The deterministic model of a base–isolated liquid storage tank subjected to horizontal base excitation can be represented by a 3–dof lumped system, cf. Fig. 1–(a), using the mechanical analogue proposed by Haroun and Housner [1] and experimentally validated in [2]. Since then, the model has been widely used for base–isolated liquid storage tanks in many research works [3, 4]. The top liquid layer is considered as sloshing mass (mc), middle layer of the liquid is considered as impulsive mass (mi), and the liquid mass moving rigidly with the tank wall at the bottom is considered as rigid mass (mr). They are considered to act at heights Hc, Hi and Hr , respectively, from the base of the tank. The isolator is considered as laminated rubber bearing with linear force–deformation behavior as described in [5, 6], cf. Fig. 1–(b), where Fb is the isolator restoring force.