10

CHAPTER 2

LITERATURE REVIEW

2.1 STRUCTURAL CONTROL

The protection of civil engineering structures, including their

material contents and human occupants, is without doubt a worldwide

priority. Such protection may range from reliable operation and comfort, on

the one hand, to survivability on the other. In like manner, events that cause

the need for such protective measures are earthquakes, winds, waves, traffic,

lightning, and today, regrettably deliberate acts. Indications are that control

methods will be able to make a genuine contribution to this problem area,

which is of great economic and social importance (Spencer and Sain 1997).

In an ideal situation, completely safe structures can be designed if

exact information is known concerning loads and strengths involved during

the lifetime of these structures, and exact methods of structural analysis are

available. In the real world, uncertainties exist in this information as well as in

the method of analysis. If the level of this uncertainty is extreme, the system

may even be driven to instability. In the context of structural control,

performance degradation and instability imply excessive vibration or even

structural failure. Robust control has typically been applied to the issue of

model uncertainty through worst-case analyses (Field et al 1996). To account

for these uncertainties, various factors of safety have been used in the design

of structures. Techniques in structural analysis are being refined continuously.

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The concept of structural control is an alternative approach to the safety

problems of structural engineering (Yao 1972).

The primary goals when applying optimal control theory to civil

structures are the maintenance of stability and the achievement of specific

performance criteria, including control efficiency, in the face of random

disturbances. Two important issues in achieving these goals are the

consideration of nonlinear actuator saturation effects and unknown, time-

varying, parametric uncertainties. Most importantly both of these issues must

be addressed concomitantly within the same control design. Robust H∞ state

feedback controllers are developed here that achieve the desired H∞ norm

bound while accounting for pre specified bounds on the time-varying

parametric uncertainties. Stability of these controllers in the presence of

nonlinear actuator saturation can be proven through the construction of a

Lyapunov function for the saturated control system using a nonlinear state

space model and new mathematical programming techniques (Geoffrey Chase

and Allison Smith 1996). Spencer and Sain (1997); Suharidjo et al 1992;

Spencer et al (1994) have used H2 and H control to their research.

Continuous sliding mode control (CSMC) methods, which do not

have an undesirable chattering effect, are presented for applications to

seismically excited linear structures. These control methods are robust with

respect to parametric uncertainties of the structure. The controllers have no

adverse effect should actuators be saturated due to unexpected extreme

earthquakes. Static output controllers using only a limited number of sensors

without an observer are also presented. When a controller is installed in each

story unit of a building, a complete compensation of the structural response

can be achieved and the response state vector can be reduced to zero

(Yang et al 1995).

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An extension to classical covariance control methods, introduced

by Skelton et al (1995), is proposed specifically for application to the control

of civil engineering structures subjected to random dynamic excitations. The

covariance structure of the system is developed directly from specification of

its reliability via the assumption of independent (Poisson) out crossings of its

stationary response process from a polyhedral safe region. This leads to a set

of state covariance controllers, each of which guarantees that the closed-loop

system will possess the specified level of reliability.

An optimal polynomial controller for reducing the peak response

quantities of seismically excited non-linear or hysteretic building systems was

introduced by Yang. A performance index, that is quadratic in control and

polynomial of any order in non-linear states, is considered. The performance

index is minimized based on the Hamilton-Jacobi-Bellman equation using a

polynomial function of non-linear states, which satisfies all the properties of a

Lyapunov function. The resulting optimal controller is a summation of

polynomials in non-linear states, i.e. linear, cubic, quintic, etc. Gain matrices

for different parts of the controller are determined from Riccati and Lyapunov

matrix equations (Yang et al 1996).

Fuzzy control was developed by Nagarajaiah (1994), Venini and

Wen (1994); Ghaboussi and Joghataie (1995) used neural control and

Agrawal and Yang (1996) used nonlinear control. Modeling issues in control

of civil engineering structures were discussed by Smith and Schemmann

(1996); Dyke et al (1995). Spencer et al (1997) developed benchmark studies

A good reference to the current state-of-the-art in active control can be found

in Housner et al (1997).

