Seismic Time History Analysis of Liquid Storage Tanks

Abstract

Liquid storage tanks are considered as an important part of liquid transmission and distribution systems. In the industrial sector, ground-supported steel tanks are used to store hazardous, toxic and inflammable liquids. Under seismic event, the liquid inside the tank vibrates along with the tank resulting into uncertain and amplified response. The failure or major damage of the tanks can cause fire, environmental pollution, and leakage of toxic liquids. Therefore, the tank should be designed and analysed such that it can withstand seismic forces. Various national and international codes are available for seismic analysis and design of the liquid storage tanks. These codes mostly include equivalent static method for estimating seismic forces. However, under seismic excitation liquid storage tank behaves dynamically due to its dynamic properties, sloshing of the liquid and fluid structure interaction. Thus, to determine critical response of the liquid storage tank, linear dynamic analysis is essential. It has been found that parameters like density of the liquid, size of the tank, boundary conditions at the base, the height of the liquid inside the tank and near and far-fault earthquakes can influence dynamic behaviour of the liquid storage tanks. The present study includes code based seismic analysis of the liquid storage tank specifically API 650 IS:1893(Part 2)-2014. Seismic response in terms of base shear and base moment are determined using both seismic codes and the same are compared. It has been found that API 650 provisions yield about 28% lower values as compared to IS:1893(Part 2)-2014 provisions. Computational models for liquid storage tank with and without roof are developed using commercial software SAP 2000 using Housner’s spring-mass model. Natural frequencies and corresponding mode shapes are determined by performing free vibration analysis and dominant frequencies constituting more than 90% of modal mass participation are identified. Linear time history analysis is performed for liquid storage tank and seismic response parameters like peak displacement, peak acceleration, base shear and base moments are determined under strong and pulse type of seismic excitations. It has been found that base shear and base moment are governed by impulsive component of the liquid in the tank. Parametric studies in terms of effect of different height of the liquid, effect of different densities of the liquid, effect of different H/D ratio and effect of near and far-fault records for open roof liquid storage tank are carried out. It has been found that displacement and the base shear of the tank increase substantially when tank has liquid more than 50% of its height. The density of the liquid influences the dynamic behaviour of the tank when height of the liquid in the tank is more than 70%. The H/D ratio of 0.4 shows resonance response under El Centro seismic excitations. Generally near-fault seismic excitation yields higher seismic response of the liquid storage tanks.