journal of vibration and control 2015 saaed 919 37

Article

A state-of-the-art review of structuralcontrol systems

Tarek Edrees Saaed1, George Nikolakopoulos2,Jan-Erik Jonasson1 and Hans Hedlund3

Abstract

The utilization of structural control systems for alleviating the responses of civil engineering structures, under the effects

of different kinds of dynamics loadings, has become a standard technology, although there are still numerous research

approaches for advancing the effectiveness of these methodologies. The aim of this article is to review the state-of-the-

art technologies in structural control systems by introducing a general literature review for all types of vibrations control

systems that have appeared up to now. These systems can be classified into four main groups: (a) passive; (b) semi-active;

(c) active; and (d) hybrid systems, based on their operational mechanisms. A brief description of each of these main

groups and their subgroups, with their corresponding advantages and disadvantages, is also given. This article will

conclude by providing an overview of some innovative practical implementations of devices that are able to demonstrate

the potential and future direction of structural control systems in civil engineering.

Keywords

Seismic protection, structural control system, vibration mitigation

1. Introduction

The common method for designing buildings and mostcivil engineering structures is to use the static approach,which is based on the design to resist gravitationalloads throughout structure lifetime. The determinationof these loads is very straight forward based on occu-pancy requirements, and can signicantly simplify thedesign process.

Structural engineers tend to deal with lateral forceslike seismic loads and wind loads in a similar mannerby using equivalent static loads, which are allowed bymany design codes (De la Cruz, 2003). A seismic designrelies on a combination of strength and ductility in away that the structures are expected (or designed) toremain in an elastic range for normal earthquakesoccurrence. In the case of large earthquakes, the struc-tural design should depend on the ductility of the struc-tures to prevent building damage. For this reason, thelateral resisting force system should have the ability toabsorb and dissipate energy in a stable manner throughplastic hinge regions of beams and column bases for alarge number of cycles. These plastic hinges are part of

irreparable, concentrated, and accepted damage togravity load carrying systems, providing that the col-lapse is prevented and life safety is conrmed. Suchstructures depend almost upon their specic stinessto withstand seismic forces and on their limited mater-ial damping to dissipate dynamic energy resulting fromthese random and variants dynamics loads. A dramaticimprovement could be achieved by taking into accountthe dynamical behavior of structures to overcome thexed capacities of load resistance and energy

1Department of Structural Engineering, Lulea University of Technology,

Lulea, Sweden2Department of Computer Science, Electrical and Space Engineering,

Lulea University of Technology, Lulea, Sweden3Skanska Sverige AB Technology, Bridge and Civil Engineering, Goteborg,

Sweden

Corresponding author:

Tarek Edrees Saaed, Department of Structural Engineering, Lulea

University of Technology, SE 97187 Lulea, Sweden.

Email: [email protected]

Received: 31 August 2012; accepted: 8 January 2013

Journal of Vibration and Control

2015, Vol 21(5) 919937

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DOI: 10.1177/1077546313478294

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dissipation, which exist in the classical code designmethods (Constantinou et al., 1998). In addition,recent buildings and important structures also shouldhave the ability to maintain their functions after severeearthquakes, not just preventing collapse (Kobori,1988), and may contain expensive equipment, such aselectronic and industrial machineries, that are very sen-sitive to motion (Fisco and Adeli, 2011). Moreover,there are a large number of old buildings that are notdetailed properly as ductile structures and have a lim-ited lateral resistance. In order to enable the structuresto resist such loadings, an increase in both structuralstrength and ductility is essential, while at the sametime, this approach will increase the overall construc-tion cost. In addition, increasing member sizes willattract more demanding forces on them, and theremay not be any benets from such solutions. In add-ition, it is very dicult to change the damping ratio forconstruction materials, such as reinforced concrete orsteel. Due to all of these limitations, researchers haveused natural and developed man-made materials withunusual properties, called smart materials, and systemsthat can be automatically adjusted to dierent kinds ofexcitations, called adaptive systems; later on, this led tothe innovative concept of smart structures (Chenget al., 2008).

Smart structure systems or structural control sys-tems for civil engineering structures are a suitable solu-tion to overcome these limitations and to provide saferand more ecient designs, through reecting andabsorbing the energy produced by dierent dynamicloadings such as seismic, wind and trac eects. Inthis case, protection is achieved by allowing the struc-tures to be damaged (Christenson, 2001).

The aim of this article is to review the state-of-the-art technologies in structural control systems by intro-ducing a general literature review for all types of vibra-tions control systems that have been appeared up tonow and present the current state of the art for eachof them, while providing an overview of some innova-tive practical implementations, which are able to dem-onstrate the potential and future direction of structuralcontrol systems in civil engineering. A limited numberof similar surveys have appeared in the literature thathave focused on specic categories of structural controlsystems, while there has never been a full overview ofall the existing technologies and a correspondingcomparison.

The overall aim of this article is to provide a detaileddescription and relative applications of the mostimportant technological advancements in the eld,while acting as a full reference for interested readers.A survey of applications of passive energy dissipationsystems for seismic protection of structures and theirbasic principles up to 2005 was presented by Symans

et al. (2008). A very good survey of structural controlsystems that covers the period from 1990 to 1996 waspresented by Housner et al. (1997), where valuableinformation was provided on passive, semi-active,hybrid, active control theories, sensors, and smartmaterials; the present article, the concentration will beon control systems devices. Therefore, the descriptionof passive devices will be extended to include base iso-lation devices, other types of energy dissipation devices,and active control devices that have appeared up tonow. The development in semi-active devices andtheir implementation in full-scale structures for theperiod 1996 to 2003 were covered by Spencer andNagarajaiah (2003). Ikeda (2009) presented anextended list of 52 practical applications of active con-trol systems to buildings in Japan from 1989 to 2007.Starting from this article, the list of applications will beextended to include recent applications to date.

The present article is structured as follows. InSection 2, a detailed description of structural controlbased on the operational mechanism is provided,this includes: passive, semi-active, active, and hybridcontrol approaches, while some recent and char-acteristic applications of these devices are presentedin Section 3. Finally, in Section 4, conclusions aredrawn.

2. Structural control systems

Structural control systems can be utilized to reduce theresponse of structures under dierent types of dynamicloads such as earthquakes, winds, trac, and otherkinds of service loads, thus, these systems have alsobeen characterized in the literature as motion controlsystems. In general, these devices can be classied intofour main groups (passive, active, semi-active, andhybrid) based on their operational mechanisms, as pre-sented in Figure 1 (Cheng et al., 2008; Christenson,2001; Constantinou et al., 1998; De la Cruz, 2003;Housner et al., 1997; Marko, 2006; Spencer andNagarajaiah, 2003; Symans et al., 2008). In the follow-ing subsections, the main groups and subgroups ofstructural control systems are presented.

2.1. Passive control systems

Passive control systems aim to dissipate part of theinput energy and include mainly isolation and energydissipation devices. In the past, these systems have beenconsidered as smart systems because they can generatea larger damping force when the structural responsegets higher (Cheng et al., 2008). Constantinou et al.(1998) presented a comparison among the perfor-mance-based categories of passive energy dissipationsystems, while Symans et al. (2008) presented a

920 Journal of Vibration and Control 21(5)


Figure 1. Structural control system categorization.

