Stability of Imperfect Steel Conical Tanks under Hydrostatic Loading

The work presented in this paper was motivated by the collapse of an elevated conical shaped water tower structure in Fredericton, Canada, in December of 1990. It is analytically based and involves nonlinear stability analysis of liquid-filled conical steel vessels possessing geometric imperfections and residual stresses. A finite-element formulation based on a newly developed consistent shell element that includes both geometric and material nonlinearities is used. Elastic stability analyses of conical shells with different geometric imperfection patterns are undertaken, and the results indicate that the presence of axisymmetric imperfections leads to the lowest limit load for the structure. The sensitivity of the hydrostatically loaded conical vessels to geometric imperfections and residual stresses is investigated by considering inelastic analyses of three cases: (1) perfect vessels; (2) same as case 1, but with axisymmetric geometric imperfections of the order of the thickness of the shell; and (3) same as case 2, but with the addition of residual stresses due to welding. Results of these analyses indicate that liquid-filled conical shells are quite sensitive to geometric imperfections and that yielding precedes elastic buckling for tanks having practical dimensions.