I am currently in the process of slowly jacking the floor in my house with a beam/jack post combination in the basement.
The dining room floor in my 1922 home (recently moved into) had sagged in one quarter of the house approximately 2"-3" from the walls. The problem appears to be the result of the house construction - the load bearing walls on half of the house are offset from the main structural beam (which is visible in the unfinished basement) by about 4'. Apparently, the builder recognized the problem and doubled up the 2x10 floor joists on this quarter of the house; however, the floors have still sagged considerably. The joists span about 18' from the 6x8 beam to the outside wall (there is no sag on the opposite side because the span is only 12'). After much research on how to jack the floor, I have placed a beam across the sagging joists (midway under the 18' span) and have started the slow process (quarter turn of jack posts a week). The first turn of the jacks resulted in lots of snapping and cracking from the old house. My concern now is that the floor joists may not straighten as intended, but may actually crack from the extreme new stress, especially underneath the load bearing wall (there is now an 8' span from the center beam to my new beam, with the load bearing wall about midway). The joists are very old, hard and brittle (you can hardly drive a nail in them). Any advice based on experience with jacking old, brittle wood? Will it straighten or is cracking a concern?
not much experience in jacking floors but from building houses it seems to me that the beam should be under the bearing wall as well as half way on the 18 foot span. the old wood will probably stay in the shape as it is meaning crooked and sagged. as for the cracking sounds im not there to say. id call a local contractor and show him your concerns and see what he would do. sorry not so much help.
You could try "sistering" new floor joists to the old ones. Place a new joist next to the old. Jack the new joist to take the sag out of the floor and then nail the new joist to the old. The flooring will separate from the old joists in places of sag, but the new will support it when in place.(simplified version).