Saaed et al. 921


summary of construction, hysteretic behavior, physicalmodels, advantages, and disadvantages of passiveenergy dissipation devices for seismic protection appli-cations. In general smart structures, which utilize pas-sive systems, can be considered as systems with alimited intelligence because these structures areunable to adapt to the excitation and global structuralresponse, and thus are characterized by a limited con-trol capacity. They are optimally tuned to protect thestructures from a specied dynamic loading, but theireciency will not be the optimal one for other casesand other types of dynamic loadings. The energy dissi-pation mechanism is totally dependent on the relativestructure movement, and it is related only to the localstructure response (Cheng et al., 2008). However, pas-sive control devices are inherently stable, do not requireany external energy to operate or structural responsemeasurements and are simpler to design and construct(Christenson, 2001). A classication of these devicesfollows.

2.1.1. Seismic isolation devices. Seismic isolation devicesbase their operation on the principle of introducing alayer that is exible in the horizontal direction and verysti in the vertical direction for increasing the horizon-tal exibility or increasing the rocking stability andthus, part of the input energy will be absorbed by theisolation system before the dissipation of energy. Dueto the horizontal exibility, seismic isolation deviceswill introduce a new vibration mode that does not con-tain signicant inter-story drifts to the main structure.This capability will lengthen the fundamental periods ofthe structures and will keep them away from the mainperiod contents of the excitation (De la Cruz, 2003).Seismic isolation devices are suitable for short tomedium height buildings, with dominant vibrationmodes lying within a specied range. For instance, itis impossible to provide the horizontal exibilityrequired for structures under seismic excitations thathave long period components (Marko, 2006). Seismicisolation is ecient against vibrations transmittedthrough the ground, such as trac and seismic vibra-tions, but it is not ecient to resist wind loading due tothe exibility in the horizontal direction (De la Cruz,2003). Isolation devices can be implemented at dierentlocations within structures.

There are many common types of isolation systemswhose mechanisms and structural behavior are wellknown (Marsico, 2008): elastomeric-based systems,low-damping natural and synthetic rubber bearings(LDRBs), lead-plug bearings (LRBs), high-dampingnatural rubber (HDNR) systems, isolation systemsbased on sliding, Teon Articulated Stainless Steel(TASS) systems, friction pendulum systems (FPSs),and sleeved-pile isolation systems (SPISs).

The elastomeric-based systems consist of large nat-ural rubber blocks without steel reinforcement, whichwere introduced in 1969. Later, steel plates were addedto improve behavior and increase vertical stiness, andmany buildings were built with these devices. LDRBscontain two thick steel plates at the end and many steelshims. These bearings have been utilized in Japan,together with supplementary damping devices. LRBsare the same as LDRBs, but with a preformed hole(or holes). A lead plug is press-tted into that hole,which will deform under horizontal movement in apure shear, yields at low level stress and dissipates theenergy in a hysteretic manner, which is almost stableover a number of cycles (De la Cruz, 2003). This systemwas used successfully to protect many buildings duringearthquakes. In 1982, HDNR systems were introducedusing extra-ne carbon black, oils or resins and otherproprietary llers in the UK, which led to an increase ofthe inherent damping of the natural rubber compound;this, in turn, eliminated the need for supplementarydamping devices.

The isolation systems based on sliding owe theiroperation to the utilization of materials such as poly-tetrauoroethylene (PTFE or Teon) on stainless steel.The eciency of these devices is inuenced by factorssuch as temperature, interface motion velocity, degreeof wear, and cleanliness of the surface. Theoretical ana-lysis has been carried out on structures equipped withthese devices (Cheng et al., 2008).

TASISEI Corp. in Japan developed the TASSsystem. In this system, the entire vertical load is carriedusing Teonstainless steel elements and re-centeringforces provided by laminated neoprene bearings carry-ing no load. The FPS combines a sliding action and arestoring force by geometry. It consists of an articu-lated slider on a spherical surface that is covered witha polished stainless steel overlay. Due to the movementof the slider over the spherical surface, the mass will riseand provide the restoring force for the system. Finally,the SPIS is a solution available for reducing vibrationtransmission by piles through providing horizontalexibility. In this system, the pile is enclosed in a tubewith a suitable gap for clearance.

2.1.2. Energy dissipation devices. The main role of theserelatively small elements, which are located between themain structure and the bracing system, is to absorb ordivert part of the input energy. This will help to reducethe energy dissipation demand in the main structure.These devices can be classied as follows.

2.1.2.1. Hysteretic devices. As is clear from the name,these devices dissipate the energy by a mechanism thatis independent of loading rate and which can be dividedinto two groups: metallic dampers, which utilize the



yielding of metals to dissipate the energy, and frictiondampers, which generate heat by dry sliding friction(Constantinou et al., 1998).

The very eective metallic damper mechanism wasrst used by Kelly et al. (1972) and made good progressin the following years. These dampers are based oninelastic deformations of metallic substances such asmild steel or lead to dissipate the energy. Two import-ant factors must be addressed carefully in designing thistype of damper. First, is the post-yielding deformationrange in order to ensure that the damper will have asucient loading cycle without premature fatigue?Second, are the dampers stably hysteretic underrepeated inelastic deformation? In addition, thesedevices have long-term reliability, relative insensitivityto temperature changes, stable properties in the longterm, and are inexpensive. The disadvantages of thesedevices are the limited number of working cycles andtheir non-linear response (Marko, 2006). X-shaped andtriangular plate dampers have received signicantattention. The installation of these parallel plate deviceswithin a frame bay between a chevron brace and theoverlaying beam will make the dampers mainly resistthe horizontal forces associated with inter-story driftvia exural deformation of the individual plates.Supplemental energy dissipation will result due to theplates yielding after a certain level of force. The taperedshape of the plate ensures uniform yielding throughoutthe length of the device (Constantinou et al., 1998).Another type of device is the yielding steel bracingsystem, which was developed in New Zealand in 1980,and which has undergone many modications in Italy.This device is fabricated from round steel bars forcross-braced structures. The energy will dissipatethrough the inelastic deformation of the rectangularsteel frame in the diagonal direction of the tensionbrace (Marko, 2006). The nal type of metallicdamper is the lead extrusion damper (LED), whichwas suggested for the rst time in New Zealand byRobinson in 1987 by introducing the two types. Theconcept of these dampers is to extrude the lead by for-cing a lead piston through a hole or an orice (createdeither by constriction in the tube wall for the rst typeor by bulge on the central shaft for the second type),thereby changing its shape. The LED has a long lifeand does not need to be replaced or repaired afterearthquake excitation due to the leads capacity torestore its original shape after excitation. In addition,LEDs are insensitive to environmental and aging eects(Marko, 2006).

In friction dampers, the dissipation of energy is pro-vided by the friction between two solid bodies sliding toone another (solid sliding friction mechanism). The aimof these devices is to slow down building motions bybracing rather than breaking (Constantinou et al.,

1998). Friction dampers have good performance char-acteristics and can dissipate a large amount of energy.They are less aected by load frequency, number ofload cycles, or changes in temperature, and exhibitrigid plastic behavior (Marko, 2006). It is very import-ant to ensure that the estimated friction response of thedamper will be maintained during the life cycle of thedamper. This response is inuenced by surface condi-tions to a large extent, which are then aected by envir-onmental eects (Constantinou et al., 1998). Sometypes of friction dampers include (Marko, 2006): (a)the X-braced damper; (b) the bracing damper system;(c) the improved Pall friction damper; (d) the uniaxialfriction damper; (e) the energy dissipating restraint(EDR); (f) the slotted bolted damper; and (g) the con-centrically braced frame.