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In recent years, considerable attention has been paid to research and

development of structural control devices, with particular emphasis on

alleviation of wind and seismic response of buildings and bridges. In both

areas, serious efforts have been undertaken in the last three decades to develop

the structural control concept into a workable technology (Spencer and

Nagarajaiah 2003).

The structural control systems fall into four basic categories:

passive, active, semi active and hybrid. These systems are well understood

and are accepted by the engineering community as a mean for mitigating the

effects of dynamic loading such as strong earthquakes and high winds.

However, these passive device methods have the limitation of not being able

to adopt to structural changes and to varying usage patterns and loading

conditions. Active, semi active and hybrid control systems have the ability to

adapt to various operating conditions (Shirley Jane Dyke 1996). As the active

control systems are using external power source, they are able to balance the

structural systems during the unexpected loading conditions. Also we can

have the control over the balance force required at the loading conditions.

Because of these reasons the active control strategies are used in the course of

this study.

2.2 PASSIVE CONTROL

Passive control is a widely used form of structural control. Passive

control uses the building’s response to develop control forces. The main

advantage of passive control is it requires no power source.

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Passive devices can be categorized into isolation systems, Energy

dissipation devices and mass dampers.

2.2.1 Isolation Systems

The main objective of the isolation systems is to ‘release’ the

structures from the influence of ground acceleration by using foundations that

are flexible in the horizontal direction. The isolation system absorbs partially

some of the input energy before this energy is transmitted to the structure. The

net effect is a reduction of the energy dissipation demand of the structural

system. The net effect is an increase of its performance. Energy dissipation

devices and mass dampers are the main isolation systems.

An idea of passive base isolation of buildings is explored, using

inclined rubber base isolators or inclined “soft” first-story columns. Such a

system behaves as a physical pendulum, “pivoted” above the center of mass,

and is more stable than the standard system. Another advantage of the

inclination is that the inertia forces of the structure due to rotation about the

pivot point cancel to some degree the inertia forces due to the base translation

(Maria I. Todorovska 1999).

Henri P Gavin et al (2003), presented the benchmark problem

definition for seismically excited base-isolated buildings to provide a well

defined base isolated building with a broad set of carefully chosen parameter

set and control algorithm.

Glenn J. Madden et al (2004) evaluated a smart base isolation

system for seismic response of a single-story steel building frame. The

isolation system consists of sliding bearings combined with an adaptive fluid

damper. The damping capacity of the fluid damper can be modulated in real

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time based on feedback from the earthquake ground motion and

superstructure response. The adaptive capabilities of the fluid damper enable

the isolation system displacement to be controlled while simultaneously

limiting the inter-story drift response of the superstructure.

Lightly reinforced and unreinforced masonry buildings have not

performed well in earthquakes. Evaluation of past performance of masonry

structures has led to more stringent design and construction requirements in

the current building codes, and has raised concerns about the performance of

existing lightly reinforced and unreinforced masonry buildings in future

earthquakes. Base isolation has been shown to be effective in reducing

damage to large building structures, and appears to be particularly effective in

protecting stiff masonry structures (Lisa A. Wipplinger 2004).

Baris Erkus and Erik A. Johnson (2004) presents a tutorial control

design for the base isolated benchmark building with bilinear hysteretic

bearings (e.g., lead-rubber bearings) by addressing the coupled problem of

finding a good linear model for the controlled nonlinear system and designing

a linear optimal controller.

Chin et al (2006) present a completed prototype of the AIGO

seismic vibration isolation system. The design has been developed to satisfy

the isolation requirements for the next generation of interferometer GW

detectors. The system relies on passive isolation and includes multiple Ultra

Low Frequency (ULF) stages to achieve minimal low frequency residual

motion. Two complete isolators are being installed at the high power test

facility located in Gingin, Western Australia. The performance of individual

mechanical stages is continually being tested and improved. Currently it is

expected that residual motion close to 1 nm at 0.3 Hz will be achieved.

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Analytical seismic response of multi-storey buildings isolated by

lead-rubber bearings (LRB) is investigated under near-fault motions. The

superstructure is idealized as a linear shear type flexible building. The force-

deformation behavior of the LRB is modeled as bilinear with viscous

damping. The governing equations of motion of the isolated structural system

are derived and the response of the system to normal component of six

recorded near-fault motions is evaluated by step-by-step numerical method

(Jangid 2007).

