CarolC, I think I see your confusion.

The steel box-beams (that I think you are seeing and mistaking for the entire core structure) are just an element of the core structure. They rose up on the outside of, or were integrated into, the core-walls. I would need to check, but I would expect that these boxes were also filled with re-bar and concrete as the pouring progressed upwards. The box-beams are NOT the beginning and end of the core structure, there is an awful lot more to it than that. It is a reinforced concrete structure - even probably inside the boxes that have caused you so much confusion.