Performance Evaluation of Semi-Active Equipment Isolation for Seismic Protection Using MR Dampers A. Bakhshi and L. Zebarjad Civil Eng. Department, Sharif University of Technology, Tehran, Iran E-mails: bakhshi@sharif.edu & lzebarjad@yahoo.com SUMMARY: Sensitive equipment, are vulnerable to strong earthquakes, and failure of these equipment may result in a heavy economic loss. To control the seismic response of the equipment, magnetorheological (MR) dampers are appropriate devices, because of their high adaptability and reliability. This paper presents the performance evaluation of passive and semi-active control in the equipment isolation system for earthquake protection. In order to reduce the vibration of the equipment, in a seismically excited steel frame building, an MR damper is placed in parallel with the sliding friction pendulum isolation system, between the equipment and the floor. For the MR damper, phenomenological model based on Bouc-Wen model is used. In the analysis, two semi-active control algorithms, clipped-optimal and the maximum energy dissipation, and two passive control algorithms, passive-on and passive-off are employed. Simulation results demonstrate that the control technique is effective in protecting vibration-sensitive equipment from both far-field and near-field earthquake excitations. Keywords: MR damper, Sensitive equipment, Semi-active control, Earthquake excitation 1. INTRODUCTION Because the performance of highly sensitive equipment in hospitals, communication centers, and computer facilities can be easily disrupted by moderate acceleration levels and even permanently damaged by higher excitations, efforts have turned toward the use of isolation for protection of a building’s contents [1]. Passive isolation and damping systems have been shown to preserve structural integrity under demanding earthquakes [2]. Typically these equipment isolation systems are a friction-pendulum, or rolling-pendulum type [3]. Passive equipment isolation systems perform extremely well during low- level seismic events [4]. However, during high-amplitude, long-period, ground motions excessive isolator displacements could damage the isolators or overturn the equipment [5]. Seeking to develop isolation systems that can be effective for a wide range of ground excitations, hybrid control strategies, have been investigated by a number of researchers. The advantages of hybrid base isolation systems are high performance in reducing vibration, the ability to adapt to different loading conditions and control of multiple vibration modes of the structure. One class of hybrid base isolation systems employs semi-active control devices, often termed ‘‘smart’’ dampers. Semi-active control systems are unconditionally stable, have modest power requirements, and can reduce vibration transmissibility for long period excitations without increasing the transmissibility for short periods [6]. MR dampers are semi-active dampers in which the damping forces are controlled by magnetic field [7]. These dampers are well suited for semi-active control of seismically loaded civil structures because of their low power requirements, high force capacity [8], high dynamic range and mechanical simplicity. Also even when battery power fails, MR dampers will still have some passive damping performance, albeit at a much lower level. Thus it is expected that the hybrid control strategies, consisting of a passive isolation system combined with semi-active control devices could solve the large base drift problem of the passive-type base isolation [9].