That has been my experience as well. I'm wondering if this was some bomb/bullet proof building like the CIA or Pentagon. I am aware of some installations using safety glass (at least on the observation deck) and particularly thick glass in some instances, but nothing like 4-6"

I remember one building (I think it was Saint Louis) where on the upper deck, the windows were installed at an angle, so one could lean over and look down. They were incredibly thick, shatterproof, and I was told that they could carry an extra 2000 pounds of weight should one of the observers accidentally fall past the VERY strongly built railing. Even that didn't look more than 2" thick, and the framework was pretty impressive as well.