Oil storage tank foundation can be considered as ring wall if the storage tank doesn’t lay on the weak soil. The ring wall foundation dimension (thickness) can be calculated base on below consideration:
1. The I.E. Boberg Equation
T = 24 W / (q.H-80h)
where:
W = weight of shell supported on the ring wall (lb/ft)
H = height of tank shell (ft)
h = height of ring wall (ft)
q = weight of storage product lb/ft3)
T = minimum thickness of ring wall foundation (in)
2. API 650 (Appendix B.4.2.2)
When a concrete ring wall designed, it shall be proportioned so that the allowable soil bearing is not exceeded. The ringwall shall not be less than 300mm (12in) thick.
Sorry SynClaire but the ring ground beam of an API 650 AST is not the foundation of the tank .
The tank foundation is the earth and according to API 650 the concrete ring beam is not mandatory but recommended only for large tanks
Because the concrete ring wall purpose is only to stiffen the bottom of tank shell and to ensure a less prejudictible dsitribution of tank shell deformation on the earth foundation
A lot of people do not really know how does work an API 650 tank foundation