"Fires do not destroy skyscrapers. Never in the history of steel frame
structures has a single one been destroyed by fire."
uh...thats a total lie...steel does not fair well in fires at all...one of the reasons for encasing steel in concrete is to protect it from fire because as it heats up it loses its stiffness and buckles...fire fighters know when to get the hell out of a building before it collapses by watching the steel expanding...they call it the foregiving metal...
the reason the towers collapsed was because the jet fuel burns so much hotter than a normal fire feeding on paper or wood...when the planes hit the buildings, the fuel was everywhere on those floors, and theres no doubt the structural columns were damaged...the heat from the fire caused the steel to expand beyond its ultimate strength point...if you remember where the planes hit, it was about a third to a quarter of the way down the building...the structural supports failed on those floors that were hit which created a dynamic condition, resulting in the supports not being able to sustain the dead load of the rest of the building above the damaged areas and the top part of the towers collapsing on the lower floors, which in turn couldn't support the dynamic load and the entire structure going down...