[Rizzo]

^ That's what I figured, though the workshop zone of the building appears to be more of a hybrid which is unexpected.

Reinforced concrete construction in older buildings corbels up in some way around the columns to diffuse the forces either with the conical "mushrooms" as ardecila mentioned or angled haunches , which isn't really the case with the post office. Plus there's a ton of steel spaced closely together, meaning the slab doesn't have all that much tensile strength.

While reinforced concrete construction was around at the time, it was still a new construction method, and there were probably not too many skilled contractors around to be commissioned on a building this large. I would imagine the US Post Office and their engineers proceeding with reliable and known methods of construction to deliver the project on time. I don't know what concrete encased around steel beams would be called as far as terminology, but it's not the common construction we see in modern times where grids or strands of rebar lend that tensile strength.

Anytime you see steel frame in an older building, odds favor that the slab will be clay tile.