Optimal design and seismic performance of tuned fluid inerter applied to structures with friction pendulum isolators

The hydraulic realization of the inerter, called fluid inerter, is a piston-cylinder device designed to convey a fluid through an external helical channel, thus producing rotational inertia of a fluid mass. This paper proposes a novel base-isolation scheme that combines friction pendulum isolators with a Tuned Fluid Inerter (TFI), wherein a grounded fluid inerter benefits from the interaction with an auxiliary set of low-damping rubber isolators as tuning elements. Based on experiments on a small-scale prototype, the fluid inerter is modeled as a linear inerter in parallel with a nonlinear dashpot, whose power-law damping term is related to the pressure drops in the helical channel. Instead, the hysteretic behavior of friction pendulum isolators is described by a standard Coulomb-type frictional model. Optimal design of the fluid inerter is performed within a probabilistic framework, by modeling the base acceleration as a Kanai-Tajimi filtered random process and handling the non-linearities of fluid inerter and friction isolators through statistical linearization. An extensive parametric study is conducted to assess the influence of different characteristics of isolators and earthquake excitation and, above all, of the nonlinearity of the fluid inerter. The effectiveness of the TFI is demonstrated through nonlinear response history analysis under a suite of 100 ground-motion records. The proposed isolation scheme not only significantly reduces the displacement demand of the isolators, but also mitigates the acceleration and interstory drift response of the superstructure. Moreover, the benefits of the inherent nonlinear damping effect produced by the fluid inerter proves to be particularly effective to reduce the peak response under pulse-like ground motions that may occur in near-field earthquake events, which indeed represent critical excitations for structures with friction isolators.