[geoschmo]

WTC7 fell for the same reason the twin towers fell, uncontrolled fire burned long enough to weaken the structrural support members to the point of collapse. A fire in any structure that is left to burn will eventually weaken and destroy enough supports to cause it to collapse. This is true for steel frame buildings as well as wood ones. It just take a little longer for steel and concrete structures. You don't see buildings collapse during fire much these days because the fire departments do a pretty good job of putting them out before they get to the point of structural collapse. But on 9/11 the fires were at such a height and and burned so hot because of the jet fuel that the fire department couldn't get to it in time.
The collapse of the north and south towers, on top of killing hundreds of fireman, choked off the are with debris so that noone that was left could get into a position where they could fight the fires burning in the surrounding buidlings.