Dolce Mauro et al (2007) presents three different isolation systems

(IS's) for bridges All of them are made of steel-PTFE sliding bearings (SB) to

support the weight of the deck and auxiliary devices, based on different

technologies and materials (i.e. rubber, steel and shape memory alloys), to

provide re-centering and/or additional energy dissipating capability. An

extensive numerical investigation has been carried out in order to (i) assess

the reliability of different design approaches, (ii) compare the response of

different types of IS's, (iii) evaluate the sensitivity of the structural response to

friction variability due to bearing pressure, air temperature and state of

lubrication and (iv) identify the response variations caused by changes in the

ground motion, bridge and isolation characteristics.

Pocnschi (2007) developed a base isolator with hardening behavior

under increasing loading for medium-rise buildings (up to four stores) and

sites with moderate earthquake risk. Following the construction of the

isolator, the behavior of a prototype has been investigated in the laboratory

under dynamic loads. Based on the experimental results the linear equivalent

stiffness and damping properties of the device are determined.

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2.2.2 Energy Dissipation Devices

Energy dissipation devices consist of relatively small devices

installed in the structure to dissipate energy. The main function of these

energy dissipation devices is to absorb or divert part of the input energy;

therefore, this action reduces the energy dissipation demand of the main

structure and minimizes the structural damage.

In last few decades, use of energy dissipation devices in structural

system has gained momentum. Several researchers have carried out

theoretical and experimental studies on passive and semi-active vibration

control systems with different device configurations.

Structures with added energy dissipation devices will generally

exhibit inelastic behavior when they are subjected to strong ground motion;

therefore, the assumption of linear structural responses has to be eliminated in

the design of such devices for the purposes of structural upgrade and retrofit.

Shen and Soong (1996) addressed, the importance of the energy concept in

the design of energy dissipation devices for structural applications and a new

design method is proposed based on the concept of damage control.

Constantinou et al (1997) tested a half length scale single storey

steel model using a novel energy dissipation system configuration termed the

toggle brace-damper. The concept, theoretical development, and experimental

and analytical results for this model were presented in Constantinou et al

(1997) and in the M.S. thesis of Hammel (1997). Analytical study of the same

has been done using SAP2000 by Scheller and Constantinou (1999).

Constantinou et al (2001) presented three new configurations that

utilize the toggle brace mechanism to substantially magnify the effect

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damping devices. So they can be utilizing effectively in the application of

small structural drift. They derived the formula to calculate the magnification

factor for different toggle brace configuration based on orientation of damper.

Ian D. Aiken et al (1993) presented, overview of seven different

passive energy dissipation systems describing the different types of devices,

with the shake table experiments, and associated analytical work. Four of the

systems studied are friction systems, and of these, three (Sumitomo, Pall, and

Friction-Slip) are based on Coulomb friction. The fourth is the Fluor-Daniel

Energy Dissipating Restraint, which is a device capable if providing self

centering friction resistance that is proportional to displacement. The three

other systems have different energy dissipation mechanisms.

Muthumani (2002) has studied the performance of structures fitted

with visco-elastic damping devices subjected to base excitation. A computer

programme was developed for the seismic analysis of three dimensional

framed structures with truss-type visco-elastic dampers, using step by step

time integration procedure. After suitably validating the program, two typical

framed structures namely, a low frequency and high frequency structures with

visco-elastic braces are analyzed for ElCentro ground acceleration and for

various damping values.

The possibility of a correlation between dynamic pore pressure

increase p and dissipated energy density D in soils subjected to earthquake

shaking has been the subject of speculation for nearly 20 years. Davis and

Berrill (2001) presented evidence to support the D-p model from two real

earthquakes. Acceleration records from two earthquakes are analyzed to

obtain approximate histories of shear stress, shear strain, and dissipated

energy over a range of depths.

