Combined primary–secondary system approach to the design of an equipment isolation system with High-Damping Rubber Bearings Anna Reggio n , Maurizio De Angelis Department of Structural and Geotechnical Engineering, Sapienza Università di Roma, via Eudossiana 18, 00184 Rome, Italy article info Article history: Received 6 August 2012 Received in revised form 9 December 2013 Accepted 10 December 2013 Handling Editor: H. Ouyang Available online 23 January 2014 abstract Isolating acceleration-sensitive equipment from the motion of the supporting structure represents an effective protection from earthquake damage. In this paper, a passive equipment isolation system composed of High-Damping Rubber Bearings (HDRB) is designed by adopting a coupled approach in which the supporting structure and the isolated equipment are considered as parts of a combined primary–secondary system and analyzed together. This allows for taking into account their dynamic interaction when significant and non-negligible according to the mass ratio and to the frequency ratio. The design methodology is developed by resorting to a reduced-order 2-DOF model of the combined system, a linear visco-elastic constitutive model of the isolation system and to a modal damping constraint depending upon the damping properties of the HDRB and their rubber compound. A 1:5 scale experimental model, consisting of a two-storey steel frame and a heavy block-type mass isolated from the second floor, is subsequently used to exemplify the design methodology and to perform shaking table tests. The dynamic properties of the experimental model are identified and the seismic performance of the equipment isolation system is discussed under a wide selection of seismic inputs, both artificial and natural. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction As recognized due to past and recent severe earthquakes (Northridge, 1994, USA; Kocaeli, 1999, Turkey; L'Aquila, 2009, Italy; Tohoku, 2011, Japan), nonstructural damage may result in serious threats to life safety and in major direct and indirect economic losses. In the worst case of critical facilities (hospitals, government buildings, nuclear power plants, etc.), the failure of equipment strongly impacts on the post-earthquake functionality, causing the loss of essential services or businesses, and may pose a catastrophic risk to the environment or to a large number of people [1]. In view of these considerations, current building codes in high seismicity countries (e.g., United States [2,3], New Zealand [4], Italy [5,6]) have devoted an increasing attention to the development of seismic design requirements, aiming at harmonizing the performance levels between structural and nonstructural components and particularly equipment. Intended to provide, in a form as simple as possible, conservative estimates of the seismic forces, such provisions adopt a static lateral force method that is easy to implement and sufficiently accurate for designing light ordinary equipment in Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jsvi Journal of Sound and Vibration 0022-460X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsv.2013.12.006 n Corresponding author. Tel.: þ39 06 44585109. E-mail addresses: anna.reggio@uniroma1.it (A. Reggio), maurizio.deangelis@uniroma1.it (M. De Angelis). Journal of Sound and Vibration 333 (2014) 2386–2403