Effect of vessel base rotation on the seismic behaviour of conical shaped steel liquid storage tanks

Abstract

Liquid storage tanks in the form of truncated cone steel vessels are widely used to provide water supply or to store other liquids. For the case of elevated conical tanks, the effect of the tank vessel base rotation has been assumed to be negligible in previous studies related to seismic behaviour of such tanks. In this study, this assumption is assessed by studying the effect of this rotation on the seismic behaviour and the vibration characteristics of elevated steel conical tanks using finite element models where a fluid-added mass matrix incorporating this effect is derived and then incorporated in dynamic and free-vibration analyses. It is found that the percent reduction in the natural frequency of the first impulsive mode is almost constant for the cases of conical tanks with angle of inclination of 30° and 45°, while it is lower for tanks with inclination angle of 60°. Regarding the base forces, it is found that the percent increase can reach up to 37% for the overturning moment and 34% for the base shear compared to the case of rotation-prevented tanks. In addition, a mechanical analog that simulates the forces acting on an elevated conical tank subjected to a horizontal excitation including the effect of this base rotation is developed taking into consideration the flexibility of the tank walls. Different parameters of the mechanical model are displayed in the form of charts for different tank dimensions. Finally, an example is provided to show how the developed mechanical model is applied.