in 1994 and Kobe, Japan in 1995 demonstrated the importance of mitigating these hazards in the design of new structures. Also, should seismic code requirements change because of a destructive earthquake, or due to discovery of a nearby active fault as recently identified beneath downtown Los Angeles, reanalysis and upgrading of seismic resistance of structures will be required.
Design for strength alone does not necessarily ensure that the building will respond dynamically in such a way that the comfort and safety of the occupants is maintained. For ex- JOURNAL OF ENGINEERING MECHANICS / SEPTEMBER 1997/897 J. Eng. Mech. 1997.123:897-971. Downloaded from ascelibrary.org by Henan University of Technology on 03/05/13. Copyright ASCE. For personal use only; all rights reserved.
ample, during the 1989 Loma Prieta earthquake, a 47-story building in San Francisco experienced peak accelerations of about lO%g in the basement and 45%g on the top floor, which indicates that harmful accelerations in the upper stories can result from strong ground accelerations. Similar comments can be made regarding the behavior of structures during the recent Northridge and Kobe earthquakes. In fact, the requirements
for strength and for safety can be conflicting. Thus, alternate means of increasing the resistance of a structure while maintaining desirable dynamic properties, based on the use of various active, semiactive, passive, and hybrid control schemes, offers great promise.
The notion of structural control as currently defined can trace its roots back more than 100 years to John Milne, a professor of engineering in Japan, who built a small house of wood and placed it on ball bearings to demonstrate that a
structure could be isolated from earthquake shaking. The development of linear system theory and its application to the
field of vibration, and in particular structural dynamics, required much of the first half of the twentieth century. The
driving force for much of this development was the internal combustion engine, used in both automobiles and airplanes, which inherently produced significant dynamic force levels at connection points. It was during the second world war that
concepts such as vibration isolation, vibration absorption, and vibration damping were developed and effectively applied to aircraft structures.
The structural engineering community first embraced this technology in the 1960s, and since then has pursued a number of different paths; one example is base isolation for low-rise
and medium-rise structures and bridges. The objective is to
mount the structure on a sufficiently flexible base that filters out the high frequencies of the ground motion and lengthens the natural period of vibration to approximately 2 s. However, this would be unsatisfactory if the earthquake spectrum had a significant amount of energy in the neighborhood of a 2-s period. The alternative would be to soften the base isolation until the natural period was 3 or 4 s, but this would lead to large amplitude motions that would be objectionable. Also, in recent earthquakes a large velocity pulse has been recorded in the
near-fault region and this may also make ordinary base isolation impractical. Certain structures because of their shape,
for example slender high-rise buildings, may not be suitable
for base isolation. Isolators, whose purpose is to filter out the higher frequencies in the ground motion, have been used to
protect the fragile contents of hospitals, computer facilities, and so on. This technology had been readily accepted and
several dozen base-isolated structures, new or retrofitted, are currently in use or under construction in the United States. For flexible structures such as tall buildings, particularly those susceptible to strong winds, auxiliary dampers have been successfully employed. The damping devices, either viscous, viscoelastic, or plastic are deployed throughout the structure,
providing a significant increase in energy dissipation and reduction of motion. Buildings currently employing auxiliary
dampers include the World Trade Center in New York City and several buildings in Seattle and in California.
Another passive approach applied to taller buildings to reduce wind induced vibrations is the tuned mass damper
(TMD). This device is a classical dynamic vibration absorber, consisting of an auxiliary mass on the order of 1% of the mass of the total structure, located at the top of the building and connected through a passive spring and damper. The auxiliary system is tuned to reduce the amplitude of building motion. While this is a particularly effective strategy for stationary, narrow band motions, it is less so for broadband excitations such as earthquakes, where transient effects are dominant. However, the designer has several parameters, including mass 898/ JOURNAL OF ENGINEERING MECHANICS / SEPTEMBER 1997 ratio and absorber damping ratio, with which the bandwidth and attenuation capability of the device can be controlled. TMDs have been employed in the United States since the 1970's; examples can be found in the John Hancock Tower in Boston and in the Citicorp Building in New York City.
