision the worst possible case and build it to handle DOUBLE that! But that's just me. My wife says I build everything to last for the millennium. Why not?