Originally Posted by JCE

His comments, as I recall, were that the 1-1.5" of cementacious monokote (sp?) fire retardent applied to the structural support beams and vertical floor struts was probably damaged and/or shaken off by the impact. In the absence of this monocoat the structural steel was exposed. The steel frame was designed with a specific geometry and tensile strength. The fires from the jet fuel were enough to heat the steel and welds enough to cause it to start loosing tensile strength. That, and the fact that some vertical struts suspending the floors were probably damaged by the impact of the plane, caused the structural integrity of that floor to be damaged and start to deform. The deformation put additional stress on other beams and struts, far exceeding their load factor safety margins, causing further deformation and stresses, eventually leading to failure of the struts and/or beams on the floor where the fire occured. Once one floor fails, the rest of the floors above pancake down in a domino effect, massivly exceeding the load factor on the lower floors, causing them to pancake as well. This is the essence of what I remember from the conversation, and I hope I am not mis-stating his comments. He also said that the fire (and probably the police) personnel were usually aware of this potential for failure in this type of situation, and were even more brave than the media portrayed, as they knew the building could/would collapse!

I also asked him why the Empire state building did not collapse when hit by a B25 during WW2. As I recall, he said the B25 was several hundred mph slower and therefore hit with much less energy (KE=1/2 mV^2, with the energy going up as the square of the velocity). He also said that the B25 contained avgas instead of jet fuel with different burn properties, and that the Empire state building was a rivited steel structure instead of welded, and therefore stronger in his opinion.