State of the Art of Structural Control B. F. Spencer Jr. Nathan M. Newmark Professor, Dept. of Civil and Environmental Engi- neering, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801. E-mail: bfs@uiuc.edu S. Nagarajaiah Associate Professor, Dept. of Civil and Environmental Engineering, Rice Univ., Houston, TX 77005. E-mail: nagaraja@rice.edu In recent years, considerable attention has been paid to research and development of structural control devices, with particular em- phasis on alleviation of wind and seismic response of buildings and bridges. In both areas, serious efforts have been undertaken in the last two decades to develop the structural control concept into a workable technology. Full-scale implementation of active con- trol systems have been accomplished in several structures, mainly in Japan; however, cost effectiveness and reliability consider- ations have limited their wide spread acceptance. Because of their mechanical simplicity, low power requirements, and large, con- trollable force capacity, semiactive systems provide an attractive alternative to active and hybrid control systems for structural vi- bration reduction. In this paper we review the recent and rapid developments in semiactive structural control and its implemen- tation in full-scale structures. Introduction Supplemental passive, active, hybrid, and semiactive damping strategies offer attractive means to protect structures against natu- ral hazards. Passive supplemental damping strategies, including base isolation systems, viscoelastic dampers, and tuned mass dampers, are well understood and are widely accepted by the engineering community as a means for mitigating the effects of dynamic loading on structures. However, these passive-device methods are unable to adapt to structural changes and to varying usage patterns and loading conditions. For example, passively isolated structures in one region of Los Angeles that survived the 1994 Northridge earthquake Nagarajaiah and Sun 2000, may well have been damaged severely if they were located elsewhere in the region Makris 1997. For more than two decades, researchers have investigated the possibility of using active, hybrid, and semiactive control meth- ods to improve upon passive approaches to reduce structural re- sponses Soong 1990; Soong and Reinhorn 1993; Spencer and Sain 1997; Housner et al. 1997; Kobori et al. 1998, 2003; Soong and Spencer 2002; Spencer 2002. The first full-scale application of active control to a building was accomplished by the Kajima Corporation in 1989 Kobori et al. 1991. The Kyobashi Center building is an 11-story 33.1 m building in Tokyo, having a total floor area of 423 m 2 . A control system was installed, consisting of two AMDs—the primary AMD is used for transverse motion and has a mass of 4 t, while the secondary AMD has a mass of 1 t and is employed to reduce torsional motion. The role of the active system is to reduce building vibration under strong winds and moderate earthquake excitations and consequently to increase comfort of occupants of the building. Hybrid-control strategies have been investigated by many re- searchers to exploit their potential to increase the overall reliabil- ity and efficiency of the controlled structure Housner et al. 1994; Kareem et al. 1999; Nishitani and Inoue 2001; Yang and Dyke 2003; Casciati 2003; Faravelli and Spencer 2003. A hybrid- control system is typically defined as one that employs a combi- nation of passive and active devices. Because multiple control devices are operating, hybrid control systems can alleviate some of the restrictions and limitations that exist when each system is acting alone. Thus, higher levels of performance may be achiev- able. Additionally, the resulting hybrid control system can be more reliable than a fully active system, although it is also often somewhat more complicated. To date, there have been over 40 buildings and about 10 bridges during erection that have em- ployed feedback control strategies in full-scale implementations Tables 1 and 2. The vast majority of these have been hybrid control systems. Although extensive analytical and experimental structural con- trol research has been conducted in both the United States and Japan in the last two decades, with the exception of one experi- mental system installed on a bridge in Oklahoma Patten et al. 1999, discussed later in this paper, none of these full-scale active control installations are located in the United States. Many possible reasons can be cited for this disparity. For example, the civil engineering profession and construction industry in the United States are conservative and generally reluctant to apply new technologies. The absence of verified and consensus- approved analysis, design, and testing procedures represent addi- tional impediments to the application of this technology. How- ever, more notable is the lack of research and development expenditures by the U.S. construction industry. This situation stands in sharp contrast to the Japanese construction industry, which invests heavily in the development and implementation of new technologies. Even in Japan, few new structures with fully active control systems are being initiated. This situation is partly due to the modest number of tall buildings and long-span bridges being planned for the near future and partly due to a number of serious challenges that remain before active control can gain gen- eral acceptance by the engineering and construction professions at large. These challenges include 1 reducing capital cost and maintenance, 2 eliminating reliance on external power, 3 in- creasing system reliability and robustness, and 4 gaining accep- tance of nontraditional technology. Despite the impediments that exist to wider application of con- trol to civil engineering structures, the future appears quite bright. Semiactive control strategies are particularly promising in ad- dressing many of the challenges to this technology, offering the reliability of passive devices, yet maintaining the versatility and adaptability of fully active systems, without requiring the associ- ated large power sources and can operate on battery power. Stud- ies have shown that appropriately implemented semiactive damp- FORUM JOURNAL OF STRUCTURAL ENGINEERING © ASCE / JULY 2003 / 845