The X-braced damper was proposed by Pall in 1982.The energy dissipation will be equal in both tension andcompression braces due to the presence of the fourlinks. This will occur only if the slippage of the deviceis sucient to completely straighten any buckledbraces. In the bracing damper system, a more detailedmodel for the Pall friction damper was proposed totake into consideration the individual axial and bend-ing characteristics for each member of the bracingdamper system. From experimental results, it hasbeen found that minor fabrication details can signi-cantly aect the overall performance of the frictiondampers. The Pall friction damper was developed andimproved in 2005 by a more detailed construction,which simplied the manufacture and assembly processfor the damper, while keeping its mechanical propertiesunchanged at the same time. The uniaxial frictiondamper developed by Sumitomo Metal Industries Ltdis based on a more sophisticated design. It depends onthe transmission of the force generated by the pre-com-pressed internal spring through the action of inner andouter wedges into a normal force on the copper alloyfriction pads. These friction pads provide dry lubrica-tion through graphite plug inserts. Experimental andnumerical tests carried out by Aiken and Kelly (1990)show that this damper is able to dissipate about 60% ofthe input energy and has regular and repeatable rect-angular hysteresis loops. In addition, this damper isunaected by loading frequency and amplitude,number of cycles, and ambient temperature. Moreover,the base shearwill not be signicantly aected by damperplacement. The device will not dissipate energy for forcessmaller than a threshold. The EDR was introduced byFluor Daniel. Although the design concept is similar tothe Sumitomodamper because it also consists of an inter-nal spring and wedges encased in a steel cylinder, but ithas very dierent response characteristics. Steel andbronze friction wedges are used to convert the axialspring force into normal pressure acting outward on

Saaed et al. 923


the cylinder wall. In this case, internal stops are usedwithin the cylinder to bind the tension and compressiongaps. For slotted bolted damper, there aremany dierentvariations basedmainly on the sliding interface material.Fitzgerald in 1989 (Marko, 2006) proposed a damperutilizing slotted bolted connections as shown in Figure2(a). The connection is composed of a gusset plate, twoback to back channels, cover plates and bolts withwashers, while the steel is utilized as a sliding interface.In 1991, Constantinou used graphite impregnatedbronze plate to improve sliding interface frictioncharacteristic.

A rotating slotted bolted friction damper withinclined slotted holes is another type of friction dam-pers that was proposed and tested successfully in 1999.Finally, the concentrically braced frame is consideredone of the most ecient lateral load resisting systemavailable due to its strength, stiness, low weight, andsimple construction. However, applying light tensiononly during an earthquake may lead to hazardoussoft-story mechanisms due to irrecoverable tensileyielding, which can be treated using specially detailedsliding plates moving in the vertical plane (Grigorianand Yang, 1993).

2.1.2.2. Viscoelastic (VE) devices. These devices includea wide range of mechanisms that dissipate energy in arate-dependent manner, i.e. their displacement charac-teristics depend on the frequency of the motion andrelative velocity between the ends of the damper. Thedamping force in these devices is proportional to vel-ocity, and the behavior is viscous. Research and devel-opment of VE devices for earthquake engineeringstarted in the early 1900s, and they are mostly used instructures where it is expected to have shear deform-ations and can be classied into two main groups asfollows.

VE solid dampers: solid VE dampers (Figure 2(b))are usually made from copolymers or glassy substancesthat dissipate energy through shear deformation in theVE material. During deformation, the VE material willundergo features of both elastic solid and viscousliquid, and it will return to its original shape aftereach deformation cycle with heat generated as a resultof energy dissipation. There are many types of VE dam-pers, for instance, the test results for bitumen rubbercompound VE damper developed by Showa andShimizu Corporations showed a 50% reduction in theseismic response, while a reduction in response of up to60% was obtained for a half-scale three-story building(super-plastic silicone rubber VE shear damper) devel-oped by Kumagai-Gumi Corporation. Recent experi-mental and numerical studies conducted by Xu (2007)conrmed the eciency of solid VE dampers in redu-cing the seismic responses of structures.

VE uid dampers: all passive devices mentioned upto this point have utilized solids to enhance structureeciency against lateral loads. Fluid can also be uti-lized to obtain enhancement in structural eciency.Signicant eorts have been directed in recent yearstowards the development of viscous uid dampersand converting the technology from military andheavy industry to structural engineering. Examples ofthese dampers include (Constantinou et al., 1998): (a)the cylindrical pot uid damper; (b) the viscous damp-ing wall system (VDW); and (c) the uid viscousdamper.

The cylindrical pot uid damper is considered to beone of the simplest types of viscous uid dampers(Figure 2(c)) that convert mechanical energy into heatduring the dissipation process. The dissipation ofenergy occurs due to the piston motion, which willdeform thick, highly viscous substances such as siliconegel. This device was used as a component in seismicisolation systems (Makris and Constantinou, 1990).Sumitomo Construction Company in Japan developedthe VDW. The VDW is composed of two main parts(Figure 2(d)). The rst one is an outer race steel casingattached to the lower oor, lled with a highly viscousuid. The second part is an inner moving steel platehanging from the upper oor and contained withinthe rst part. It is also required to protect the systemfrom re or impacts by using cover walls of reinforcedconcrete or reproof materials. Relative inter-storymotion (velocity) shears the uid, and this will workto dissipate energy. Experimental tests conducted byArima et al. (1988) on a full-scale four-story steelframe tted with a viscous wall system showed responsereductions of 6680%. Another example of this systemis the 78 m high steel frame building in Shizuoka City inJapan, which reduced the building response by up to7080% (Marko, 2006). The cylindrical pot uiddamper and VDW system mentioned above dependon deformation of a viscous uid contained in anopened container. The disadvantage of these devicesis that to obtain maximum energy dissipation, it isnecessary to use materials with large viscosities. Thiswill lead to using materials that exhibit both frequency-and temperature-dependent behavior. Fluid viscousdampers are another class of VE uid dampers thatrely upon the ow of uids within a closed containerinstead of an open one. The piston motion will force theuid to pass through small orices instead of deformingthe uid locally, resulting in high-energy dissipationlevels. These dampers are not suitable for sti struc-tures due to high damper force demand. Althoughthey have been invented for military purposes, now-adays they are utilized in seismic base isolation systems,for supplemental damping during seismic waves, andwind-induced vibrations (Constantinou et al., 1998).



Figure 2. Different types of passive devices (Marko, 2006): (a) three types of slotted bolted dampers, (b) typical solid viscoelastic

damper, (c) cylindrical pot fluid damper, (d) viscous damping wall system, (e) orificed fluid damper, (f) pressurized fluid damper, (g)

models of the Single Degree of Freedom (SDOF) structure and tuned mass damper, (h) Crystal Tower Building in Chicago with a tuned

liquid damper, (i) Crystal Tower Building in Chicago with a tuned liquid column damper, (j) tuned liquid column damper within shear

wall Sumitomo Construction Company.

Saaed et al. 925


A typical orice uid damper is shown in Figure 2(e),which is composed of a cylindrical device that containscompressible silicone oil forced to ow by a stainlesssteel piston rod with a bronze head and with a uidiccontrol orice design. For the change in volume, due tothe rod positioning, an accumulator is provided(Constantinou and Symans, 1993).The main advantageof the uid viscous damper is the capability to reduceboth the deection and stress at the same time, since thedamper force is totally out of phase with the stressesresulting from the structure exing. Moreover, thesedampers are relatively insensitive to temperaturechanges. One of the main disadvantages of these devicesis that it is dicult to reduce the peak structural responsein the early stages of loading, mainly due to the depend-ence of the dampers resisting force on the velocity; thus,the utilization of a combination of tapered-plate energyabsorbers and uid dampers was suggested to overcomethis shortcoming (Pong et al., 1994).

