This photo was taken from the basement / garage of a 35 year old home. It looks like during the remodeling of a bathroom someone wanted a tub that was low to the floor. So they boxed an opening in the floor joists, and framed another box with 2×10’s resting on the floor joists. They then tiled everything and rested the tub on the tiles. It wasn’t known how much use this tub got or how heavy the occupants were, but it didn’t look like there was much of a lip for support. It was a cast iron tub, pretty big and deep.