vibration control devices for civil structures

5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES

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Department of CE, GEC, Thrissur

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1. INTRODUCTION

In recent years, due to developments in design technology and material

qualities in civil engineering, the structures (high-rise building and long-span

bridges) become more light and slender. This has caused the structures to be

subjected to series structural vibrations when they are located in environments

prone to earthquakes or high winds. These vibrations may lead to serious structural

damage and potential structural failure.

Structural control is a diverse field of study. Structural control is one area

of current research that looks promising in attaining reduce structural vibrations

during loadings such as earthquakes and strong winds. The reduction of structural

vibrations occurs by adding a mechanical system that is installed in a structure.

The concept of employing structural control to minimize structural

vibration was proposed in the 1970s. Structural control based on various passive,

active, hybrid and semi-active control schemes offers attractive opportunities to

mitigate damage and loss of serviceability caused by natural hazards such as

earthquakes and hurricanes.

2. BUILDINGS RESPONSE TO EARTHQUAKE

2.1 Dynamic Characteristics

2.1.1 Bui lding f requency and period

To begin with the magnitude of the building response--that is, the

accelerations which it undergoes-- depends primarily upon the frequencies of the

input ground motion and the building's natural frequency. When these are near or

equal to one another, the building's response reaches a peak level.When the

frequency contents of the ground motion are around the building's natural

frequency, it is said that the building and the ground motion are in resonance with

one another. Resonance tends to increase or amplify the building's response.

Because of this, buildings suffer the greatest damage from ground motion at a

frequency close or equal to their own natural frequency. In some circumstances,

this dynamic amplification effect can increase the building acceleration to a value

two times or more that of the ground acceleration at the base of the building.

Generally, buildings with higher natural frequencies, a short natural period, tend to

suffer higher accelerations but smaller displacement. In the case of buildings with


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lower natural frequencies, a long natural period, this is reversed: the buildings will

experience lower accelerations but larger displacements.

2.1.2 Ductil ity

Ductility is the ability to undergo distortion or deformation (bending, for

example), without resulting in complete breakage or failure. One of the primary

tasks of an engineer designing a building to be earthquake resistant is to ensure that

the building will possess enough ductility to withstand the size and types of

earthquakes it is likely to experience during its lifetime.

2.1.3 Damping

Damping is due to internal friction and the absorption of energy by the

building's structural and non-structural elements. All buildings possess some

intrinsic damping. The more damping a building possesses, the sooner it will stop

vibrating (which of course is highly desirable from the standpoint of earthquake

performance). Today, some of the more advanced techniques of earthquake

resistant design and construction employ added damping devices like shock

absorbers to supplement artificially the intrinsic damping of a building and so

improve its earthquake performance.

Viscosity damping ratios of different construction materials are

Building Damping

Construction Type

Viscous Damping Ratio

Min. Mean Max.

Tall Buildings(h>~100

m)

Reinforced concrete

Steel

0.010

0.007

0.015

0.010

0.020

0.013

Buildings ( h ~ 50 m)

Reinforced concrete

Steel

0.020

0.015

0.025

0.020

0.030

0.025

Table 2.1.3 damping level in buildings


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The damping ratio is a dimensionless measure describing how oscillations in a

system decay after a disturbance. The damping ratio is a measure of describing

how rapidly the oscillations decay from one bounce to the next.

2.2 Response Spectra

The response spectrum is a plot of the maximum response of displacement,

velocity, acceleration or any other quantity of interest to a specified load function

for all possible single degree of freedom systems.

Different buildings can respond in widely differing manners to the same

earthquake ground motion. Conversely, any given building will act differently

during different earthquakes, which gives rise to the need of concisely representing

the building's range of responses to ground motion of different frequency contents.

Such a representation is known as a response spectrum. Response spectra are very

important "tools" in earthquake engineering.

Fig.2.2(a) shows a highly simplified version of a response spectrum. Even

though highly simplified, it does show how building response characteristics vary

with building frequency and period: as building period lengthens, accelerations

decrease and displacement increases. On the other hand, buildings with shorter

periods undergo higher accelerations but smaller displacements. The amount of

acceleration which a building undergoes during an earthquake is a critical factor in

determining how much damage it will suffer.


