Seismic Response of Base-Isolated Liquid Storage Tanks

We investigate the seismic response of liquid storage tanks isolated by lead-rubber bearings. The force-deformation behavior of the bearings is considered as bi-linear modeled by the Wen equation. The continuous liquid mass of the tank is modeled as a sloshing mass, impulsive mass and rigid mass. The corresponding stiffness associated with these masses has been worked out depending upon the properties of the tank wall and liquid mass. The governing equations of motion of the three-degrees-of-freedom model of the isolated liquid storage tank are derived. Since the force-deformation behavior of the bearings is non-linear, as a result, the seismic response is obtained using the Newmark step-by-step method under several recorded earthquake ground motions. The responses of two types of tanks, namely slender and broad, are compared with the corresponding response without an isolation system in order to investigate the effectiveness of the isolation system. A parametric study is also carried out to study the effects of important system parameters on the effectiveness of seismic isolation for liquid storage tanks. The various important parameters considered are the aspect ratio of the tank, period, damping and the yield strength of the isolation system. It has been observed that the seismic isolation of the tanks is quite effective and the response of isolated liquid storage tanks is significantly influenced by the above system parameters. There is an optimum value of isolation damping for which the base shear in the tank attains the minimum value. Therefore, increasing the bearing damping beyond a certain value decreases the bearing and sloshing displacements but it increases the base shear.