I know this type of question has been asked before but I am going to ask again since I couldn;t find it. The joists in the basment run 16 feet from wall to wall and are rough sawn 2X8 (they actually measure 2x8) so are sufficent for the supported span. Basically in order to address a sagging floor in my house I am going to install a support beam in the basement. So I was looking at making the beam with 2 2x10 with 3/4 osb or plywood in the middle and supporting with steel posts every 8 feet or so with 2'x2'x1' concrete and rebar footers. This sound about right?
A # 1 spruce 12x8 joist (@ 16" o.c.) will support only up to a 12'-6" span for living areas of up to 40 lbs per sq ft.
These existing joists you have (even tho they are a full 2x8 in size) are not strong enough to support this 16' span, and that is why they have sagged. Rather than putting in a supporting beam and posts why not just "sister " each joists with a normal 2x8. it would probably be just as costly but it would still keep the area "open" without any beam/posts in the way.
When installing each "sister" joist make sure you use a temporary support to take some of the sag out of the existing joists before securing the "sister" joist to the existing ones. Do one joist at a time.
I had thought of that but due to the pipes and electric running in the basement it really isn't feasible. The home was built I believe at least in the 30's and the joists may or may not be spruce. I believe they may be fir or another type wood but it's difficult to tell because of the age. Also I have to double check but I think they are closer together than 16" oc