Behaviour of Elevated Water Storage Tanks Under Seismic Events

Abstract

Liquid storage tanks are lifeline structures and strategically very important. Water tanks, in particular are important to the continued operation of water distribution system in the event of earthquakes. Most of the failures of large tanks after earthquakes are suspected to have resulted from the dynamic buckling caused by overturning moments of seismically induced liquid inertia and surface slosh waves. Recent earthquakes have shown that liquid storage tanks are found to be vulnerable to damage. The dynamic behaviour of liquid containing structures cannot be estimated by the same approach as for normal building structure as the weight of storage tanks varies in time because of variable liquid storage level. Also there exists sloshing of liquid which influences the response of the tank. Current knowledge about the behaviour of liquid storage tank is extensive, but many of the analytical and theoretical results are for tanks ground supported and open tanks. Therefore, appropriate modeling of the liquid storage tanks is essential for dynamic analysis and seismic response evaluation. In most of the research works, the liquid storage tank has been modeled using two mass model or three mass model concepts. The major difference of these two models is the number of lumped masses into which the liquid column is divided. The two-mass model is convenient for designers to use due to ease but the three-mass model predicts the seismic response more accurately. The objective of the study is to investigate the uncontrolled response of Steel and Reinforced Cement Concrete (RCC) Elevated Water reservoirs of different aspect ratio S = H/R (height of the container to its radius) and subjected to different strong ground motion earthquakes. The simulation of water tank using three-mass model concept is carried out through MATLAB. The paper deals with two H/R ratios viz. 0.6 and 1.85. The governing equations of motion are solved by State Space Method. In the first phase, the response of the tanks to different real earthquake ground excitations is investigated using simplified three mass model of the tank. In the second phase, characteristics of earthquakes are varied and emphasis is given to study their effect on response of the tanks. The response quantities such as sloshing displacement, impulsive displacement, tower displacement and base shear are measured by varying the characteristics of earthquake time history. It is observed that the sloshing displacement is highly influenced by the characteristics of the time history compared to other response quantities.