The picture shows an old mansion in Brooklyn, currently in renovation. It’s got the standard 1910 New York mansion construction: brick bearing walls, steel beams and tile arches for the floors, and terra-cotta block partitions for the room dividers.
The groin vault on the left is fake: it’s plaster on metal lath, hung from the floor above. (The flat ceiling oil the right is built and hung the same way, as can be seen its edge.) The room’s walls are just partitions, and made of terra-cotta blocks that are not intended to be structure. But…
How do you make a door opening in a terra cotta partition? In this case, using arches of more terra cotta. So the fake bearing masonry partition was using an actual bearing masonry detail to span.
