For a very long time, the New York City Building Code has required that the portion of floor directly in front of a fireplace, somewhat confusingly called the hearth, has to be of non-flammable construction. That way, if sparks or pieces of wood fall out of the fireplace onto the floor, they don’t immediately start a fire. In new buildings, that’s not particularly difficult, but it required some gymnastics in old wood-joist rowhouses and tenements. The picture above shows the gymnastics, although the structure shown here has been altered in a recent and slightly odd way.
The wall on the right contains the flues from the floors below and is where the fireplace for this floor is if my camera shot was aimed down a bit. There’s another fireplace on the floor above (above the floor framing I was focused on). The corner of the brick visible in the bottom center is where the end of the thickened wall is, at the end of the flues, and beyond that the wall goes back to its normal thickness.
Normal floor joists and the diagonally-laid plank subfloor are visible to the left of the header that splits the photo in two. To the right is a sort of wood barrel vault on very short curved joists. Above that is either cinder fill or, more likely, some attempt at a brick vault with cinder fill above it. That’s the fireproof hearth. We see these all the time, sometimes (like here) as barrel vaults, sometimes as half-vaults, curving up from the header to meet the wall at the apex of the vault.
In this this case, someone went to a lot of trouble to rebuild part of this floor. The header and trimmers have been replaced with steel beams – a channel for the header and wide-flanges for the trimmers. The rough yellow junk that’s all over the place is spray-on fireproofing for the steel beams that, as it usually does, got everywhere. In order to install those beams, the wood had to be shored while the old wood header and trimmers were removed and the new steel put in place. (It is theoretically possible that the steel beams are original and the spray-on came later, but unlikely.)