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Fig.2.2(a) Simplified Response Spectra

A response spectrum is used to provide the most descriptive representation

of the influence of a given earthquake on a structure or machine. If the ground

acceleration from an earthquake is known, the response of the structure can be

computed. Therefore, a response spectrum can be generated for that earthquake.

Maximum relative displacement, velocity, and total accelerations are found out.

Time-histories of ground accelerations from different earthquakes are quite

different; the resulting spectra will also be very different. We generate earthquake

design spectra by averaging spectra from past earthquakes to design structures to

resist earthquakes.

Fig2.2(b) Design response spectra


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3.VIBRATION CONTROL

3.1 Model of Simple Vibration Absorber

Model for simple vibration absorber consist of the two masses m1 and m2.

Here model shown in the fig is undamped two degree of freedom system excited

with a sinusoidal component of f=F0sin(wt). In the fig.3.1 m1stands for the mass

of building, m2stands for the mass of vibration absorber. k1and k2are the stiffness

coefficient of the structure and vibration absorber. And the equation can be given

as under

m111+k1x1+k2(x1-x2) = f (1)

m222 +k2(x2-x1) = 0 (2)

Fig.3.1 Model for the Analysis of Vibration Absorber

The magnitude of the frequency response is obtained from the following

equations:

As structure is excited by f=F0sin(w

t) put x1=X1sin(w

1t);x2=X2sin (w

2t);


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-m1w12X

1+ (k1+k2)X1-k2x2=F0(3)

-k2X1-w22m2X2+k2X2=0 (4)

When the forcing frequency w is equal to the natural frequency of the vibration

absorber (i.e.w2=k2/m2), we get

X1=0

X2= -F0/K2 (putting w2=k2/m2 in equation 3 and 4)

Therefore, the motion of the main mass is ideally zero and the spring force

of the absorber is at all times equal and opposite to the applied force, F0. Hence

no force is transmitted to the supporting structure.

3.2. Vibration control devices

The control of structural vibrations produced by earthquake or wind can be

done by various means such as modifying rigidities, masses, damping, or shape,

and by providing passive or active counter forces. Structural control methods that

can be used include

1- Passive control systems.

2- Active control systems.

3- Semi-active control systems.

3.2.1 passive control system

A passive control system does not require an external power source.

Passive control devices impart forces that are developed in response to the motion

of the structure. The passive control devices cannot increase the energy in a

passively controlled structural system, including the passive devices. The concept

of a tuned mass damper (TMD) as an added energy-absorbing system dates to

1909. Much analysis in vibration has related to the use of TMD (or vibration

absorbers) in mechanical engineering systems. Robert J. McNamara studied the

TMD as an energy-absorbing system to reduce wind-induced structural response of


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buildings in the elastic range behaviour.

A tuned liquid damper (TLD) is a special class of TMD where the mass is

replaced by liquid (usually water). The sloshing of the liquid mimics the motion of

the TMD mass. Tuned liquid column dampers (TLCDs) are a special type of TLDs

relying on the motion of the liquid column in a U-tube to counteract the forces

acting on the structure, with damping introduced in the oscillating liquid column

through an orifice

In order to achieve better protection for the bridge subjected to strong

vertical ground motion, helical springs are used as shock absorbers with fluid

dampers as energy dissipaters. They concluded that the response of acceleration in

an isolated damped bridge model, particularly at the mid-span, has been greatly

reduced up to 75% compared to the non-isolated case. The damping level of a

structural system isolated by fluid dampers could be over 20% with more energy

absorbed, offering a dramatic reduction in deflection at no cost of increase in base

shear. Also they noted that extra damping becomes less efficient at higher damping

levels.

However, a passive control system has limited ability because it is not able

to adapt to structural changes or varying usage patterns and loading conditions. To

overcome these shortcomings, active, and semi-active controls can be used.

Advantages

1) It can be easily installed2) selection of material of damping is easy as the characteristics of various

damper materials are well known and have been scientifically researched for

decades

3) there is no moving parts4)

it can be easily replaced

Disadvantages

1) performance of passive dampers are limited on to limited frequency band.

