EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS Earthquake Engng Struct. Dyn. 2001; 30:195–212 Active viscous damping system for control of MDOF structures Y. Ribakov 1;* , J. Gluck 1;† and A. M. Reinhorn 2;‡ 1 Structural Engineering Division; Department of Civil Engineering; Technion-Israel Institute of Technology; Technion City; Haifa 32000; Israel 2 Department of Civil; Structural and Environmental Engineering; State University of New York at Bualo; NY; 14260-4300; U.S.A. SUMMARY The development and applications of a supplemental viscous damping device with active capacity are described. The system of the dampers dened as active viscous damping system (AVDS) is presented herein. Structural control principles dened here as active control theory (ACT) are used to obtain the control forces at each time step during an excitation. Control of the damping forces is possible due to a mechanical structure of the proposed AVDS and do not require the input of large power and energy. This system can be eciently used to enhance the damping of a structure without adding in stiness and strength. The added damping forces can be adjusted in a wide range. Its eciency is demonstrated by a numerical simulation of a seven-storey building subjected to earthquakes. The simulation shows that the behaviour of the damped structure with the AVDS is signicantly improved compared to that of an uncontrolled system. Moreover, the response is better than that of adding either passive viscous dampers or electrorheological damping devices. Copyright ? 2001 John Wiley & Sons, Ltd. 1. INTRODUCTION Passive energy dissipating systems such as viscous dampers, tuned mass dampers and base isolation systems have been installed in new or existing buildings resulting in improved struc- tural response to earthquakes. However, passive systems are not always ecient in reducing response of random vibrations. Active systems have wider ranges of operation as shown in recent research outlines in state-of-the-art publications [1; 2]. Soong [3], and Agrawal et al. [4] indicate that new devices using external energy and algorithmic logic can produce more optimal reductions than passive systems. Recently, Kobori et al. [5] proposed an active variable stiness system consisting of stand-by braces * Correspondence to: Y. Ribakov; Structural Engineering Division; Department of Civil Engineering; Technion–Israel Institute of Technology; Technion City; Haifa 32000; Israel. † Professor and Member, ASCE. ‡ Professor and Fellow, ASCE. Received 7 February 2000 Copyright ? 2001 John Wiley & Sons, Ltd. Accepted 5 June 2000