If the sight of planes crashing into the World Trade Center towers was astonishing, watching them collapse into dust was quite beyond belief. Hollywood does this sort of thing with little balsa models and computer graphics. I watched it, all right; but my brain kept insisting, fiction. Pure fiction. This shit just doesn't happen.
The last time a big plane crossed paths with a New York skyscraper was in 1945, when a B-25 Bomber en route to Newark Airport crashed into the 79th and 80th floors of the Empire State Building, killing fourteen people and causing only superficial damage to the structure.
Perhaps it was that recollection which made it so hard for the mind to grasp what the eye was reporting. Perhaps it was years of adaptation to the spectacular cartoons which Hollywood passes off as motion pictures. Modern entertainment, after all, has trained us not to believe our eyes.
So what happened?
It's been noted that the two towers collapsed neatly, as if brought down by a demolition crew. This isn't a bad observation to begin with. The overall structure of the WTC towers was that of a rigid tube. The real strength was in the outer walls, an approach which frees up a great deal of interior space.
In the center of each tower was a vertical core which housed the elevators, stairwells and a fair amount of internal plumbing. Beams connected this section with the outer walls and provided the basis for each floor. The core was only strong enough to handle its own weight and structures bearing on it against the force of gravity. Immense external loads such as those caused by wind were handled by the outer walls.
When the buildings were struck by planes carrying 20,000+ pounds of fuel, and remained standing, heat became the real structural threat as all that fuel burned.
Temperatures near the wreckage may have risen to 2,000 degrees Fahrenheit, more than hot enough to melt steel. Most likely, the inner core failed from thermal stress, and as it collapsed the floors tipped downward at an angle and pulled the outer walls inward, which accounts for the implosion and the illusion of a controlled demolition.
New York architect and Cryptome Webmaster John Young offers a far more detailed discussion here. ®