It almost sounds similar to a concrete ring wall design problem except with the forces coming in from the exterior instead of the interior.
We design "belts and suspenders" with our underground structures meaning that we design the horizontal bars for hoop stress and vertical bars as a cantilever off the foundation. From another reference here on this site, I found the document "Oil Storage Tank Foundations" by Irving E. Boberg to be an excellent source for understanding the hoop stress portion of this design.
I believe that Hoop Stress though would be more applicable to the forces pushing out causing tension in the horizontal steel. In your case, I believe the design is more similar to a compression ring.
When we design these circular walls, we do design circular foundation slabs below them (although it varies if we have any kind of foundation below them depending on where the project is located). The diameter of these foundations is large enough that this isn't a problem. I imagine that it might be if the circle was small just because of the limitations of what you can form.
However, we show a circular foundation on the drawings but what ends up happening sometimes in the field is that it is a little more truncated with straight lines. Ours are underground so this usually isn't a problem. If given the option and depending on the size of the tank, a square footing would probably be simpler to form.