3.2.2 active contr ol device

An active control system is one in which an external source powers control

actuator(s) that apply forces to the structure in a prescribed manner. These forces

can be used to both add and dissipate energy in the structure. In an active feedback


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control system, the signals sent to the control actuators are a function of the

response of the system measured with physical sensors (optical, mechanical,

electrical, chemical, and so on). The generation of control forces by actuator

requires large power sources , which are on the order of ten kilowatts for small

structures and may reach several megawatts for large structures.

The primary effect of some experimentally tested active control system has

been to modify the level of damping with a minor modification of stiffness. An

overview of active structural control is provided by Spencer et al. He discussed

frequency domain optimal control strategies for active control of civil engineering

structures under seismic loading. They reported that, in contrast to previously

reported time domain based controllers; the numerical studies show that these

control techniques are capable of reducing the buildings response in both the first

and second mode response using an active mass damper. They also concluded that

the frequency domain optimal control design methods are flexible and offer a good

match between control concepts and engineering practice.

Different active control devices are: the active mass driver system (AMD),

the active tendon system and the active bracing system. The control forces can be

used to both add and dissipate energy in the structure. The control forces within the

framework of an active control system are generated by wide variety of actuators

that can act hydraulic, pneumatic, electromagnetic .piezoelectric or motor driven

ball screw actuation. The controller (e.g. computer)is a device that receives signals

from the response of the structures measured by physical sensors(within active

control using feedback) and that on basis of a predetermined control algorithm

compares the received signal with a desired response and used the error to generate

a proper control signal. The control signal is then sent to actuator. In feed-forward

control, the disturbance, not the response, is measured and used to generate the

control signals. Both the feedback and feed-forward principles can be used together

in the semi active control systems.

Since active control relies on external power, which requires routine

maintenance and thus may become potentially unstable, semi-active control have

been studied by many researchers. It combines active and passive control systems

and attempts to utilize the advantages of both methods to achieve better effects.


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Advantages

1) Significant control of vibration by imposing force on the structure2) It can be used in wide range of frequencies.

Disadvantages

1) It has got lot of moving parts2) Utilisation of high amount of input energy which may not be available at

the time of vibration occurs.

3.2.3 semiactive contr ol system

Semi-active control systems combine the features of active and passive

control to reduce the response of structures to various dynamic loadings. Semi-

active control systems are a class of active control systems for which the external

energy requirements are orders of magnitude smaller than typical active control

systems.

Typically, semi-active control devices do not add mechanical energy to the

structural system (including the structure and the control actuators), therefore

bounded-input bounded-output is guaranteed. Semi-active control devices are often

viewed as controllable passive devices.

Structures typically dissipate energy from extreme dynamic events by

allowing damage to the structure. Semi-active control provides supplemental

damping to more efficiently dissipate energy due to dynamic loads preserving the

primary structure.

Semi-active control systems include: (1) active variable stiffness, where the

stiffness of the structure is adjusted to establish a non-resonant condition between

the structure and excitation; and (2) active variable damper, where the dampingcoefficient of the device is varied to achieve the most reduction in the response.

As it has seen that new trends are more concentrates on the use of semi

active controlling device. Hence our discussion is more tends on the different

consideration in semi active device. Here MR dampers are explained in details.


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Advantages

1) Small size2) Few moving parts3) Reacts dynamically to a number of vibration frequencies

Disadvantages

1) Relatively low amount of use. This is mainly due to the fact that it isquite a new solution in the market and not very widely researched yet.

4. MAGNETO RHEOLOGICAL DAMPERS

There has been a great deal of interest in recent years in use of magneto

rheological (MR) dampers for semi-active structural control. The advantages of

using such devices include low power requirements, high reliability, ensured

stability of the control system, and higher force capacities in comparison to other

types of damping devices.

The study on the application of a MR damper for semi-active control of

bridges is conducted by a series of cyclic loading tests and shaking table tests. It

was concluded that the MR damper can be idealized with good accuracy by the

model friction and viscous elements in parallel. Correlative study was conducted

on a bridge model with the MR damper under the control algorithms represented

by the analysis with good accuracy. Magneto-rheological fluid (MRF) dampers are

also utilized to control vibration of a scaled, two-span bridge. In this work, the

focus is on a combination of passive and semi-active damping capabilities of a

bridge.

