[kylaka]

Construction of a modern skyscraper is very much like hanging a light facade on a birdcage frame. I'm astonished that they went down.

[RyanM]

I am no structural engineer, but I have read that the one thing modern skyscrapers cannot handle is high temperature fire.

Remember the movie Towering Inferno ?

Heat "melts" the steel, weakening it, leading to collapse.

[ffusco]

You are correct, most highrise buildings are built up by beams and columns stacked up on top of each other with the facade hung off the perimeter. This is called curtain wall construction. The World Trade Center was not contructed that way however. It was built by the tube method which implies that the load is carried by the skin of the building. If you notice the vertical lattice work on the exterior of the building- it was not decorative- it was structural. The terrorists knew this and knew that enough destruction to the skin of the building would cause its collapse. In 1946? the Empire State Bldg was hit by a B-24 and suffered no damage.