The WTC towers were not constructed using a standard steel frame lattice. The highest building that can be constructed using a standard steel frame lattice is 100 stories, and the WTC towers needed to be 110 stories. To get the extra height engineers had to switch to a radically different design.
The towers consist of an outer cage, which is what you see on the outside, and an inner cage surrounding the elevators shaft. The floors were suspended between the outer cage and the inner cage. This is a radically different design, unlike other steel frame high-rises.
Structural engineer G. Charles Clifton of the New Zealand Heavy Engineering Research Association posted his speculative explanation of the towers’ collapse on the internet six days after the event. I corresponded with him numerous times during the following months as more information about the tower’s design emerged and his explanation proved to be correct.David W. Deley