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Alfredo Reyes et al (2001) presented the major sources of energy

dissipation, lateral deformation, and base shear in steel frames with partially

restrained (PR) connections subjected to seismic loading are analytically

studied. The analytical study confirms, in general, the behavior observed

during experimental investigation: PR connections reduce the overall stiffness

of frames, but add a major source of energy dissipation. It is observed, in

general, that the maximum total base shear may significantly increase as the

connection stiffness increases.

The seismic performance of a post tensioned energy dissipating

(PTED) connection for steel frames was investigated analytically and

experimentally by Constantin Christopoulos et al (2002). The PTED

connection incorporates post tensioned high-strength bars to provide a

self-centering response along with energy dissipating bars that are able to

yield in axial tension and compression. The analytical study involves the

development of an equivalent iterative sectional analysis procedure to predict

the moment-rotation relationship of the PTED connection.

A simple semiactive oil damper developed for an actual

application. This device can dissipate twice as much energy imposed by wind

or earthquake forces as an ordinary passive damper by switching the opening

of the on/off valve according to a signal from a controller. Because the system

employs a decentralized control algorithm that uses only a built-in sensors’

information, each device can be equipped with all the necessary control

equipment such as sensors and a controller (Haruhiko Kurino et al 2003).

Lin and Chang (2003) discusses the rationality of the design force

and damping reduction factors adopted by a few seismic design provisions for

buildings with and without added passive energy dissipation systems. The

issue will first be pointed out that the damping reduction factors adopted by

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those provisions are derived from the effects of viscous damping on

displacement responses, but are used to reduce the design force of buildings.

Statistical results from 1053 ground motions recorded in the U.S. show that it

may lead to unconservative results, especially for systems with damping

ratios greater than 10% and periods longer than 0.15 s. Furthermore, although

there is no doubt that the additions of extra damping to a structure will always

reduce the displacement responses, many documents argue the effect of added

damping to reduce the force responses of the buildings. Therefore, this paper

also addresses the effects of viscous damping on the inertial force and elastic

restoring force in order to use the damping reduction factors correctly.

Polymer matrix composite (PMC)-infill walls hold great promise

for energy dissipation when used in retrofitting applications where seismic

activity is a consideration. . The PMC-infill wall system consists of two fiber-

reinforced polymer laminates with an infill of vinyl sheet foam. At the

interface between the laminates, viscoelastic honeycomb is used to dissipate

energy and improve the damping characteristics of the structure. As part of

this research, analytical and experimental studies were performed to explore

the effectiveness of this seismic retrofitting strategy and to examine the

behavior of the PMC-infill wall system when subjected to monotonic and

cyclic loading. A steel frame retrofitted with a PMC-infill wall was monitored

to assess the resultant enhancements to its seismic-energy resistance capacity.

In testing the PMC-infill wall system in this research, a large-scale steel frame

was used to avoid the typical uncertainties associated with scaling the

dimensions. The optimal design for the stacking sequence of a PMC-infill

wall panel was determined based on the performance and material cost using

the finite-element analysis. Finally, the observed behavior of the PMC-infilled

frame was assessed on the bases of stiffness, strength, modes of failure, and

energy dissipation output (Amjad J. Aref and Woo-Young Jung 2003).

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A systematic method for identifying the optimal damper

distribution to control the seismic response of a 20-story benchmark building

was presented by Wongprasert and Symans (2004). A genetic algorithm with

integer representation was used to determine the damper locations. Both

H2- and H∞-norms of the linear system transfer function were utilized as the

objective functions. Moreover, frequency weighting was incorporated into the

objective functions so that the genetic algorithm emphasized minimization of

the response in the second mode of vibration instead of the dominant first

mode.

Hwang et al (2004) presented a paper summarizes the feasibility

study of implementing seismic protective systems into high-tech industrial

structures in which costly vibration-sensitive facilities are housed. Due to the

fact that the micro vibration of an existing integrated circuit (IC) fab structure

plays an important role in affecting the chip probe yield of manufacturing and

the reliability of chip products, the paper has emphasized on the micro

vibration analysis and measurement of a test structure before and after the

seismic protective systems has been incorporated.

