We are remodeling the first floor in our 2 flat. We use it as a single family. Tearing out a wall between kitchen and "second" bedroom from when it was originally built in 1926. The current floor joists are 2x8 and span the 24 feet, but there is a wall mid way. If we were to double up the joists to add additional support would that work? These are 16" on center. We have steel columns in the basement supporting the house. Or do we need to do a header to support the second level? Any help would be greatly appreciated.
No, you cannot span 23 feet with conventional lumber, even doubled. It'll be as bouncy as a trampoline and won't meet code, and that's if you can find such lengths.
You'll need at least a 12" (nominal) I-joist or an engineered floor truss to span that far, or you'll need an LVL or steel beam midway.
None of us can offer more specific advice without a site visit, and most of us aren't engineers anyway. There's really no way around having a qualified local expert stop by.
Way back then, the wood was milled to 1 5/8" wide to 7 5/8" high. You say there is a wall "mid-way". Therefore, the span is 12'. So, with joists at 16" on center, they will support about 78 lbs. per square foot. (This is based on a No.2 grade Douglas Fir-Larch wood joist. Douglas Fir-Larch lumber is some of the strongest. So, a couple of factors to consider: 1) If the sources of wood is something other than Douglas Fir-Larch, then it could be up to 10% less, and 2) if the joists are fairly "sound" (fairly straight grain, without a lot of knotholes , without loose or missing knotholes, etc.), then the strength could be up to 25% - 30% more.
Assuming a "Dead Load" (subfloor, underlayment, the joist themselves, finish ceiling below, and carpet or other finished flooring) of 15 lbs. per square foot, then that leaves a "Live Load" of 63 lbs. per square foot.
The Code requires a live load of a minimum of 40 lbs. per square foot. However, at 40 lbs. per square foot, the floor will "feel" springy. We like floors designed to 75 lbs. per square foot, so the floor feels "stiff".
Other issues to consider: 1) will there be a water bed, 2) will there be a large refrigerator, 3) will books be stored in the rooms. (The Code requires a live Load for book storage of 200-250 lbs. per square foot. Other storage is 125 lbs. per square foot.) If any of these conditions apply, I'd add an extra joist in those areas.
Also, the mid-span wall must be supported down to the footings. You say, "there are steel columns supporting the house in the basement." That's good, if they support a large enough beam under the mid-span wall above. To calculate this, we'll need to know the size of the beam under the wall and how far the columns are apart.