Reply #6: World Trade Center was built with then new fangled.......

[Historic NY]

construction & engineering techniques to give more open space. Load bearing was transferred to the outer walls vs having lots of internal steel supporting columns. There probably are lots of other explanations of how it was engineered, but hot fire will melt & deform steel. I think they also found that there was inadequate fireproofing of the steel, which could have slowed structural failure.

The building did pancake on itself and that was in the design. I remember being on the 89th floor a few times on very windy days and the building actually moved and doors slammed shut. In the end I personally think it was a compounding of lots of factors, not anticipated or planned for in construction. Who would have thought then terrorists would fly large jet aircraft into the towers on purpose.