[CCs77]

I really, really doubt those slabs are made of concrete. By the time that building was constructed, reinforced concrete construction was barely in an experimental stage, so it is pretty unlikely they used concrete there.

The most common material for building floors back then was wood, not even steel was still widely used. They may have used some form of tile vault, like a catalan vault, which could've made the slab thicker, but not uniformily thick, so it would have been 3ft thick in the lower part of the vault, but much thinner in the center. That if it was indeed 3 ft thick, but I doubt it. My guess is that it is a conventional wood floor, and not 3 ft thick, maybe that figure was including the ceiling.