An overview of the present and future directions in active and

passive vibration control analysis and design methodologies used in civil

engineering structures are given by Rama Raju et al (2003). In this study, the

basic design concepts of the vibration control methodologies and their

incorporation in structures is described. The methodologies for developing

different linear control strategies using Artificial Neural Network techniques

were also discussed.

Shambhu Sinha (2004) studied the feasibility of using dissipating

fluid viscous dampers in structures to protect against seismic loads. He has

examined experimentally and analytically the benefits of using fluid viscous

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devices in steel moment frame, reinforced concrete building models and a

steel bridge model. He has also described about how fluid dampers could be

used as elements of seismic systems for enhancing their energy dissipation

capabilities.

2.2.3 Mass Dampers

Mass dampers consist of an auxiliary mass connected elastically to

the main structure. Such a connection must allow the relative motion between

the mass dampers and the structures. So the big inertia forces involved

partially cancel the external forces on the structure.

Multiple tuned mass dampers (MTMDs) consisting of many tuned

mass dampers (TMDs) with a uniform distribution of natural frequencies are

considered for attenuating undesirable vibration of a structure. The MTMD is

manufactured by keeping the stiffness and damping constant and varying the

mass. The structure is represented by its mode-generalized system in the

specific vibration mode being controlled using the mode reduced-order

method (Chunxiang Li 2003).

The dynamic properties of the dampers and structure were

identified from free and forced vibration tests. The building structure with or

without the dampers was, respectively, tested on a shake table under the white

noise excitation, the scaled 1940 El Centro earthquake and the scaled 1952

Taft earthquake. The dampers were placed on the building floors using the

sequential procedure developed by the authors in previous studies.

Experimental results indicated that the multiple damper system is

substantially superior to a single tuned mass damper in mitigating the floor

accelerations even though the multiple dampers are sub-optimal in terms of

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tuning frequency, damping and placement (Genda Chen and Jinging Wu

2003).

Mahendra P. Singh et al (2002) presents an approach for optimum

design of tuned mass dampers for response control of torsional building

systems subjected to bi-directional seismic inputs. Four dampers with

fourteen distinct design parameters, installed in pairs along two orthogonal

directions, are optimally designed. A genetic algorithm is used to search for

the optimum parameter values for the four dampers. This approach is quite

versatile as it can be used with different design criteria and definitions of

seismic inputs.

Five multiple-tuned mass damper (MTMD) models with their

natural frequencies being uniformly distributed around their mean natural

frequency and eight MTMD models with the system parameters being

uniformly distributed around their average values, respectively, have been

recently presented by Bingkang Han and Chunxiang Li (2007).

Jin-Min Ueng et al (2007) developed a new design procedure for

reducing the dynamic responses of torsionally coupled buildings, particularly

existing buildings, under bilateral earthquake excitations, by incorporating the

vibration control effectiveness of passive tuned mass dampers (PTMDs).

Some practical design issues such as the optimal location for installation,

movement direction and numbers of PTMD are considered in this study.

A tuned mass damper (TMD) is suppressing wind-induced

excitation motion of a tall building. According to the experimental and

analytical investigation, the effectiveness of the TMD increased when the

inherent structural damping was decreased in response to both along-wind and

the cross-wind directions. The parametric study revealed that a certain TMD

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damping ratio can maximize the effectiveness of the TMD (Young-Moon

Kim et al 2007).

An arrangement of tuned mass dampers, termed coupled tuned

mass dampers (CTMDs) has been introduced, where a mass is connected by

translational springs and viscous dampers in an eccentric manner. The CTMD

has coupled modes of lateral and rotational vibration, which have been

utilized to control coupled lateral and torsional vibrations of asymmetric

buildings. An efficient control strategy has been presented in this context to

control displacements as well as acceleration responses of asymmetric

buildings having asymmetry in both plan and elevation (Nagendra Babu Desu

et al 2007).

2.3 ACTIVE CONTROL

Active control systems consists of mechanisms (actuators) powered

by energy sources. The actuator is an integral component of the control

system, which generates and applies the control forces at specific locations on

the structure according to instructions from the controller (Connor 2003).

These devices are able to push the structure to counteract the input forces.