Magneto rheological fluid dampers use magneto rheological fluids thus this

can be discussed in detail as

4.1 Magneto rheological fluids

MR fluids are the magnetic analogs of electro rheological fluids and

typically consist of micron-sized, magnetically polarizable particles dispersed in a

carrier medium such as mineral or silicone oil. When a magnetic field is applied to

the fluids, particle chains form, and the fluid becomes a semi-solid and exhibits

viscoelastic behaviour. Transition to rheological equilibrium can be achieved in a

few milliseconds, allowing construction of devices with high bandwidth. MR

fluids can operate at temperatures from 40 to 150o C with only slight variations


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in the yield stress. Moreover, MR fluids are not sensitive to impurities such as are

commonly encountered during manufacturing and usage, and little particle/carrier

fluid separation takes place in MR fluids under common flow conditions. Further a

wider choice of additives (surfactants, dispersants, friction modifiers, anti-wear

agents, etc.) can generally be used with MR fluids to enhance stability, seal life,

bearing life, etc., since electro-chemistry does not affect the magneto-polarization

mechanism. The MR fluid can be readily controlled with a low voltage (e.g., ~12

24V), current-driven power supply outputting only ~12 amps.

A magneto rheological fluid (MR fluid) is a type of smart fluid in a carrier

fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly

increases itsapparentviscosity, to the point of becoming aviscoelastic solid.

Importantly, the yield stress of the fluid when in its active ("on") state can be

controlled very accurately by varying the magnetic field intensity. The upshot of

this is that the fluid's ability to transmit force can be controlled with

anelectromagnet, which gives rise to its many possible control-based applications.

MR fluid is different from aFerro fluid which has smaller particles. MR fluid

particles are primarily on themicro meter-scale and are toodense forBrownian

motion to keep them suspended (in the lower density carrier fluid).Ferro

fluidparticles are primarilynanoparticles that are suspended byBrownian

motionand generally will not settle under normal conditions. As a result, these two

fluids have very different applications.MR Fluids are non-colloidal suspensions of

magnetisable particles that are of the order of tens of 20-50 m in diameter. MR

devices are capable of much higher yield strengths when activated. The main

difference between Ferro fluids and MR fluids is the size of the polarizable

particles. In Ferro fluids, these particles are an order of magnitude smaller than

MR Fluids that is they are 1-2 m, incontrast to 20-50 m for MR fluids.

MR Fluid is composed of oil, usually mineral or silicon based, and varying

percentages of ferrous particles that have been coated with an anti-coagulant

material. Engineering notes by Lord Corporation have reported that when

inactivated, MR Fluid displays Newtonian-like behaviour when exposed to a

magnetic field, the ferrous particles that are dispersed throughout the fluid form
http://en.wikipedia.org/wiki/Apparent_viscosityhttp://en.wikipedia.org/wiki/Viscoelastichttp://en.wikipedia.org/wiki/Ferrofluidhttp://en.wikipedia.org/wiki/Micrometrehttp://en.wikipedia.org/wiki/Densehttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Ferrofluidhttp://en.wikipedia.org/wiki/Ferrofluidhttp://en.wikipedia.org/wiki/Nanoparticleshttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Nanoparticleshttp://en.wikipedia.org/wiki/Ferrofluidhttp://en.wikipedia.org/wiki/Ferrofluidhttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Brownian_Motionhttp://en.wikipedia.org/wiki/Densehttp://en.wikipedia.org/wiki/Micrometrehttp://en.wikipedia.org/wiki/Ferrofluidhttp://en.wikipedia.org/wiki/Viscoelastichttp://en.wikipedia.org/wiki/Apparent_viscosity


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magnetic dipoles. These magnetic dipoles align themselves along lines of

magnetic flux, as shown in Fig.4.1

Fig 4.1(a) Dipole alignment of ferrous particles

(Reference: A paper on design fabrication and evaluation of MR dampers

presented by A Ashfak and A Saeed at world academy of science and technology)

Fig.4.1(a) shows Dipole alignments of ferrous particles On a larger scale,

this reordering of ferrous dipole particles can be visualized as a very large number

of microscopic beads that are threaded onto a very thin string as is shown in Fig.

below.

