[yooper]

"The collapse mechanism you suggest does not account for the hour or so that passed between collision and collapse."

The hour which passed was the time needed for the steel members to heat up and transfer heat to other steel members and flammables. Once the crash level floors lost their structural integrity, the building was doomed to cascade failure.

[Rome2000]

All steel columns were sprayed with fireproofing that protected them for about an hour in that intense heat.

Somebody should have warned rescue workers about the possibility of collapse after that time.