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..
There was of course the concrete core for the elevators and uss ducts... The elevators were Minoru Yamasaki's trademark "Skylobby" elevator system that didn't use the traditional full-height shafts in a reinforced central core in the building. The center cores were steel columns, not concrete.
"The core of each tower was a rectangular area and contained steel columns running from the bedrock to the top of the tower. The large, column-free space between the perimeter and core was bridged by prefabricated floor trusses."
Construction showing a central core of steel columns but no concrete:
The floor trusses:
