The allowable buckling stress in the API standard is based on the classical value of buckling under axial load, significantly reduced by a large knock down factor due to shell imperfections and also increased to account for the effects of the internal liquid pressure. In the New Zealand recommendations, the classical buckling stress in membrane compression is calculated, then corrected according to the imperfection amplitude and the internal pressure. In addition, an elastic-plastic collapse stress is also calculated. The lower of the two stresses is used in the calculation of the factor of safety.
The critical buckling stress assumed in current standards for the tank design might lead to rather conservative estimates of the buckling strength of a free base tank subjected to rocking motions. The actual loading conditions during the uplift response were far different from those provided in static buckling tests of small cylinders under uniform axial compression, on which the current design buckling stress has been based. In addition, investigations of the uplifting behavior of the bottom plate showed that ignoring the membrane stress mechanism considerably underestimated the uplifting stiffness of the bottom plate.