First you need to design to the applicable code (API, AWWA). The code should determine the method of reinforcement. For small tanks the excess thickness in the shell may provide sufficient reinforcement. You need to carry all shell forces through to the support base and allow for stress...
I assume you have read IFR's reply in thread 1452-257294 and since you have decided against a center column, dome or umbrella, I suggest you enlist the help of a structural or civil engineer familiar with the AISC ASD Steel Manual. It is not a difficult solution but in your case could involve...
What information are you seeking, system design, equipment or blanketing gas? You might seek advice from your local inert gas or welding products vendor.
If you are concerned with the straightness, consider the deviation of the column axis from the compression axis at the ends as inducing a bending moment with the deviation being the moment arm. Then combine the axial and bending stresses by addition or sum of squares and compare to allowable...
