[T.B. Yoits]

...had studied architecture and knew it was a steel building.

Not just that it was a steel building but that it was built without a central concrete core and instead relied on an external skeleton. Once that the external skeleton was compromised, the building was coming down.

[jmacusa]

You're right to a degree. This was not the typical ‘’brick and steel’’’ design that had come before in high rise office construction.

There was of course the concrete core for the elevators and uss ducts, etc but the structural steel formed the outside perimeter of the building, 200 yards by 200 yards and the floors of poured concrete were supported(’’carried off’’) by trusses.

That was why the buildings failed. A truss can only support high heat for so long and then it fails.

That and the fact that most of the fire proofing that had been put on during the construction phase had not only falling off at impact but it had deteriorated over the yeas..