2.1.2.3. Re-centering devices. These devices possess aninherent re-centering capability due to a little residualdeformation remaining after load removal(Constantinou et al., 1998). Examples of these devicesare: (a) pressurized uid dampers and (b) preloadedspring friction dampers.

The pressurized uid dampers shown in Figure 2(f)were proposed by Tsopelas and Constantinou (1994) toprovide both damping and re-centering capabilities fora base isolation system. Many factors play an import-ant role in determining load resistance: initial pressur-ization, device stiness due to silicone oilcompressibility, seal friction, and damping due to thepassage of uid through orices. The preloaded springfriction dampers are similar to pressurized uid dam-pers, but the dampers resisting force is due to frictionalwedges in addition to a preloaded internal spring(Constantinou et al., 1998).

2.1.2.4. Phase transformation dampers. A new materialtype called a shape memory alloy (SMA) is used inpassive dampers. These metals have the ability tochange status between martensitic and austenitic crys-talline phases responding to reversible stress or tem-perature. For example, an SMA specimen will distortin an apparent manner under its low-temperature mar-tensitic phase. If the temperature rises to a certain level,this will induce transformation to the austenitic phase,and the specimen will return to its original undistortedshape (Constantinou et al., 1998). There are manyadvantages of this type of material: it provides a self-centering mechanism, it is insensitive to environmentaltemperature changes (after being properly heat trea-ted), it has excellent fatigue resistance, it is corrosionresistance, and nally it is capable of producing large

control forces, even for slow response times. Until now,there have been no practical applications for this typeof material, except for research and experimental inves-tigations. Some investigations recommended a carefuldesign for this type of damper due to the high sensitiv-ity of SMAs to earthquake excitations, which maychange the stiness and consequently alter the rst nat-ural frequency of the structure toward the earthquakedominant frequency (Marko, 2006).

2.1.2.5. Dynamic vibration absorbers. The last class ofpassive systems is the dynamic vibration absorber,which is also used to reduce energy dissipationdemand on the primary structural members underdynamic loadings. The reduction is achieved by trans-ferring (rather than directly dissipating) some of thevibrational energy to the absorber. This system includesa mass, stiness, and damping. The dynamic propertiesshould be tuned to those of the primary structure. Theprimary applications of this system are for mitigationof wind loads. There are limitations to using thesedevices for seismic applications due to detuning thatmay occur as the primary structure yields, high damp-ing level demand and an incapability to control highermodes in an ecient manner. The three main commontypes of these devices are: (a) tuned mass dampers(TMDs), (b) tuned liquid dampers (TLDs); and (c)tuned liquid column dampers (TLCDs) (Constantinouet al., 1998).

The TMD contains a mass that moves relative to thestructure, and it is attached to the structure by a springand a viscous damper in parallel, as presented inFigure 2(g). The TMD will be excited by the structuralvibrations, and the kinetic energy generated due tothese structural vibrations will be transferred from thestructure to the TMD and absorbed by the dampingcomponent of the TMD. The TMD usually experienceslarge displacements. TMDs have been successfullyapplied in mitigation of wind and harmonic loads,and have been installed in a number of buildings,while there is not a general agreement about the e-ciency of TMDs for seismic applications. The TLD hasa similar mechanism to the TMD. Improvements ofstructural behavior are achieved by applying indirectdamping to the structure. The dissipation of energywill happen due to the viscous action of uid andwave breaking. The TLDs presented in Figure 2(h) con-sist of rigid tanks lled with liquid. The sloshing motionwill absorb the energy and dissipate it through the vis-cous action of the liquid. Use of TLDs has manyadvantages compared with the use of TMDs: themotion is reduced in two directions at the same time,large stroke lengths are not required, there is no needfor any activation mechanism, and there are lowermaintenance costs. TLDs are insensitive to the



frequency ratio between the primary and secondary sys-tems (Marko, 2006). In addition, the TLCD has a simi-lar mechanism to the TMD, and the improvements ofstructural behavior will be achieved by applying indir-ect damping to the structures. The TLCD consists of aliquid-lled tube-like container that is rigidly attachedto the structure (Figure 2(i)). The dissipation of energywill happen due to the passage of liquid through anorice in the tube with inherent head loss characteris-tics. It is possible to tune the vibration frequency of thedevice by changing the liquid column length. Majoradvantages of this system are the simplicity of imple-mentation in existing buildings without any conictwith vertical and horizontal load paths (as shown inFigure 2(j)), there is no space requirement for largestroke lengths, and the damping can be controlled byadjusting the orice opening. Research has shown thatreduction in the displacement and accelerationresponses could be up to 47% (Marko, 2006). De laCruz (2003) made a comparison between the eciencyfor each of the isolation systems, energy dissipaters,and mass dampers. Tait (2008) proposed a preliminarydesign procedure for initial TLD sizing and an initialdamping screen design for a TLD equipped with damp-ing screens. Love and Tait (2012) outlined a designmethodology for designing a TLD tank under spacerestrictions conditions.

2.1.2.6. Other energy dissipators. According toHousner et al. (1997), there are many newly developedinnovations that can be classied as passive energy dis-sipation devices in the following methods. (a) Usingdampers to connect two adjacent structures at theroofs. This method was used to mitigate the vibrationin high-rise buildings during strong earthquakes byoptimizing the mass and stiness ratio between thestructures. (b) High-damping rubber damper. Themain part of this device is unvulcanized rubber, whichhas low stiness and high energy absorption abilitycompared with the vulcanized high-damping rubbermaterial used for rubber bearings. (c) Rubber compos-ite damper used for cable-stayed bridges. (d) V-stripe orU-stripe congurations of bridge cables. Moreover,Korkmaz et al. (2011) showed that using a dampingtrench at a certain distance from a building is eectiveto mitigate the trac-induced vibrations on structuralbehavior of masonry buildings. The current state ofpassive control systems and their characteristics aresummarized in Table 1.

2.2. Semi-active control

Semi-active control devices are a natural extension ofpassive devices. They are commonly called controllableor intelligent dampers because they include adaptive

systems to increase intelligence and eciency. In orderto improve the performance, their adaptive system regu-lates the damper behavior based on the collected infor-mation of excitation and structural response. Thecomponents of this system include: sensors (measurethe input and/or output), a control computer (processesthe measurement and generate a control signal to theactuator), a control actuator (acts to regulate the behav-ior of the passive device), and a passive damping device.The actuator in semi-active devices is used to control theproperties of passive devices instead of directly applyinga force to the structure. Therefore, this kind of devicesrequires a small power supply, such as batteries, which isa very important merit as the main power supply mightfail during an earthquake and result in structuredestabilization.

In spite of the complexity of these devices, whencompared with the passive devices, they are still easyto manufacture, fail-safe, reliable to operate and cap-able of acting better than passive ones. The disadvan-tage of these devices is the limited control capacitybecause they are still working within the capacity ofcorresponding passive devices. Overall, these devicesare very promising since they combine the positivemerits of both passive and active devices. Examples ofsemi-active devices are given in the following para-graphs (Cheng et al., 2008).

2.2.1. Semi-active TMDs. These were developed in 1983to control wind-induced vibrations in tall buildings andare still in the research stage. The damper is composedof a TMD and an actuator installed on top of the mainstructure. The characteristics of the TMD are: mass(md), damping (cd), and stiness (kd), while the mainstructure characteristics are: mass (m), damping (c),and stiness (k). SA represents the actuator and u isthe control force generated by the actuator. TheTMD damping will be continuously adjusted by theactuator control force u, while a small amount of exter-nal power is required to make this adjustment, as themass of the TMD (md) is much smaller than the mainstructure mass (m) and the active control force is uti-lized to change the damping force of the TMD, which isless than the inertial force of the TMD (Cheng et al.,2008).

