Recent severe earthquakes (1994 Northridge, USA, 1999 Koaceli, Turkey; 2001 Nisqually, USA; 2009 L'Aquila, Italy) have clearly shown that a correct analysis of the seismic risk posed to critical facilities should consider structural failure as well as secondary system damage, including damage to non-structural elements and to equipment [1]. In many cases, the monetary value of the equipment damage even exceeds the value of damage to the structure. Hence, for facilities having high-risk contents (e.g. thermal or nuclear power plants) or high-value contents (e.g. high-tech facilities, data centers), the need to protect non-structural parts under strong earthquakes may be equally important than the seismic design of the structure itself. An effective way to protect seismic vulnerable equipment consists in implementing an isolation system between the secondary system to be protected, which can be an individual floor [2] or a single equipment itself [3], and the supporting structure. The absolute acceleration of the equipment is considerably reduced so damages due to excessive inertial forces can be prevented.

The use of linear and nonlinear vibration isolation has been recognized for a long time to be very effective in protecting building structures, industrial plants as well as seismic vulnerable equipment from earthquake ground motion [4]. In this work, an innovative passive isolation system with nonlinear hysteretic behaviour is proposed. The study addresses the problem of formulating a constitutive model that can be used in the design phase: the system behaviour is strongly nonlinear and shows challenging aspects, including the modelling of nonlinear stiffness and damping, the deep understanding of the hysteretic energy dissipation mechanism and the large deformation effects. The aforementioned model is then used for the optimal design of an isolation system for the seismic protection of equipment. A case study is chosen which refers to floor-mounted block-type equipment, a class comprising a significant portion of the mechanical, electrical and electronic equipment found in critical facilities (e.g. emergency generators, transformers, computer cabinets, chillers, ...). The seismic performance of the proposed isolation system is finally investigated through numerical simulations.

[1] FEMA-P 750, NEHRP Recommended Provisions for the Seismic Regulations for New Buildings and Other Structures, 2009, Building Seismic Safety Council.

[2] Hamidi M., El Naggar M.H. On the performance of SCF in seismic isolation of the interior equipment. Earthquake Engineering and Structural Dynamics, 36:1581-1604, 2007.

[3] De Angelis, M., Perno, S., Reggio, A., De Canio, G., Ranieri, N. Prove sperimentali su tavola vibrante di un telaio in scala 1:5 dotato di isolamento di piano. Atti del XIII Convegno dell'Associazione Nazionale Italiana di Ingegneria Sismica ANIDIS, Bologna, Italia, 28 giugno - 2 luglio 2009.

[4] Ibrahim R.A. Recent advances in nonlinear passive vibration isolation. Journal of Sound and Vibration, 314: 371-452, 2008.