This photo was taken from the basement / garage of a 35 year old home in Knoxville, TN. 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 2x10'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 Kohler cast iron tub, pretty big and deep.

Photo taken by Jack Feldmann of Clayton Inspection Service Inc