-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
1/
Department of CE, GEC, Thrissur
1
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
2/
Department of CE, GEC, Thrissur
2
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
3/
Department of CE, GEC, Thrissur
3
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.
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
4/
Department of CE, GEC, Thrissur
4
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
5/
Department of CE, GEC, Thrissur
5
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);
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
6/
Department of CE, GEC, Thrissur
6
-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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
7/
Department of CE, GEC, Thrissur
7
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
8/
Department of CE, GEC, Thrissur
8
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.
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
9/
Department of CE, GEC, Thrissur
9
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.
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
10
Department of CE, GEC, Thrissur
10
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
11
Department of CE, GEC, Thrissur
11
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
12
Department of CE, GEC, Thrissur
12
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]
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
13
Department of CE, GEC, Thrissur
13
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]
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
14
Department of CE, GEC, Thrissur
14
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
15
Department of CE, GEC, Thrissur
15
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 ,
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
16
Department of CE, GEC, Thrissur
16
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
17
Department of CE, GEC, Thrissur
17
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
18
Department of CE, GEC, Thrissur
18
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]
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
19
Department of CE, GEC, Thrissur
19
Fig 5.9Equivalent damping coefficients vs. voltage[2]
Fig 5.9 Force vs displacement [2]
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
20
Department of CE, GEC, Thrissur
20
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
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
21
Department of CE, GEC, Thrissur
21
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.
-
5/26/2018 VIBRATION CONTROL DEVICES FOR CIVIL STRUCTURES
22
Department of CE, GEC, Thrissur
22
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.