Why Did the WTC Towers Collapse?
Immediately after the twin towers of the World Trade Center were struck, it looked as though the buildings might remain standing. While the plane crashes had taken huge chunks out of both towers, the overall structure seemed to be intact, at least to the observers on the ground and the millions of Americans watching the catastrophe on television. But within an hour, World Trade Center 2 had collapsed, followed by World Trade Center 1 only 40 minutes later.
Like all skyscrapers, the World Trade Center was held up by a sturdy metal framework. Steel and aluminum beams were riveted end-to-end to form long vertical columns, which were spaced about 3 feet apart all around the outside of the building. At each floor level, these vertical columns were bound together by many horizontal girder beams. The towers had a similar steel tube framework in the center of the building, which housed the elevators. This tube design left a large area of open space on each floor, with no vertical support columns.

In this giant three-dimensional grid -- called the super structure -- all the weight in each building was transferred directly to the vertical columns. The downward force caused by the weight of the buildings was concentrated into a relatively small area -- the point where the columns rested at the base of the building. At this point, the concentrated force was spread out across the substructure under the building (see How Skyscrapers Work to learn about substructures.)

When the planes hit the two towers, the collisions damaged the structures in two major ways:

They knocked out a large number of vertical columns around the edge of each building and presumably at each building's core.
They ignited the planes' fuel supplies, causing a massive explosion that set each building on fire.
The crash did not take out enough columns to immediately break apart the superstructures; the remaining columns were strong enough to hold up the top portion of each building. But the raging fire heated these standing columns to extremely high temperatures, twisting and weakening them. In the World Trade Center towers, the support structure was designed to withstand a typical building fire for a few hours or more, to give rescue workers time to evacuate the occupants and possibly extinguish the fire. But because of the intensity of the explosions on September 11, and the large amount of fuel feeding the fires, temperatures climbed well above expected levels.
As key structural elements melted, more of the columns buckled. Without adequate column support at the crash site, the top part of each building collapsed onto the lower part of the building.

Essentially, this was like dropping a 20-story building on top of another building. Before the crash, this upper structure exerted a constant downward force -- its weight -- on the superstructure below. Obviously, the lower superstructure was strong enough to support this weight. But when the columns collapsed, the upper part of the building started moving -- the downward force of gravity accelerated it. The momentum of an object, the quantity of its motion, is equal to its mass multiplied by its velocity. So when you increase the velocity of an object with a set mass, you increase its momentum. This increases the total force that the object can exert on another object.

To understand how this works, think of a hammer. Resting in your hand, it doesn't hurt you at all. But if you drop it on your foot, it can do some damage. Similarly, if you swing the hammer forward, you can apply enough force to drive nails.

When the upper structure of each World Trade Center tower fell down, its velocity -- and therefore its momentum -- increased sharply. This greater momentum resulted in an impact force that exceeded the structural integrity of the columns immediately underneath the destroyed area. Those support columns gave way, and the whole mass fell on the floors even farther down. In this way, the force of the falling building structure broke apart the superstructure underneath, crushing the building from the top, one floor at a time.

To put it another way, the potential energy of the building mass, the energy of position it had due to its height and the pull of gravity, was converted into kinetic energy, or energy of motion. This is the same basic principle that professional demolition blasters use to bring down unoccupied buildings.

World Trade Center 2, the second tower hit, actually collapsed before World Trade Center 1. Most likely, this occured because of the planes' relative impact points. The plane that hit World Trade Center 2 crashed lower on the building than the plane that hit World Trade Center 1. Consequently, the strained support columns in World Trade Center 2 had a greater load pressing down on them then the strained columns in World Trade Center 1, so they reached the buckling point more quickly.


http://www.howstuffworks.com/sept-eleven7.htm