Without a Mr. Man intervention, I would sob and flail my arms, but luckily, I live with an architect (with a stamp and everything!) so instead I only sobbed and cried a little and then turned to he who understandeth buildings and building structure. Ahem. Without further ado, Mr. Man:
"It is called a Sawsall and the name pretty much says it all. It cuts joists, beams, floors, and even columns without much trouble, even if they are still holding something up. It is a favorite tool of pretty much everyone in construction including plumbers. And a Sawsall is likely the tool that was used to make the plumbing in the upstairs bathroom function . . . by cutting out large sections of the floor joist that were in the way of the pipes (see photo above.)
Opening up the floor was originally an exercise in cleaning up the missmash of flooring under the sink and toilet. It was not until I pulled a few boards up that I noticed the cut joist. Looking more closely at the surrounding floor, it was apparent the floor had sagged noticeably due to the cut joist and that it was one of several joists bearing the weight of not only the sink and toilet, but the five foot cast iron tub as well. The joist and floor had most likely been this way for 30 or more years, naturally redistributing the loads to other joists with out a disaster, so an actual failure was not that likely. On the other hand, even small odds of finishing a hot bath with broken bones in the dining room is not a pleasant thought.
Ideally the plumbing area would be boxed out with wood by doubling up adjacent joists and putting headers around the opening. But that would require removing a large part of the ceiling below, which isn't in the budget. So I cut this:
Thanks Mr. Man!