Originally Posted by Ron Bennett

The plane hit at a lower point of that tower = more weight above the impact area compared to the other tower. Basic physics; not surprising it would fall first.



Steel loses much of its strength well below its melting point. And that matters a lot in regards to why the towers fell due to how they were built ... see my next response below...



The towers were tube-frame construction unlike most, especially older, skyscrapers which are box framed construction. The difference is that box-frame is akin to a grid with each floor being supported by numerous internal columns. Whereas, in tube-frame, there is an inner core and the outer walls with few to no internal columns...



The towers, also including building 7, were tube-frame construction verses box-frame ... as the steel weakened from the heat, the floor supports sagged and pulled at the connections between the core and outer walls ...

Eventually, as the stress built up, portions of affected floors began to fall onto the floor beneath. At some point, the load limit of the floors below was reached, starting a cascade of floors falling onto the floors below; pancaking effect.



Heat up some steel to even half of that for awhile and one sees plainly it loses much of its strength; bends much easier.



Much of the jet fuel was rapidly consumed and hence the temperature it burns at is of little consequence. 1200F is plenty hot enough to ignite materials within the building, which then burned for an extended period of time, weakening the steel, leading to an unstoppable pancaking of floors, and ultimately, the destruction of the buildings.

In short, the difference between tube-frame (Towers 1, 2, and 7), which allowed for large open floor plans, and box-frame (ie. Empire State Building) is ultimately what did the towers in ... lack of structural redundancy / robustness compared to traditional box-frame structures.

Ron