Samrt Structures and Systems, Vol. 11, No. 5 (2013) 435-451 DOI: http://dx.doi.org/10.12989/sss.2013.11.5.435 435 Copyright © 2013 Techno-Press, Ltd. http://www.techno-press.org/?journal=sss&subpage=5 ISSN: 1738-1584 (Print), 1738-1991 (Online) Semi-active damped outriggers for seismic protection of high-rise buildings Chia-Ming Chang 1 , Zhihao Wang 2 , Billie F. Spencer, Jr. ∗3 and Zhengqing Chen 4 1 Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA 2 School of Civil Engineering and Communication, North China Institute of Water Conservancy and Hydroelectric Power, Zhengzhou, China, 450011 3 Department of Civil Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA 4 College of Civil Engineering, Hunan University, Changsha, China, 410082 (Received June 11, 2012, Revised November 25, 2012, Accepted November 30, 2012) Abstract. High-rise buildings are a common feature of urban cities around the world. These flexible structures frequently exhibit large vibration due to strong winds and earthquakes. Structural control has been employed as an effective means to mitigate excessive responses; however, structural control mechanisms that can be used in tall buildings are limited primarily to mass and liquid dampers. An attractive alternative can be found in outrigger damping systems, where the bending deformation of the building is transformed into shear deformation across dampers placed between the outrigger and the perimeter columns. The outrigger system provides additional damping that can reduce structural responses, such as the floor displacements and accelerations. This paper investigates the potential of using smart dampers, specifically magnetorheological (MR) fluid dampers, in the outrigger system. First, a high-rise building is modeled to portray the St. Francis Shangri-La Place in Philippines. The optimal performance of the outrigger damping system for mitigation of seismic responses in terms of damper size and location also is subsequently evaluated. The efficacy of the semi-active damped outrigger system is finally verified through numerical simulation. Keywords: semi-active damped outriggers; MR dampers; seismic control; high-rise buildings; LQG/clipped-optimal control 1. Introduction Buildings have continued to soar skyward with the development of new materials and construction technologies. However, such flexible structures may fall victim to excessive levels of vibration caused by strong winds and earthquakes. As a result, structural modifications ranging from alternative structural systems and aerodynamic changes to utilization of passive and active control devices have been suggested for protecting flexible structures against external disturbances (Kareem et al. 1999). ∗Corresponding author, Dr., E-mail: bfs@illinois.edu