The resistance to overturning is provided by the weight of the tank shell and by the weight of a portion of the tank content which depends on the width of a bottom annular ring that lifts off the foundation. To determine this width, an elemental strip of the bottom plate perpendicular to the shell which can be lifted off the ground is considered in the API standard. The content weight which may be utilized to resist overturning is based on calculated reaction at the tank shell of this strip. It is assumed that two hinges are developed in the bottom plate, one at the junction of shell and bottom plate, and the other at some distance inward from shell. Neglecting hydrodynamic pressure on tank bottom plate, the equilibrium of the strip leads to the unknown value of the compressive stress. It is noted that neither the deformability of the tank wall nor the flexibility of the underlying soil are considered in the model. In addition, only small deformation behavior of the bottom plate is taken into account. At higher levels of ground excitation, the capability of the plate to resist the applied loads would not be fully accounted for, leading to conclusions of global instability (overturning). One other discrepancy noted in the API procedure is that the overturning moment at tank base and the uplift force are independent of each other which is unrealistic as the overturning moment produces the uplift force! It is noted that the API standard restricts the maximum value of uplift width of the bottom annular ring to 0.035 of tank diameter, otherwise, the tank must be anchored.
In the New Zealand recommendations, the stress in the tank shell is calculated using global equilibrium of the shell and its base. It is assumed that the bottom plate remains in contact with the foundation on a circular area of a radius slightly less than the radius of the tank, and that the edge of shell rests on an arc of the tank's perimeter of unknown central angle. The two unknowns of the problem, the maximum compressive stress in tank shell and the central angle of contact arc, are found by solving two nonlinear algebraic equations which govern both global vertical force equilibrium and global moment equilibrium. One disadvantage of this model is that it does not take into account the deformation behavior of either the tank wall or the bottom plate. Furthermore, it neglects the variation of dynamic pressure on bottom plate and uses a constant value equal to the static pressure.