I recently purchased a brick house built in the 1870s. We have a second floor sag right in the center of the house that I like to straighten out a bit because I'm going to have a tough time putting down flooring. For the life of me I can't figure out what type of framing is in this house. The joist in the basement rests on brick and spans the entire basement without any support? Is the second floor like this?
Brick homes built in the late 1800s were usually solid brick walls, 8" thick or thicker, 12" is not unusual. As the walls went up the floor joists were laid into the walls, bearing directly onto the brick. Depending on the span, the joists are continuous, from one wall to the other. This could easily be up to 24 feet, no reason why it couldn't be more since wood was plentiful and cheap back then. There would be no exterior walls supporting the second floor, unless it was added as a necessary upgrade during one of the remodels. I have had a part in that in homes.
If your floor space is open from exterior wall to exterior wall, then you have joists that are one continuous length from one wall to the other. If you have a wall, which it sounds like you don't in the basement at least, on the first floor, that may be a bearing wall for the second floor, even though there may be no means of support for the bearing wall in the basement. The first floor joists could be heavy enough to allow the needed support for the second floor joists..... or not....
Either way in an old home such as yours, you would not be able to straighten the original floors by simply jacking the up, at least not to a large degree, not without putting a bigger strain on another part of the floor from trying to remove what over a century of gravity has done. It would require removing the ceilings, since they cannot handle the forces of straightening the ceilings in any fashion. The joist could be cut in mid span and add a beam inline to keep a flush ceiling and that will cut the sag in half. That is a lot to do for the sake of flattening a floor.
But if a flat floor is what you want, that is what it takes. There are other ways to do it too, like removing all the flooring if possible, (I just answered another post about a second floor flooring issue and what it meant to pull up all the flooring) sister the floor joists so they are straight, plywood and lay new floor. Quite difficult to do if you have walls on the second floor. If you have an open floor plan, then that is a good thing. All in all, still no cheap proposition.