Saw an interview with a structural engineer some days after the NYC atrocity. He explained the collapse in terms even I could understand.
The towers survived the initial collisions with no problems, except from the 'local damage' caused by the aircraft, even though they had never been designed for that type of impact. Once the method became available to release the kinetic energy stored in the structure - in this case a fuel fed fire to weaken the steel - gravity would do the rest.
At least that's how I understood it.