In almost buildings, the water storage tanks would be placed. Among the various shape tanks, many rectangular tanks would be found. The water storage tanks are usually made from FRP (Fiber Reinforced Plastics) which would possess lower rigidities than other structural materials. The rectangular water storage tanks have large capacities in comparison with other shape tanks. Accordingly, it is worried that the rectangular tanks may present low stiffnesses. There is an anxiety about the big earthquake to take place in future. Thus, the earthquake-resistant tanks, which reserve water to be considered most vital at the time of disasters, are required. However, in the recent earthquakes; Near Izu-Oshima earthquake on Jan. 14th, 1978 and two earthqukes taken place off Miyagi Prefecture on Feb. 20th and Jun. 12th, 1978, many water storage tanks were damaged. In these reasons, it should be necessary to study the dynamic behaviors of rectangular tanks. The dynamic behaviors of rectangular tanks were already studied by G. W. Housner and Fumiki Kito. In the formula developod by Housner, the elasticity of the side walls was not considered. Dr. Kito analyzed the natural frequencies of elastic side walls, by the energy method, assuming the simply supported edges and clamped edges, and so on. This paper describes the theoretical and experimental studies concerning rectangular water storage tanks. In the theoretical study, the Kito's method is employed. The bending displacements of the walls perpendicular to an excitational direction (which are called the pressured walls), and the shear displacements of the walls parallel to this direction (which are called the side walls), and the displacement of a base frame are assumed. The water in the tank is regarded to have the velocity potential that satisfies the condition of free surface, the continual conditions of velocities on the pressured walls, and the three dimensional Laplace's equotion. From these assumptions, the kinematic energies and the potential energies would be obtained. The vibrational equations of this system would be expressed by substituting these energies into the Eular's equation. After all, this analysis would be concluded in solving the multi-degrees vibrational equations. The dynamic tests concerning the water storage tank assembled by the steel panels with dimensions of 1m × 1m, were conducted. This rectangular tank has the capacity of 48t (width×length×height=3m×4m×4m). The large-scale shaking table of National Research Center for Disaster Prevention in Tsukuba Newtown was used in this experiment. The measurements for this experiment were made in the purposes to obtain the frequency properties of responses and hydrodynamic pressures. The mathematical simulation for this experiment were carried out in the frequency and time domains. The results of theoretical analyses for this tank would agree with the results of the experiment, partly.
