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I want to use a 4x4 8' with two floor jacks on each side holding it up against the floor joists in the middle of the length. The purpose is for support.

We added a lot of heavy mud weight for tiles, that I was considering this support.

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Without much more details this is impossible to answer. Any attempt to try and answer this with so little information might be dangerous. You might want to consider bringing in an engineer and get his opinion. –  maple_shaft Oct 13 '13 at 23:21
I agree with maple_shaft; you need to get something like this engineered. You likely need a beam under all of the floor joists, then proper footings to hold the weight under the posts. –  Eric Gunnerson Oct 14 '13 at 3:39
I agree with @maple_shaft but very briefly I would say no. When you are supporting joists in the basement usually most engineers will want the load carried all the way through - not just 8 feet. Also I don't think a 4x4 is acceptable other than as temporary support. –  DMoore Oct 14 '13 at 5:30
I don't think a 4x4 is the right choice here. A 4x4 would be strong in a vertical orientation (carrying loads on top of it), but not so strong in a horizontal orientation. You'd be better off with 2 sistered 2x6s, but likely if you're carrying a floor you'll want sistered 2x10 or 2x12. –  Tester101 Oct 14 '13 at 11:59
A common problem I have seen in DIY supports is very common to this question. Someone has an issue - lots of times it is tiling or bathtub or fireplace. They put in a support structure as noted here. Said structure does OK until the jacks sink into the concrete from the pressure - maybe just a 1/2 inch to an inch. Tile cracks, fireplace cracks, something minor happens but is usually costly. Not a huge deal but if you are doing something then just do it right. If for you - so you don't have to deal with it again in 5 years, if for someone else - so it passes inspection. –  DMoore Oct 14 '13 at 14:16

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