*sigh* It was not just "slip off", and it was not "brackets".... certain areas collapsed when structural beams lost integrity and allowed the weight of the roof and upper floors to start downward. This was not a 'totally' symmetrical collapse, but due to fire having 40 or so minutes to heat beams which had no fireproofing left, it was close....and those beams were connected to the core! It was not two separate structural entities....but the ROOF, when it fell, hit ALL areas.