just to get your interest (or not as the case may be ) here is a quote
"Whatever irregularities caused the top of the tower to tilt, subsequent pictures show the tower falling mostly within its own footprint. There are no reports of this cube of concrete and steel from the upper floors (measuring 200 ft. wide, 200 ft. deep, and 250 ft high) falling a 1000 feet onto the buildings below.
Implosion expert Mark Loizeaux, president of Controlled Demolition, Inc. of Phoenix, MD, was also misled by the picture. Having observed the collapses on television news, Loizeaux said the 1,362-ft-tall south tower failed much as one would fell a tree ( www.civil.usyd.edu.au/wtc_enr.htm or: public-action.com/911/jmcm/USYDENR ).
I have recently seen a videotape rerun of the south tower falling. In that take, the upper floors descend as a complete unit, tilted over as shown on the BBC page, sliding down behind the intervening buildings like a piece of stage scenery.
That scene is the most puzzling of all. Since the upper floors were not collapsed (the connection between the center columns and the platters were intact), this assembly would present itself to the lower floors as a block of platters WITHOUT a central hole. How then would a platter without a hole slide down the spindle with the other platters? Where would the central columns go if they could not penetrate the upper floors as the platters fell?
If the fire melted the floor joints so that the collapse began from the 60th floor downward, the upper floors would be left hanging in the air, supported only by the central columns. This situation would soon become unstable and the top 30 floors would topple over (to use Loizeaux's image) much like felling the top 600 ft. from a 1,300 ft. tree.
This model would also hold for the north tower."