I am getting really tired of people saying that steel frame buildings don’t collapse like the World Trade Center, so it must have been rigged for demolition. How many bats to the ears does it take to accept that the twin towers were not steel-frame buildings like, say, the Empire State Building. They were tube shaped with a central spindle. The towers fell straight because the floors slid down along those spindles. You can even see the south tower start to topple over sideways then spring back to verticle, as the tube collapsed on one side and then the central spindle sprang back.
It was very nicely shown by an old episode of Nova entitled Why the Towers Fell, which is available through WGBH, by the way.
A fatal flaw in the design of the towers is that the fireproofing applied to the steel structure wasn’t sticky enough, and came off during the impacts. Thus the steel had no protection from the fires and inevitably would soften and fail. In the south tower the tube snapped on one side, whereas in the north tower what probably gave way first were the attachments of floors to the central spindle-like structure, so the antenna tower starts falling visibly just before the sides of the building do.