Have you ruled out foundation problems? Maybe you could do an elevation survey of the top of the slab to either support your theory or rule out a foundation problem. Are their any other signs of disstress in the othe parts of the building or is that one and only separation in the whole building?
If you can't find a comparable SJI joist in the historical tables, then you will need to analyze the joists. You could do it by hand or use an off the shelf FEM Program such as RISA-3d or RISA-2D to model the joists. The biggest problem is knowing what type of steel the joists are made of.
Or...
what hokie66 said, sounds to be a Geopeir. Check them out on WWW.Geopiers.com. I have never used this product, but sound like it may be an answer for your situation.
Connor M,
I believe a good reference would be “Foundation behavior and repair residential and light commercial” by Robert Wade Brown. I believe it's in its second edition still. Another good reference for the practicing engineer would be the Texas ASCE “guidelines for the evaluation and repair...