Earthquake response of isolated elevated liquid storage steel tanks

Abstract

Earthquake response of elevated liquid storage steel tanks isolated by the linear elastomeric bearings is investigated under real earthquake ground motion. Two types of isolated tank models are considered in which the bearings are placed at the base and top of the steel tower structure. The continuous liquid mass of the tank is modelled as lumped masses known as sloshing mass, impulsive mass and rigid mass. The corresponding stiffness constants associated with these lumped masses have been worked out depending upon the properties of the tank wall and liquid mass. The mass of steel tower structure is lumped equally at top and bottom. Since the damping matrix of the isolated tank system is non-classical in nature, the seismic response is obtained by the Newmark’s step-by-step method. The response of two types of tanks, namely slender and broad tanks, is obtained and a parametric study is carried out to study the effects of important system parameters on the effectiveness of seismic isolation. The various important parameters considered are the tank aspect ratio, the time period of tower structure, damping and the time period of isolation system. It has been shown that the earthquake response of the isolated tank is significantly reduced. Further, it is also observed that the isolation is more effective for the tank with stiff steel tower structure in comparison to flexible towers. In addition, a simplified analysis is also presented to evaluate the response of the elevated steel tanks using two-degrees-of-freedom model and two single-degree-of-freedom models. It is observed that the proposed analysis predicts the seismic response of elevated steel tanks accurately with significantly less computational efforts.