[geoschmo]

The guy is wrong. He doesn't understand how the building was designed and what sort of stresses were involved.

Modern structures can't be efficently built to withstand stress in every possible direction. Skyscrapers are designed to withstand downward stress from the weight of the building and occupants, and lateral stress from wind and earthquakes. The structure is carefully designed so that each component does it's share and all the pieces support each other.

Basically all the weight of the floors on those buildings was supported by the steel tubing that ran vertically up the outside. All those individual tubes were tied together in effect forming one giant square cylinder. All the floors were mounted to the inside of that cylinder.

As the floors fell there was a force downard of kinetic energy and gravity, but there is also a force outward against those tubes from debris and the explosive outrush of air. The hardware mounting the floors to the tubes wasn't designed to resist that kind of stress pushing outwards. In effect the mass of debris falling down inside this tube was pushing the walls outward and breaking loose several floors at a time.

So, basically he is right that the floors weren't coming loose one at a time, but he is wrong when he says explosives would have been needed to cause that.

Geoschmo