2.2.2. Semi-active TLDs. The semi-active concept wasdeveloped for both the TLDs and TLCDs mentionedpreviously and it is still in the research and develop-ment stage. For TLDs, a set of rotatable baes wereadded in the liquid tank of a sloshing TLD. The orien-tation of these baes is adjusted by an actuator accord-ing to application-specic algorithms. Since the naturalfrequency of the contained liquid changes with tanklength, tuning of the TLD will be controlled by rotating

Saaed et al. 927


the baes to a desired inclined position. When the baf-es are in the horizontal position, the tank maintains itsfull length, while the tank will be divided into a numberof shorter tanks if the baes are in the vertical position.In this manner, the actuator is required only for rotat-ing the light-weight baes (Cheng et al., 2008).

For TLCDs, Yalla (2001) used a variable orice tomaintain the optimal damping conditions. Accordingto the proposed control algorithm, an electropneumaticactuator is used to control a ball valve in order tochange the cross section of the TLCD. This willimprove the damping properties of the damper with arelatively low cost.

2.2.3. Semi-active friction dampers. The rst form of thisdevice was developed by (Constantinou et al., 1998)using an electromechanical actuator. Another typewas developed by Chen and Chen (2004) usingPiezoelectric and Translators (PZT) actuators in orderto improve the eciency of passive friction dampers.PZT is a smart material that is able to produce a sig-nicant amount of stress when exposed to an electricaleld under restrained motion. This device is called a

piezoelectric friction damper (PFD). It consists offour preloading units, four PZT stack actuators (toapply normal forces), a friction component (containinga thin steel plate with brake lining as a friction materialbonded to its top and bottom surfaces) and a steel boxfor housing other components. The friction generateddue to the relative movement of the isolation plate andthe bottom plate will dissipate energy. The normalforces generated by the PZT stack actuator will be con-trolled by adjusting the electric eld on the PZT actu-ators and thus the friction force is regulated to enhancereal-time eciency with rather a low cost (Cheng et al.,2008). This device can be adapted to varying excita-tions, caused by weak and strong earthquakes, whilefurther improvements in force generation are needed(Chen and Chen, 2004).

2.2.4. Semi-active vibration absorbers. These types ofdevices are still under research, and their operatingmechanism depend on adjusting both the stiness anddamping properties. Examples of this system are semi-active vibration absorbers (SAVAs), which are alsocalled semi-active hydraulic dampers (SAHDs).

Table 1. State-of-the-art summary of passive control systems.

Control system Key features Applications

Seismic isolation -LDRB, LRB, HDNR, TASS system, FPS, SPIS Many buildings built with these

devices

devices -Safer and more economic than traditional structural systems

Hysteretic devices -Metallic dampers, friction dampers Mostly used in structures

-Energy dissipation independent of loading rate

-Long-term reliability

-Fabrication details significantly affect overall performance of

friction dampers

Viscoelastic devices -Viscoelastic solid dampers, viscoelastic fluid dampers Mostly used in

-Displacement characteristics depend on frequency of motion and

relative velocity between ends of damper

structures

Re-centering devices -Possess inherent re-centering capability Many buildings built with these

devices

Phase transformation

dampers

-Use shape memory alloys Still in research stages

-Self-centering mechanism

-Insensitive to environmental temperature changes

-Excellent fatigue resistance

-Corrosion resistance

-Capable of producing large control forces

Dynamic vibration

absorber

-TMD, TLD, TLCD Successfully applied in mitigation

of wind loads in a number of

buildings-Reduction achieved by transferring some vibrational energy to

absorber

-Dynamic properties should be tuned to those of the primary

structure

-Detuning may occur

-No need for any activation mechanism

-Less maintenance costs



The damping capacity comes from the viscous uid,while the stiness is regulated by the opening of theow valve (Cheng et al., 2008). A new device calledan accumulated semi-active hydraulic damper(ASHD) was proposed by Shih et al. (2004). It is com-posed of a hydraulic jack, a directional valve and anaccumulator. The optimal rate of energy dissipation isachieved by controlling the ow of the oil in thehydraulic jack (by a controlling algorithm) to regulatethe acting direction of the device. Extended test resultsprove that the energy dissipation of ASHDs is extre-mely good with minimum energy requirements.

2.2.5. Semi-active stiffness control devices. A semi-activevariable stiness (SAVS) device was developed byKobori in Japan. It consists of a balanced hydrauliccylinder, a double acting piston rod, a normallyclosed solenoid control valve and a tube that connectsthe two cylinder chambers. The uid will ow freelyand will unlock the beam brace connection when thevalve is open, thus decreasing structural stiness. Incontrast, when the valve is closed, the uid cannotow and thus eectively locks the beam to the brace,resulting in increasing structural stiness (Cheng et al.,2008; Spencer and Nagarajaiah, 2003).

2.2.6. Electrorheological (ER) dampers. This type of deviceuses smart ER uids that contain dielectric particlessuspended within non-conducting viscous uidsabsorbed into the particles. The dielectric particlespolarize and become aligned when they are exposedto electrical elds, resulting in resistance to the ow.ER uids have the ability to undergo dramatic revers-ible increases to the ow in milliseconds. Thus, adjust-ing the electrical eld will simply control the behaviorof ER uids. ER dampers were rst proposed byMakris et al. (1995), where the smart properties ofER uids were utilized to adjust damping force gener-ation. This damper consists of a cylinder containing abalanced piston rod and a piston head.

Adjustment of the voltage V changes the electric eldand therefore controls the behavior of the ER uid,while regulating the capacity of the ER damper. Theenergy dissipation is a result of two eects: (a) EReects due to shearing of the uid and (b) frictioneects owing to oricing of the viscous uid. Thereare three factors that limit the utilization of the ERdampers for the response control of large structures:(a) limited yield stress (of the order of 510 kPa); (b)manufacturing impurities may reduce the applicability;and (c) high-voltage demand (about 4000 V) to controlthe ER uid (Cheng et al., 2008).

2.2.7. Magnetorheological (MR) dampers. MR uid wasdiscovered by Jacob Rabinow in the early 1950s. This

smart material has the ability to adapt its uid proper-ties when it is exposed to a magnetic eld (Larrecq,2010). MR uid is a magnetic equivalent of ER uidand typically composed of micron-sized, magneticallypolarizable particles dispersed in a viscous uid suchas silicone oil. The particles in the uid polarize whenthe MR uid is exposed to a magnetic eld, and theuid displays viscoplastic behavior, thus oering resist-ance to uid ow.

When subjected to a magnetic eld, the MR uid,such as an ER uid, has the ability to reversibly changefrom a free-owing linear viscous uid to a semi-solidone in milliseconds. The control force produced by theMR uid can be adapted by varying the strength of themagnetic eld according to a control scheme. The mag-netic eld is applied perpendicular to the direction ofuid ow. When compared to ER uids, the advan-tages of MR uids are: (a) high yielding strength (ofthe order of 50100 kPa); (b) stability over a broadrange of temperatures (Cheng et al., 2008); (c) low pro-duction cost due to insensitivity to contaminants; and(d) low power requirements (2050 W) (Jansen andDyke, 2000). Experimental tests carried out by Wuand Cai (2006) showed that the MR damper is appro-priate for cable vibration control in cable-stayedbridges. Moreover, the eciency of implementing MRdampers for vibration suppression for a space trussstructure using a fault-tolerant controller was con-rmed by Huo et al. (2011).