Studies into active control of civil engineering structures have

flourished since its introduction to the field by Yao (1972). For approximately

three decades, researchers have investigated the possibility of using active

control methods to improve upon passive approaches to reduce structural

responses. Active structures, that reacts to changes in their environment

(loads, movements, etc.) through performing specific tasks. These tasks are

i) measurement of behavior, ii) evaluation using knowledge bases, iii) use of

computational control to modify structural characteristics and iv) use of past

events to improve performance. (Ian F.C. Smith et al 2000). Unlike passive

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control systems, active control systems have the ability to control different

vibration modes and accommodate different loading conditions, such as

pulse-type loadings. “Properly designed optimal active control systems are

highly effective in reducing peak structural vibrations during earthquakes”

(Marzbanrad 2004).

A verity of active control mechanism have been suggested by the

researches, some of them are (1) active tendon system (2) active bracing

system (3) active tuned mass damper and (4) active aerodynamic appendage

mechanism.

2.4 SEMI-ACTIVE CONTROL

Semi active control system is a system which requires a small

power source i.e a battery for operation and utilizes the motion of the

structure to develop the control forces, the magnitude of which can be

adjusted by the external power source. Control forces are developed based on

the feedback from sensors that measures the excitation and/or the response of

the structure which may be measured at location remote from the location of

the semi active control system. Semi active controllers combine the desirable

features of both active and passive control systems.

Semiactive control systems combine the following features of

active and passive control to reduce the response of structures to various

dynamic loadings: (1) active variable stiffness, where the stiffness of the

structure is adjusted to establish a nonresonant condition between the

structure and excitation; and (2) active variable damper, where the damping

coefficient of the device is varied to achieve the most reduction in the

response (Fahim Sadek and Bijan Mohraz 1998).

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2.5 HYBRID CONTROL

Hybrid control system is typically defined as one which employs a

combination of two or more passive or active devices. The hybrid control

system consists of a base-isolation system connected to either a passive or

active mass damper. The base-isolation system, such as elastomeric bearings,

is used to decouple horizontal ground motions from the building; whereas the

mass damper, either active or passive, is used to protect the safety and

integrity of the base-isolation system (Yang et al 1991). Because multiple

control devices are operating, a hybrid control system could alleviate some of

the restrictions and limitations that exist when each system is acting alone

(Kyu-Sik Park et al 2003).

A combined use of active and passive control systems, referred to

as the hybrid control system, is more effective, beneficial, and practical, in

some cases, for reducing the building response under strong earthquakes than

an active or passive system used alone. However, the use of hybrid control

systems involves active control of nonlinear or hysteretic structural systems.

Yang et al (1992) presented a refined version of the instantaneous optimal

control algorithms for nonlinear or hysteretic structural systems. The main

advantage of the proposed algorithms is that the control vector is determined

directly from the measured response state vector without the necessity of

tracking a time-dependent system matrix, as suggested previously.

Satish Nagarajaiah et al (1993) presented an experimental and

analytical study of hybrid control of bridges using sliding bearings, with

recentering springs, in parallel with servo hydraulic actuators. A new control

algorithm with absolute acceleration feedback, based on instantaneous optimal

control laws, is developed. The developed control algorithm is implemented

in a shake-table study of an actively controlled sliding-isolated bridge.

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Hybrid mass damper (HMD) system was developed to reduce

building response during strong winds and earthquakes of up to medium

strength. The HMD consists of an auxiliary mass supported by multi-stage

rubber bearings and actuators driven by AC servomotors. Tomoo Saito et al

(2000) presented the dynamic characteristics of both the building and the

HMDs, through forced vibration tests using the HMD system. The

effectiveness of the HMD system was confirmed by these tests and both wind

and earthquake observation data.

Ichiro Nagashima and Yuzo Shinozaki (1998) studied a systematic

design procedure and an algorithm variable gain feedback (VGF) control of

buildings using active mass damper (AMD) systems. The limit of the stroke

length of the auxiliary mass, which was considered to be one of the most

important physical constraints for application of the AMD systems to the

actual structures.

