Earthquake protection of liquid storage tanks by sliding isolation bearings

Liquid storage tanks are critical components of industrial facilities since damage to such structures may cause spreading of hazardous material and environmental pollution. Tanks exhibit mainly two different seismic behaviors one of which is the long period movements due to sloshing of the liquid and the other is the impulsive vibrations generated as a result of the fluid structure interaction phenomena at higher frequencies. The overall base shear is the combination of these two loads. The seismic base isolation aims to control the impulsive load as it has appreciable amount of contribution to the base shear values. Among various types, the curved surface sliding bearings (FPS) are commonly used in liquid tanks since provide isolation periods which is independent of the tank weight (liquid height). In this paper a parametric analysis has been performed to investigate the efficiency of FPS bearings. The numerical model is based on the Haroun and Housner's simplified lumped parameter model in which the sloshing and fluid-tank interactions are modeled by convective and impulsive masses, respectively. The effectiveness of the isolation system was investigated under a series of ground motions, isolation periods and tank aspect (slenderness) ratios. Results indicated that depending on the characteristics of the ground motion, the response of the isolated tank can be reduced in appreciable amounts as compared to the conventionally constructed one. On the other hand, some detrimental effects were also observed in lower isolation periods (Tb=2s) particularly in medium slender tanks under near fault ground motions. This undesirable situation was avoided by using higher isolation periods (Tb =3s) without much affecting the bearing displacements.