2.2.8. Semi-active viscous fluid dampers. This device wasevaluated by Shinozuka et al. (1992) for bridges. It util-izes a normally closed solenoid valve to control theintensity of the uid through a bypass loop.Adjusting the valve opening according to the prede-ned control algorithm will control the damper behav-ior with a low power requirement. The uid can easilyow through the valve when the opening of the valve islarge, and hence develops less damping force. In con-trast, when the opening is small, the damper provides agreater control force due to the diculties in the ow.The energy dissipation mechanism is due to the frictionbetween the ow, the bypass loop and orices in thepiston head (Cheng et al., 2008). The current state ofthe art in semi-active control systems are summarized inTable 2.

2.3. Active control systems

In spite of the cost eciency and reliability of the pas-sive and semi-active devices, these systems have a lim-ited capacity and/or intelligence for structural seismicresponse control. For instance, passive systems havesimple mechanisms and are easy to manufacture, butthey are unable to adapt to ever-changing excitation

Saaed et al. 929


because they neither sense excitation and response noruse external power.

In addition, some are eective for controlling onedominant vibration mode and are ineective for othermode or loading types. Semi-active devices, as men-tioned previously, are adaptable to excitations, buttheir eciency is restricted within the limit of the max-imum capacity of the passive devices on which they arebased.

This need has led to the development of an area ofactive control systems (Cheng et al., 2008). An error-activated control system, proposed by Yao (1972), hasthe ability to automatically supply a force into the struc-ture to counteract the unpredictable vibrations due todierent kinds of dynamic loadings, thus reducing thedynamic response for dierent vibration modes.

Depending on the measurement of the global systemresponse, active control systems can have optimal e-ciency compared with passive control systems, whichdepends on the local responses only. Active control sys-tems require signicant energy to counteract thedynamics loadings, which cannot be ensured duringsevere natural hazards due to failure of the energysupply during such events. Moreover, active controlsystems are complicated; they require sensors and con-troller equipment, and give a shift in the dynamicbehavior of the structure by adding or removingenergy from it; this may result in an unwanted or

even unstable condition (Christenson, 2001). Theadvantages of active control systems are (Chenget al., 2008): (a) enhanced control eectiveness: thismeans there are no theoretical limits on the active con-trol eciency; (b) adaptability to ground motion: theyrefer to the system ability to sense the excitation andautomatically adjust their control eorts; (c) selectivityof control objectives: the system can be designed fordierent objectives such as structural safety or humancomfort; and (d) applicability to dierent excitationmechanism: this system can be used for a wide fre-quency range.

Interested readers can nd valuable informationconcerning structural control theories and control stra-tegies in Casciati et al. (2012), Chen et al. (2012) andHousner et al. (1997). Active control systems can alsobe categorized as follows:

2.3.1. Active mass damper (AMD) systems. As a result ofthe limited capability of TMDs and their applicabilityfor structures with the rst mode dominant (e.g. struc-tures under wind loads), in the early 1980s, researchersevolved the AMD from TMDs that have utilized anactive control mechanism in order to improve theirapplicability for a wide frequency band. The actuatorinstalled between the primary (structure) and the aux-iliary (TMD) system has been utilized to adjust themotion of the TMD according to predened control

Table 2. State-of-the-art summary of semi-active control systems.


Semi-active tuned mass dampers -TMD actuator Still in research stagesSemi-active tuned liquid dampers -(TLD actuator) or (TLCD actuator) Still in research stagesSemi-active friction dampers -Electromechanical actuator Still in research stages

-PZT actuators

-Adapted to varying excitations

Semi-active vibration absorbers -SAVA, SAHD, ASHD Still in research stages

-Adjusting both stiffness and damping properties

-Energy dissipation extremely good with minimum

energy requirements

Semi-active stiffness control devices -SAVS Still in research stages

-Adjusting the stiffness SAVS

Electrorheological dampers -Use smart ER fluids Still in research stages

-Limited yield stress

-Manufacturing impurities may reduce applicability

-High-voltage demand

Magnetorheological dampers -Use smart MR fluids Still in research stages

-High yielding strength

-Stable over broad range of temperatures

-Low production cost

-Small power requirements

Semi-active viscous fluid damper -Utilizes normally closed solenoid valve Still in research stages



algorithms. Analytical and experimental studiesshowed that AMDs have an economic benet in full-scale structures because both the control force andactuation are much smaller than other types of activesystems. This is mainly because the actuator in otheractive systems generally acts on the structure directly,while the actuator in an AMD is utilized to drive theauxiliary mass only. However, the eciency of thisdevice is limited to the fundamental frequency, andless so for higher frequencies.

2.3.2. Active tendon systems. These devices are composedof a set of pre-stressed tendons whose tension is con-trolled by electrohydraulic servomechanisms. Thesystem is installed between two stories of a building.The actuation cylinder is xed on the lower oor.One end of the tendon is attached to the devicepiston, while the other end is connected to the upperoor. The inter-story drift resulting from earthquakeexcitations drives the relative movement of the actuatorpiston to the actuator cylinder and thus the changes inthe tension of the prestress will apply a dynamic controlforce to the structure. Both experimental and simula-tion results indicated that an important reduction instructure response can be achieved by the utilizationof active tendon systems. The advantages of thissystem are the applicability in both the pulsed and thecontinuous time modes and the possibility of usingexisting structural members, which will minimize theneed for modications in the structure.

2.3.3. Active brace systems. These devices can also utilizeexisting structural braces to install an active controldevice (actuator) onto a structure. The device can beintegrated within the three common types of bracing

systems: diagonal, K-braces, and X-braces. A largecontrol force can be generated by the servo valve-con-trolled hydraulic actuator, which is mounted on thebracing systems between two adjacent oors. Thissystem is composed of a servo valve, a servo valve con-troller, a hydraulic actuator, a hydraulic power supply,sensors, and a control computer with a predened con-trol algorithm. The control computer utilizes the con-trol algorithm to process the sensor measurements andthen order the control signal. This signal will be used bythe servo valve to adjust the ow direction and inten-sity, thus the pressure dierence will yield in the twoactuator chambers. This pressure dierence will pro-duce the required control force to resist seismic loadson the structure.

2.3.4. Pulse generation systems. These devices produce theactive control forces using pulse generators, whichdepend on pneumatic mechanisms that utilize com-pressed air instead of hydraulic actuators that usehigh-pressure uids. Protection of a smart structurecan be fullled by installing pulse generators at severallocations within the structure. The pneumatic actuatorwill be triggered when a large relative velocity isdetected at any pulse generator location, and a controlforce opposite to the velocity is applied to the structure.Tests conducted by Masri and Caughey (1988) showedthat pulse generators were a promising device for seis-mic response control. The disadvantages of this systemare: (a) even though the compressed gas energy used bypulse generators is cheap, it may not be powerfulenough to drive full-scale buildings and (b) the highnonlinearity of these devices.

A current state-of-the-art summary of active controlsystems is given in Table 3.

Table 3. State-of-the-art summary of active control systems.