A hybrid mass damper system with convertible active and passive

modes using a hydraulic actuator (hereinafter referred to as APMD) has been

installed to an actual slender tall building and observation of behaviors

against moderate wind or some earthquake excitations were carried out to

investigate the response control performance of the mass damper system.

From the analysis, it was confirmed that on the active mode due to the

vibration tests, the damping factors in the 1st mode were about

8-11 per cent, the satisfactory vibration control effect was obtained, and no

spillover in the second mode occurred (Morimasa Watakabe et al 2001).

A large-scale hybrid mass damper (HMD) system was developed to

reduce building response during strong winds and earthquakes of up to

medium strength. The HMD consists of an auxiliary mass supported by

multi-stage rubber bearings and actuators driven by AC servomotors. Two

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HMDs were installed on the top floor of a target building to suppress both

translational and torsional vibration. The system was installed in two high-rise

buildings. One was a 50-storey, 200 m high, steel-frame building. The

dynamic characteristics of both the building and the HMDs were identified

through forced vibration tests using the HMD system. The effectiveness of the

HMD system was confirmed by these tests and both wind and earthquake

observation data (Tomoo Saito et al 2001).

A hybrid mass damper (HMD) using an electric servomotor for

vibration control of buildings has been installed in three buildings in order to

improve habitability during strong winds and small-to-moderate earthquakes

and speculated that its high performance, compactness, and high reliability are

important factors in its practical application to actual buildings (Nakamura

et al 2000).

A large-scale hybrid mass damper (HMD) system was developed to

reduce building response during strong winds and earthquakes of up to

medium strength. The HMD consists of an auxiliary mass supported by multi-

stage rubber bearings and actuators driven by AC servomotors. The

effectiveness of the HMD system was confirmed by these tests and both wind

and earthquake observation data (Tomoo Saito et al 2001).

Ahlawat and Ramaswamy (2004) presented the third generation

benchmark control problem for seismically excited nonlinear buildings is an

effort to evaluate the developed control strategies in order to apply them in

field applications. As the fuzzy logic control systems have been applied

effectively in various fields, including vibration control of structures, a

multiobjective optimal fuzzy logic control system has been proposed.

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Classical control algorithms such as the linear quadratic regulator

(LQR) and linear quadratic Gaussian (LQG) algorithms, used for structural

control problems suffer from a number of fundamental shortcomings. They

are susceptible to parameter uncertainty and modeling error. Hongjin Kim

and Hojjat Adeli (2004) developed a hybrid feedback-least mean square

(LMS) algorithm is presented for control of structures through integration of a

feedback control algorithm such as the LQR or LQG algorithm and the

filtered-x LMS algorithm. The algorithm is applied to the active tuned mass

damper system. It is shown that the hybrid feedback-LMS algorithm

minimizes vibrations over the entire frequency range and thus is less

susceptible to modeling error and inherently more stable.

He and Agrawal (2004) proposes an innovative hybrid control

system consisting of a passive fluid viscous damper installed in parallel to a

semi-active friction damper for applications to structures subject to near-field

earthquakes. The hybrid control system combines the best features of both

control devices and possesses the great advantage that it is capable of quickly

responding to external excitations.

Control of irregular highrise building structures under various

seismic excitations is investigated using a hybrid control system consisting of

a passive supplementary damping system and a semi-active tuned liquid

column damper (TLCD) system. Equations of motion for the combined

building and the TLCD system are derived for multistorey building structures

with rigid floors and plan and elevation irregularities. Major steps involved in

optimal control of three-dimensional irregular buildings equipped with a

hybrid damper-TLCD system are delineated (Hongjin Kim and Hojjat Adeli

2005).

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The effectiveness of passive dampers and a hybrid control system

consisting of passive viscous dampers installed in parallel with semi-active

dampers is investigated by Wan-Long He and Agrawal (2005) and found

hybrid control system not only reduces response quantities, but also protects

passive dampers by reducing force demand on passive dampers during very

strong earthquakes.

2.6 SUMMARY

In comparison with passive systems, the research and development

of active control strategies is more recent. The advantages typically cited for

active control system are: enhanced effectiveness in motion control depending

upon the capacity of the system, applicability to multi-hazard applications and

selectivity of control objectives.