Complications in Free Vibration Analysis of Tanks

A simple and computationally effective method for computing the dynamic characteristics of ground-supported, cylindrical liquid storage tanks has been developed. A generalization of this method is presented herein to include some complicating factors which affect these characteristics. It has been shown that the initial hoop stress due to the hydrostatic pressure and the in-plane rigidity of the roof system may affect considerably the cosnΘ-type modes of the tank wall. In addition, the flexibility of the foundation soil can reduce measurably the fundamental natural frequency of the cosΘ-type modes due to the rocking motion. It is also concluded that the coupling between liquid sloshing modes and shell vibrational modes is weak; and consequently, the convective pressure can be evaluated with reasonable accuracy by considering the tank wall to be rigid, and the impulsive pressure can be determined by analyzing the liquid-shell system and neglecting the sloshing motion. The validity of the method of analysis has been confirmed by both scale model testing and field measurements of the vibrational characteristics of full-scale tanks.