Active mass damper -TMD actuator Analytical and experimental studies-Efficiency limited to fundamental frequency

-Economical in full-scale structures

Active tendon systems -Pre-stressed tendons electrohydraulic servomechanisms Analytical and experimental studies-Applicability in both pulsed and continuous time modes

-Possibility to use existing structural members

(minimize the cost)

Active brace systems -Utilize existing structural braces actuator Analytical and-Can be integrated within three common types of bracing

systems

experimental studies

Pulse generation systems -Pulse generators Analytical and experimental studies

-Promising device for seismic response control

-Cheap

-Not powerful enough to drive full-scale buildings

-High nonlinearity

Saaed et al. 931


2.4. Hybrid control devices

As presented previously, active control systems are uti-lized to compensate for the restricted capacity and intel-ligence of passive and semi-active dampers. Theiroperation depends mainly on an external powersupply, which limits their applications, requires compli-cated sensing, requires a signal-processing system,which reduces control reliability, and nally requireslarge force-generating equipment, which cannot beachieved within reasonable costs (Cheng et al., 2008).For these reasons, the three main groups of controlsystems (passive, active, semi-active) can be groupedinto series or parallel combinations in order to selectthe best advantage of each group to yield the fourthgroup, which are called hybrid control devices, a cat-egory that has become an attractive solution since the1990s.

The passive devices in this group can be used toachieve the major part of the response reduction neces-sary to keep the structures within the required perform-ance range, while the active ones will be necessary totune and nally adjust the response, for instance, tominimize the displacements and accelerations in orderto protect the sensitive equipment in the structures.Hybrid devices have a larger capacity and greater e-ciency than a passive system, and cost less (Cheng et al.,2008). Moreover, they are more reliable and require lessenergy than active devices since there is no need forlarge control forces, but they still require signicantenergy (Christenson, 2001; De la Cruz, 2003).According to Wu (2011), hybrid control systems arevery ecient in protecting structures from dierenttypes of excitation with dissimilar intensity and fre-quency content. In addition, research carried out byYan and Wu (2011) conrmed the reliability and e-ciency of this kind of system. Owing to these importantfeatures, hybrid control systems have become very pro-mising for seismic response reduction of civil engineer-ing structures. More information about the merits ofhybrid control systems and recent applications can befound in Khodaverdian et al. (2012) and Love et al.(2011). Typical hybrid control systems are shownbelow, and will be analyzed in the following.

2.4.1. Hybrid mass damper (HMD). This device is com-posed of either a combination of a passive TMD andan active control actuator, or a combination of anAMD to a TMD. The connection of the AMD to theTMD instead of the main structure will work to min-imize the mass of the AMD, which will be 1015% ofthat of the TMD. The energy and forces required toactivate an HMD are far less than those related with afull AMD system with similar performance, mainly dueto the fact that the AMD is designed to increase control

eciency for higher modes of the structure only, whilethe TMD is tuned to control the fundamental mode ofthe structure. Due to this feature, which makes HMDsan inexpensive control solution, these systems havebeen the most common control devices employed infull-scale building structure applications. However,space limitations can impede the use of an HMDsystem (Cheng et al., 2008).

2.4.2. Hybrid base isolation system. This type of devicerepresents the majority of the hybrid controlapplications in the USA, which can be subdividedinto two types (Housner et al., 1997). The rst systemwas proposed and tested by Yoshioka et al. (2002), thissystem uses MR uid dampers on the superstructureinstead of the active tendon in the second system. Thesecond was studied and tested by Cheng and Jiang(1998); this system is composed of a base isolationsystem between the foundation and the structure, andan active tendon control system on the superstructure.

2.4.3. Hybrid damper actuator bracing control. In the early1990s, the hybrid damper actuator bracing controlmounted by K-braces on the structure was developedby Cheng and Jiang (1998). Many passive devices canbe used in this system, such as liquid mass dampers,spring dampers, and viscous uid dampers. Owing totheir powerful force-generating capacity, hydraulicactuators are proposed as the active device for thesystem. Extensive experiments and studies showedthat this system has greater capacity than a passivesystem in decreasing seismic structural response, andit needs less active control force than an active controlsystem to achieve a control objective. Moreover, themajor advantage of this system is the possibility ofeither combining the damper and the actuator, orseparating them. In addition, it is possible to use exist-ing structural braces for xing of control devices, andthe active control force is applied directly to the struc-ture. Thus, a hybrid bracing system costs less than abase isolated/actuator system and oers further controlcapacity than an HMD (Cheng et al., 2008). Thecurrent state of the art in hybrid control devices isgiven in Table 4.

Finally, a comparison of the dierent kinds of struc-tural control systems is presented in Table 5.

3. Recent applications

According to Ikeda (2004), research and developmentin the eld of active and semi-active vibrationcontrol of civil engineering structures in Japan can becategorized into four stages. In the rst stage up to thelate 1980s, the fundamental dynamics properties ofactive control were understood theoretically and



experimentally from the civil engineering viewpoint.Applying AMDs to a building for the rst time in1989 marked the start of the second stage. The 1995Hyogo-ken Nanbu (Kobe) Earthquake opened thedoor to the third stage where buildings can be semi-actively controlled, even under large earthquakes. Theintegration of structural control and health monitoringdeclared the start of the fourth stage. As a result, thereare about 70 active and semi-active control systems thathave already been implemented to actual buildings inJapan since 1989, while Ikeda (2009) presented a list of52 practical applications of active control systems tobuildings in Japan from 1989 to 2007. The rst full-scale application of active control to a building wasapplied to the Kyobashi Center Building in Tokyo. Inaddition, the same researcher presented another surveycontaining practical applications of semi-active controlto buildings in Japan from 1990 to 2006. Spencer andNagarajaiah (2003) presented a detailed summary ofcontrolled buildings in Japan for the period 1989 to2002 with information about control systems usedand their actuation mechanism. In their survey, theNihon-Kagaku-Miraikan, the Tokyo NationalMuseum of Emerging Science and Innovation, whichwas constructed in 2001, is considered as the rst imple-mentation of MR dampers for civil engineering struc-tures, while the Dongting Lake Bridge in Hunan,China, constructed in 2003, represents the rst full-scale application of MR dampers for bridge structures.Kareem et al. (1999) presented a report regarding thenumber of installations for dierent types of dampingdevices in Japan during the 1990s, and give detailedexamples of applications of dierent types of controlsystems in Australia, Canada, China, Japan, andthe USA.

The base isolation system, which came into practicaluse in Japan from the 1980s, has been implemented inmore than 2000 buildings, and its eciency has beentested in magnitude 7 earthquakes.

According to Shinozaki et al. (2010), base isolationin Japan was used not only to improve the seismic per-formance, but to also allow more exible architecturalplanning. Shinozaki et al. (2010) proposed the worldsrst semi-active base isolation system in a tall buildingand implemented it in the Metropolitan building inTokyo in order to ensure high stability for the buildingand maintain its function. The system was composed ofrubber bearings, 12 variable oil dampers, and 12 pas-sive oil dampers.

Another example of an innovative isolation system isthe core-suspended isolation system (CSI) that has beendeveloped and implemented for the rst time byNakamura et al. (2011). The developed system alsosatises the architectural requirements such as trans-parent facades for suspended structures and functionaland attractive open spaces underneath the suspendedbuilding. The CSI system consists of a reinforced con-crete core on top of which a seismic isolation mechan-ism composed of a double layer of inclined rubberbearings is installed to create a pendulum isolationmechanism. A multi-level structure is then suspendedfrom a hat-truss or an umbrella girder constructed onthe seismic isolation mechanism. Fluid dampers areplaced between the core shaft and the suspended struc-ture to control the motion of the building. The buildingis a four-story building with a total area of 213.65m2

located in Tokyo.In China, since the rst application of rubber bearing

in 1993 for an eight-story building, the application ofseismic isolators proved that these systems are safer and

Table 4. State-of-the-art summary of hybrid control devices.


Hybrid mass damper -TMD active control actuator Most common control device employed infull-scale buildings-or (AMD TMD)

-Inexpensive

-Space limitations

-Low activation forces

Hybrid base isolation

system

-Base isolation system active tendoncontrol system.

