The response of the two tanks was estimated using the large deflection assumption and the tank base plate was considered supported on a tensionless elastic foundation of a uniform stiffness of 1000 lb/in/in2 in compression. Table (5.7) shows the response of these tanks to El Centro and Northridge earthquakes, respectively. The table shows a reduction in the uplift displacements of the base plate and an increase in the contact area of the base plate with the foundation due to the membrane effect that increased its uplifting stiffness. This also has caused the dominant period of the rocking motion to decrease to 0.21 sec for the broad tank and 0.55sec for the tall tank. In addition, the membrane effect of the tank shell has reduced the deflection and acceleration at top of the broad tank. As a result, the impulsive acceleration was less and the resulting hydrodynamic forces were also less. For a tall tank, which acts more like a cantilever, this did not have much effect on the tank shell acceleration and the hydrodynamic forces were also were not altered much.
Axial stresses at the bottom of the tank shell were less than those using the small deflection assumption. This is attributed to the reduction in the uplift displacements as well as the increase in the contact area of the tank base plate.