Fig 4.1(b) String and beads analogy of MR fluids[2]


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One can picture this thin string stretching from one magnetic pole to the other and

perpendicular to each paramagnetic pole surface.

4.1.1 Str ing and beads analogy of activated MR f luid

In this analogy, the spherical beads represent iron particles and each string

represents a single flux line. One can picture many of these strings of beads placed

closely together much like the bristles of a toothbrush. Once aligned in this

fashion, the ferrous particles resist being moved out of their respective flux lines

and act as a barrier to fluid flow. Typically, MR fluids can be used in three

different ways, all of which can be applied to MR damper design depending on

the dampers intended use. These modes of operation are referred to as squeeze

mode, valve mode and shear mode.

4.2 Types of MR dampers

There are three main types of MR dampers. These are the mono tube, the

twin tube, and the double-ended MR damper. Of the three types, the mono tube is

the most common since it can be installed in any orientation and is compact in

size. A mono tube MR damper, shown in Fig.5.3, has only one reservoir for the

MR fluid and an accumulator mechanism to accommodate the change in volume

that results from piston rod movement. The accumulator piston provides a barrier

between the MR fluid and a compressed gas (usually nitrogen) that is used to

accommodate the volume changes that occur when the piston rod enters the

housing.

Fig 4.2(a) Mono tube MR dampers[2]


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The twin tube MR damper is one that has two fluid reservoirs, one inside

of the other, as shown in Fig. 5.4. In this configuration, the damper has an inner

and outer housing. The inner housing guides the piston rod assembly, in exactly

the same manner as in a mono tube damper. The volume enclosed by the inner

housing is referred to as the inner reservoir. Likewise, the volume that is defined

by the space between the inner housing and the outer housing is referred to as the

outer reservoir. The inner reservoir is filled with MR fluid so that no air pockets

exist.

Fig 5.4 Twin tube MR dampers[2]

To accommodate changes in volume due to piston rod movement, an outerreservoir that is partially filled with MR fluid is used. Therefore, the outer tube in

a twin tube damper serves the same purpose as the pneumatic accumulator

mechanism in mono tube dampers. In practice, a valve assembly called a foot

valve is attached to the bottom of the inner housing to regulate the flow of fluid

between the two reservoirs. As the piston rod enters the damper, MR fluid flows

from the inner reservoir into the outer reservoir through the compression valve,

which is part of the foot valve assembly. The amount of fluid that flows from the

inner reservoir into the outer reservoir is equal to the volume displaced by the

piston rod as it enters the inner housing. As the piston rod is withdrawn from the

damper, MR fluid flows from the outer reservoir into the inner reservoir through

the return valve, which is also part of the foot valve assembly. The final type of

MR damper is called a double-ended damper since a piston rod of equal diameter

protrudes from both ends of the damper housing. Fig. 9 shows a section view of a

typical double-ended MR damper. Since there is no change in volume as the


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piston rod moves relative to the damper body, the double-ended damper does not

require an accumulator mechanism. Double-ended MR dampers have been used

for bicycle applications gun recoil applications, commercial applications and for

controlling building sway motion caused by wind gusts and earthquakes.

Fig 5.5 Double ended MR dampers[2]

4.3 MR damper mathematics

MR fluid behaves in two distinct modes: off state and activated state. While

Newtonian like behaviour is common in the off state, the fluid behaves as a

Bingham plastic with variable yield strength when activated. Though the fluid

does have the departures from this model, this gives a good reference as to the

behaviour of the fluid . The shear stress associated with the flow of MR fluid can

be predicted by the Bingham equations

= y(B ) + , >y (1)

In equation (1), is the fluid shear stress,yis the fluids yield stress at a given

magnetic flux density B, is the plastic viscosity(i.e. viscosity at B=0), and is

the fluid shear rate. This equation holds for fluid stresses above the field

dependent yield stress. However, for fluid stresses below y, the MR fluid behaves

as a visco-elastic material:

