Snap Through of a Shallow Spherical Dome of a Prestressed Concrete Tank

An investigation of the stability of the roof of an 85-ft diameter cylindrical prestressed concrete wastewater treatment tank is presented. The roof of the tank is a very thick, shallow spherical dome. With a 1/16 rise to span ratio, 2 ft thickness, and an 18 ft diameter central opening at its pole, the dome falls outside the range of parameters for which the thickness design equation for domes in the ACI 350 code applies. The shallowness parameter of the dome is in the range where axisymmetric snap-through buckling can be expected. A finite-element model was used to compute the nonlinear axisymmetric deflection of the dome up to its limit point, the snap-through to its inverted configuration, and the post snap-through load deflection. Bifurcation from the symmetric deformation to a nonsymmetric buckling mode below the limit point was investigated with a different finite-element model than was used for the evaluation of the snap-through. The finite-element results are compared to a form of the design equation for dome thickness in ACI 350. The dome thickness equation in ACI 350 was used to calculate dome load capacity, given the thickness, and was found to be significantly higher than the capacity computed with the finite-element model.