Nuclear Engineering and Design 241 (2011) 700–713 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes Double variable frequency pendulum isolator for seismic isolation of liquid storage tanks D.P. Soni a,∗ , B.B. Mistry b , V.R. Panchal a a Civil Engineering Department, Sardar Vallabhbhai Patel Institute of Technology, Vasad 388 306, Gujarat, India b Engineering College, Tuwa 389 001, India article info Article history: Received 11 April 2010 Received in revised form 30 November 2010 Accepted 9 January 2011 abstract The paper describes the behaviour of liquid storage slender and broad tanks isolated by the double variable frequency pendulum isolator (DVFPI). The DVFPI is a double sliding isolation system having elliptical sliding surfaces. The geometry and coefficient of friction of top and bottom sliding surfaces can be unequal. The governing equations of motion and energy balance equation of the tank-isolation system subjected to bilateral ground excitation are derived and solved in the incremental form. In order to investigate the behaviour of the DVFPI, the response is obtained under different parametric variations for a set of 20 far-field earthquake ground motions. Four different combinations of the DVFPI design cases having different isolator geometry and coefficient friction at top and bottom sliding surfaces are studied and the criterion to optimize its performance is proposed based on minimum responses and energy quantities. Further, influences of the initial time period, coefficient of friction and frequency variation factors at the two sliding surfaces and the tank aspect ratio are investigated. It is found that the performance of the DVFPI can be optimized by designing the top sliding surface with high initial stiffness relative to the bottom one and the coefficient of friction of both sliding surfaces to be equal for a slender tank whereas both surfaces should be designed with equal initial stiffness and coefficient of friction for a broad tank. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The liquid storage tanks are most commonly used in water distribution systems, industries and nuclear power plants for the storage of drinking water, toxic and flammable liquid and nuclear fuel, respectively. Since the damage of liquid storage tanks due to an earthquake may cause secondary hazards, they are strategically very important structures. Thus, the protection of liquid storage tanks against severe seismic events has become crucial. There have been several reports on large-scale damage to liquid storage tanks such as during the 1960 Chilean earthquake (Steinbrugge and Rodrigo, 1963), the 1964 Alaska earthquake (Hanson, 1973), the 1977 San Jan Argentina earthquake (Manos, 1991), the 1979 Imperial County earthquake (Gates, 1980), the 1980 Livermore earthquake (Niwa and Clough, 1982), the 1983 Coalinga earthquake (Manos and Clough, 1985), the 1994 Northridge earthquake (Hall, 1994) and the 2001 Bhuj earthquake (Rai, 2001). The failure of tanks has been found mainly due to buckling of the tank wall, failure of piping system and uplift of anchorage system. ∗ Corresponding author. Tel.: +91 9879399106; fax: +91 2696274540. E-mail addresses: soni svit@yahoo.com (D.P. Soni), bbm 7@yahoo.co.in (B.B. Mistry), vijay svit@yahoo.co.in (V.R. Panchal). For over three decades, seismic isolation technology has been recognized as one of the promising alternatives for protecting liquid storage tanks against severe earthquakes. Several investi- gations have been made to study seismic response of non-isolated (Housner, 1957; Rosenblueth and Newmark, 1971; Veletsos and Yang, 1977; Haroun and Housner, 1981; Haroun, 1983) and base- isolated liquid storage tanks (Chalhoub and Kelly, 1988; Bo and Jia-xiang, 1994; Kim and Lee, 1995; Malhotra, 1997; Wang et al., 2001; Malhotra et al., 2000; Shrimali and Jangid, 2002a,b, 2009; Christovasilis and Whittaker, 2008; Panchal and Jangid, 2008; Jadhav and Jangid, 2006). It has been observed that the isolation is mainly effective in reducing the impulsive component of the response while convective component is slightly increased which do not significantly influence the over all performance of the iso- lated system. In spite of the above studies, there have not been attempt to investigate the dynamic behaviour of liquid storage tanks iso- lated by double sliding bearing. Recently, there has been growing interest in the development of double sliding bearing principally owing to two reasons (Fenz and Constantinou, 2006): (1) the dis- placement capacity of such isolator is twice that of the traditional isolator with a single sliding surface of identical plan dimensions permitting it to accommodate large sliding displacement imposed by severe earthquake ground motions, and (2) there is the capa- 0029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2011.01.012