I think that people keep completely overlooking how the towers were constructed and the role it played in the collapse. The towers were built to withstand dynamic loading, ALL buldings have to be built to withstand SOME dynamic loading, but engineers made some critical mistakes when factoring in the expected forces from a jet slamming into the towers. When weight and speed were factored into the calculations for a plane strike, the largest, fastest passenger liner at the time, the 707 was used. What wasn't factored in was the fuel. They didn't factor in the heat which would be generated from the fires that would surely ensue from a jetliner striking the buildings, AND they didn't factor in the aircraft fuel weight. These engineers actually forgot to get the weight of a fuel laden jet, and made their calculations off of the Boeing spec sheets!!!!!!
Which don't factor in ANY fuel in their weight!
They also didn't account for impacts shearing off fire resistive material on trusses and columns. Huge mistakes, but regardless, the towers performed extraordinarily well considering that the gross weight of jets wasn't factored in. The buildings actually withstood, and may have survived without collapsing altogether had there not been the fires that weakened the bolts and fasteners which held the floor trusses to the core and perimiter columns. Remember, there were NO structural elements which ran continuous from the ground to the tops of the towers. None of the structural elements went more than a floor or 2 up.