=G ,


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the fluid. Pressure driven flow mode has two components to the pressure drop:

pressure loss due to viscous drag, and pressure loss due to the field dependent

yield stress, as shown in equation (3)

P =Pn +Pt

=12QL/ g w+ c L/g (3)

In equation (3), P is the total pressure drop, P is the viscous pressure loss,

P is the field dependent yield stress pressure loss, is the fluid viscosity, Q is

the flow rate, L is the pole length, w is the pole width, g is the fluid gap, and yis

the field dependent yield stress. Many of these dimensions are illustrated in Fig.

below. The variable changes from a minimum value of 2 (for P/P


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are taken out through the hallow piston rod. The configuration is simple and easy

to manufacture. The design involves both magnetic circuit designs along with

previously mentioned mechanical design. The design also based upon type of MR

fluid used in the damper. Fig below illustrates the conceptual design of the MR

damper. Spool of magnet wire, Shown with the vertical hash marks, generate

magnetic flux within the steel piston. The flux in the magnetic circuit flows

axially through the piston core of diameter Dc, beneath the winding, radially

through the piston poles of length Lp, through a gap of thickness tg, in which the

MR fluid flows, and axially through the cylinder wall of thickness t w. Our MR

damper design involves six different physically dimensioned parameters. They are

the diameter of the cylinder bore, Db, the diameter of the piston rod, Dp, the

thickness of the casing wall, tw, the diameter of the piston core, Dc, the inside

piston diameter, Dh, the pole length, Lp and the thickness of the gap, tg.

Fig 5.7 Design of MR dampers [2]

4.5 testing and analysis

Testing of MR dampers is done for the analysing the efficiency.Fig.5.8 shows the

variation of force with time at different applied voltage for typical MR dampers.

Fig.8 shows the equivalent damping coefficients vs. voltage. As the voltage

increases the damping force increases for the constant interval of time. Fig.9

shows the variation of force versus displacement of the damper. Fig.10 shows the


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variation of force versus velocity. These plots show that the damping force is

very low for zero current and it increases gradually as the current is increased.

Also the yield stress part of the damping force dominates the viscous force. This

means we have very good control over the damping force, which is necessary for

semi-active control. Also the controllable force is not zero at zero current which

means the yield stress is never zero.

Fig 5.8 Force vs. time [2]


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Fig 5.9Equivalent damping coefficients vs. voltage[2]

Fig 5.9 Force vs displacement [2]


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Fig 5.10. Force vs velocity[2]

5. CONCLUSION

A review of various vibratory control devices has been made.

Building response to the dynamic vibration is discussed. Different dynamic

characteristics of building such as building frequency and period ,ductility and

damping response were discussed. Model of simple vibration absorber is

considered and the theory involved in the vibration absorber is noted for a

particular excitation. There are three different type of vibratory control devices as

passive, active and semi active. Passive doesnt use any external energy for its

function. In active control devices external energy are used. As during earthquake

power failure is common, this could limit the use of active devices. Thus semi

active devices come into use as it combines the action of both active and passive

devices to reduce the response of structures to various dynamic loadings. Semi-

active control systems are a class of active control systems for which the external

energy requirements are orders of magnitude smaller than typical active control


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systems. Magneto rheological dampers are the commonly used semi active device

which gives a good result as the semi active vibratory control device. MR dampers

contain magneto rheological fluids. Viscoelastic characteristics of MR fluids are

discussed. MR fluids contains ferrous particles and it align under magnetic field to

give its particular property. Different designs involved in MR dampers are

discussed.


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REFERENCE

[1]. Mario Paz, Structural dynamics theory and computation second

edition,CBS publishers.

[2]. A Ashfak and A Saeed , A paper on design fabrication and evaluation of

MR dampers presented at world academy of science and technology.

[3]. Aly Mousaad Aly, A thesis on vibration control in structures due to

earthquake effects using MR damper, submitted to the Department of

Mechanical Power Engineering at Alexandria university.

[4]. Kerla A Villarreal , paper on effects of MR dampers on structural

vibration parameter, dept. of civil and environmental engg .FAMU FSU

college of engineering, host institution Tokyo university.

vibration control devices for civil structures

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