As pointed out in the foreword to the proceedings of the
First World Conference on Structural Control, \control has distinctive features that govern the direction of research\First, civil engineering structures are anchored and, thus,
are statically stable. The addition of purely active control carries with it the possibility of destabilization and is thus suspect. This is in contrast to space structures which, when deployed,
require active control for stability. Further, environmental disturbances that we associate with civil engineering structures,
for example wind and earthquakes, are highly uncertain with
respect to magnitude and arrival times, while the characteristics of mechanical loads are fairly well documented. Also, performance requirements that we associate with civil engineering
structures are generally coarse by comparison with those of,
say, aircraft and spacecraft. These are but a few of the differences. The buildings and bridges that have been constructed using structural control show that it is a valuable engineering tool and, in addition, possible combinations of methods and more sophisticated methods show promise of extended use. In fact,
structural control in civil engineering seems now to be developing into a special form of vibration problem involving large and massive bodies whose motions are to be controlled by modifying the vibrational properties in a variety of ways or by applying counterforces. At the present stage of development it is desirable to summarize the state of the art and to point the way to future developments. 1.2 Recent International Developments
The U.S. National Workshop on Structural Control, held at
the University of Southern California in 1990 under the auspices of the National Science Foundation (NSF) sponsored
U.S. National Panel on Structural Control, attracted nearly 100 participants, including several representatives from Canada, China, Germany, Italy, Japan, Mexico, and Spain. This was followed by several additional meetings, including the Japan National Workshop, the U.S.-Italy Workshop in 1992, and an international workshop held in Hawaii in 1993. Perhaps the
most significant event took place at the Tenth World Conference on Earthquake Engineering in Madrid, Spain in 1992,
which saw several technical sessions dedicated to topics in structural control. Here, the decision to form an international association and to hold a world conference dedicated to structural control was made. The IASC was formed the next year,
with Professor G. Housner as president, Professor T. Kobori
as vice president, and Professor S. Masri as secretary-treasurer. Their efforts led to the successful First World Conference, where 337 participants from 15 countries met to present and
discuss the results of their research. Additional activity in the field also took place at various international meetings, several of which held special symposia and theme sessions devoted to structural control. These included both the 10th and 11 th World Conferences on Earthquake Engineering, held in Madrid and in Acapulco, respectively. Most recently, the
First European Conference on Structural Control was successfully held in Barcelona, Spain, attracting more than 100 international participants. The Second International Workshop
on Structural Control, which was held in Hong Kong in December of 1996, attracted the participation of 85 scientists and engineers from 10 countries. The forthcoming Second World Conference, to be held in Tokyo in 1998, promises to J. Eng. Mech. 1997.123:897-971. Downloaded from ascelibrary.org by Henan University of Technology on 03/05/13. Copyright ASCE. For personal use only; all rights reserved.
extend the frontiers of this exciting new area of civil engineering. 1.3 Scope
The motivation behind this tutoriaVsurvey paper is twofold. First, it is meant to provide a concise point of departure for researchers and practitioners alike wishing to assess the current state of the art in the control of civil engineering structures. Second, and perhaps more important, it provides a link between structural control and other fields of control theory,
pointing out both differences and similarities and where future research and application efforts are likely to prove fruitful. The paper is organized in the following way: section 2 deals with passive energy dissipation; section 3 deals with active control; section 4 deals with hybrid and semiactive control
systems; section 5 discusses sensors for structural control; section 6 deals with smart material systems; section 7 deals with health monitoring and damage detection; and section 8 deals
with research needs. An extensive list of references is provided in the references section.
Given the very broad and interdisciplinary nature of the field
of structural control and monitoring of civil infrastructure systems, it is not feasible to discuss or cite all relevant publications and applications. The writers have done their best to
present a balanced view of the developments in the field of structural control and monitoring, however, only a limited number of references could be cited. Consequently, absence of
a citation of a particular work should not be construed as implying anything about the publication's merit. Where appropriate,
publications in technical journals were preferred for inclusion over related publications in proceedings. Also, when
discussing control theory, emphasis was placed on those issues related to the physical behavior of civil structures as opposed to sophisticated developments in control system theory. 1.4 Definitions
For convenience, definitions of some key terms will be provided.
1.4.1 Active Control An active control system is one in which an external source
powers control actuator(s) that apply forces to the structure in a prescribed manner. These forces can be used to both add and dissipate energy in the structure. In an active feedback control system, the signals sent to the control actuators are a function of the response of the system measured with physical sensors (optical, mechanical, electrical, chemical, and so on).
1.4.2 Passive Control A passive control system does not require an external power
source. Passive control devices impart forces that are developed in response to the motion of the structure. The energy in
a passively controlled structural system, including the passive devices, cannot be increased by the passive control devices.
1.4.3 Hybrid Control The common usage of the term \
the combined use of active and passive control systems. For example, a structure equipped with distributed viscoelastic damping supplemented with an active mass damper on or near
the top of the structure, or a base isolated structure with actuators actively controlled to enhance performance.
1.4.4 Semiactive Control Semiactive control systems are a class of active control systems for which the external energy requirements are orders of
magnitude smaller than typical active control systems. Typically, semiactive control devices do not add mechanical energy
to the structural system (including the structure and the control actuators), therefore bounded-input bounded-output stability is guaranteed. Semiactive control devices are often viewed as controllable passive devices.
1.4.5 Structural Health Monitoring Health monitoring refers to the use of in-situ, nondestructive sensing and analysis of structural characteristics, including the structural response, for the purpose of detecting changes that may indicate damage or degradation.