William B
every 4 feet

hollifieldrobert
would depend on thickness of joists but if you put them every 8ft you cant go wrong if there at least 2x6

Carpenter
Well, I would recommend having an inspector come out and take a look. He may be able to give you some better ideas than me. But if you are just looking to support the floor you will and to install a beam perpendicular to the floor joists. The beam should run the length of the house in the middle of the floor joists. You could buy a LVL, but I doubt that you need one. Take and nail 3 2x6 together the length of the house and I would think that to be sufficient. Then you will need to put your floor jacks under the beam and on solid bases and tighten them. I hope this helps! Good Luck! Here I found a picture of what you need to do, Sorry about the size, its the best i could do.