Majority of hybrid control applications in USA

-Base isolation system MR fluid dampersHybrid damper actuator

bracing control

-Hydraulic actuators passive device Analytical and experimental studies-Mounted by K-braces on structure

-Many passive devices can be used in this system

-Possibility to either combine damper and

actuator, or separate them

-Costs less than a base isolated/actuator system

-Offers further control capacity than HMD

Saaed et al. 933


Table

5.Comparisonbetw

eendifferentkindsofstructuralco

ntrolsystems.

Seismic

isolationdevices

Passive

controlsystems

Semi-active

controlsystems

Activeco

ntrolsystems

Hybridco

ntroldevices

-Dissipatepartofinput

energy

-Absorb

ordivergepartofinput

energy

-Naturalextensionofpassive

devices

-Automatically

supply

aforce

into

thestructure

-Mixture

-Increasehorizontal

flexibility

-Dependentonrelative

movement

-Includeadaptive

systems

-Dependsonglobalresponse

-Mixture

Lengthenfundam

ental

periodsofstructures

-Relatedonly

tolocalstructure

response

-Ability

tosense

excitationand

automatically

adjust

control

efforts

-Nostructuralresponse

measurements

Suitable

forshort

to

medium

height

buildings

Optimally

tunedto

specified

dynam

icloading

-Betterthan

passive

systemsand

less

than

active

systems

-Optimal

efficiency

compared

withpassive

controlsystems

-Suitable

foralltypesof

structures

-Efficientagainstvibrations

transm

ittedthrough

ground

-Notoptimalforothertypesof

dynam

icloadings

-Capable

ofactingbetterthan

passive

ones

-Designedfordifferentobjective

-Largercapacitythan

passive

system

-Unable

toadaptto

excitation

andglobalstructuralresponse

Lim

itedco

ntrolcapacity

-Notheoreticallim

itson

efficiency

-Notefficientto

resist

wind

-Lim

itedco

ntrolcapacity

-Widefrequency

range

-Greaterefficiency

than

passive

system

-Safe

-Inherentlystable

-Fail-safe

-Detuningmay

occur

-More

reliable

-Reliable

-Eco

nomic

-Noenergyrequirement

-Littlepowerrequirement

-Significantenergy

-Costsless

than

active

system

-Sim

plerto

design

andco

nstruct

-Easyto

manufacture

-Complicated

-Very

promising

-Very

promising



more economic than traditional structural systems.They have been successfully used in more than 500full-scale implementations for buildings and bridges(Li and Huo, 2010). The Isolation House Buildings onSubway Hub located in the center of Beijing are con-sidered as the largest isolated area in the world. There isa very large platform composed of a two-story RCframe that is used by the railway hub. The platformdimensions are 1500 m wide and 2000m long. Overthe top oor of the platform, there are 50 house build-ings 7 9 Reinforced Concrete RC frames built ona layer of rubber bearing. The total oor area isapproximately 480,000 m2 (Zhou et al., 2006).

In China, the TMD system may be formed by addingone or more stories supported by rubber bearings on theroof of main building structures to obtain good seismic-reduction eectiveness ondisplacements andaccelerations.

The AMD control system was designed and imple-mented in the Nanjing Communication Tower inChina. The physical size of the damper was limited toa ring-shaped oor area with inner and outer radii of 3m and 6.1 m, respectively. The damper was elevated othe oor by steel supports with Teon bearings to allowfree access to the oor area (Li and Huo, 2010).

The hybrid TMD control system, which consists oftwo water tanks used as a mass block, was installed inGuangzhou TV and Sightseeing Tower. The tower,which is 610 m high, is a very exible structure with arst period of about 10.03 s and is susceptible to thewind.

Taipei 101 Tower was completed at the endof 2004, and is a famous application of TMDsin Taiwan. This tower is 508 m in height and has a101-story structure with a ve-story basement. Thedesign of such a building is a challenge because bothseismic and wind-induced eects should be taken intoconsideration. Therefore, the idea of a mega structure isutilized for the design of the structural system. For thelateral-resistant system, a combination of bracedframes in the core, outriggers from core to perimeter,shear walls, supercolumns, and moment resistingframes has been designed to resist the lateral forces.Most importantly, the passive TMD system has beenapplied to improve the wind-resistant ability of such atall building. This passive TMD system is composed ofa ball-shaped mass block of 600 tons in weight with 41layers of 125 mm thick round plates, eight sets of high-strength steel cables from the 92nd oor to the 87thoor for suspension of the mass block, eight primaryviscous dampers allocated around the cradle for energyabsorption, a bumper ring and eight sets of snubbeddampers installed underneath the mass block on the87th oor for the control of unexpected oscillationamplitudes. In addition, there are two TMDs installedat the top of the pinnacle to control fatigue induced bycross-winds (Chang et al., 2009).

The mid-story isolation technique is among increas-ing practical applications in Taiwan. In mid-story iso-lation, the isolation system is typically installed on thetop of the rst story of a building in order to satisfy thearchitectural concerns of aesthetics and functionality.Moreover, this concept can improve the constructionfeasibility in highly populated areas where xing theisolation system beneath the base of a building is verytough if the building separation and property line are ofparticular concern (Chang et al., 2009). Importantinstallations of TMDs, TLDs, and TLCDs aroundthe world are presented in Chaiviriyawong andPrachaseree (2010). The concept of coupled buildingshas been extensively studied and implemented onShanghai Shimao International Plaza in China by Luet al. (2007). This tall building is composed of a mainbuilding that is 60 stories, of total height of 333 m, andits surrounding large podium structure, 10 stories oftotal height of 49 m. A set of 40 linking viscous uiddampers were used successfully to link the podiumstructure to the main building in order to decrease seis-mic torsional response of the podium structure, result-ing from large eccentricity of stiness and massdistribution in podium structures.

Another innovative example of a coupled buildingwas proposed and implemented by Koike et al. (2004)in Japan. This newly active vibration control system,which is called the active damping bridge, has beendeveloped to reduce vibration by connecting adjacenthigh-rise buildings and applied to the three high towersof Harumi Island Triton Square in Tokyo. The testresults showed that this technique would improve thedamping of buildings two to three times in comparisonwith buildings without the bridge.

4. Conclusions

The state of the art for structural control systemswas reviewed by briey summarizing all the categoriesof these devices; these include: (a) passive; (b) semi-active; (c) active; and (d) hybrid systems. To demon-strate the structural control system potential andfuture directions in civil engineering, an overview ofsome innovative practical implementations of thesedevices was provided. The state of the art clearly indi-cates the huge capability for these systems and theirimportance in modern buildings. These systems canbe used to achieve architectural requirements in add-ition to their original functions of controlling structurevibrations.

Funding

This research received no specic grant from any fundingagency in the public, commercial, or not-for-prot sectors.

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Appendix 1

Abbreviations

AMD active mass damperASHD accumulated semi-active hydraulic damper

CSI core-suspended isolationEDR energy dissipating restraintER electrorheologicalFPS friction pendulum system

HDNR high-damping natural rubberHMD hybrid mass damperLDRB low-damping natural and synthetic rubber

bearingLED lead extrusion damperLRB lead-plug bearingMR magnetorheologicalPFD piezoelectric friction damperPZT piezoelectric and translators

SAHD semi-active hydraulic damperSAVA semi-active vibration absorberSAVS semi-active variable stiffnessSMA shape memory alloySPIS sleeved-pile isolation systemTASS teflon articulated stainless steelTLCD tuned liquid column damperTLD tuned liquid damperTMD tuned mass damperVDW viscous damping wall

VE viscoelastic

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