Earthquake Mitigation Using Active Dampers

8/7/2019 Earthquake Mitigation Using Active Dampers

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Earthquake mitigation: Using

Active dampers & other devices

Supervised under,Prof. Girish Kumar

Presented by,

Vidushi Jain(05004011)


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Abstract

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. Flexible structures may fall victim to excessive levels of vibration underthe action of wind, earthquake, adversely affecting service ability and occupant comfort. To

ensure the functional performance of flexible structures, various design modifications are

possible, ranging from alternative structural systems to the utilization of passive and active

control devices. This paper presents an overview of state-of-the-art measures to reduce

structural response of buildings using active control devices.

Introduction

Natural Disasters & Calamities like earthquake causes widespread destruction, deaths and

damage to property and infrastructure We can not avoid such natural calamities but by

monitoring their upcoming and subsequently implementing effective measures, we can reduce

their vicious effect. Engineering structures like dams, bridges and sky high buildings are the

key factors of development. The race toward new heights has not been without its challenges.

Despite engineers put all their efforts to ensure safety of a structure, some time these

structures undergo large deformation due to calamities like earthquake which results in their

collapse and loss of lives and property

As a measure of earthquake mitigation we apply following way: changing rigidity, mass,

damping, shape, or applying passive or active control forces. Following are the facts related to

various measures adopted by different countries in the direction of earthquake mitigation:

> 20 full scale active control appl. in Japan

Passive base isolation used in USA.

Recently rapid developments have been occurring in the area of controlled civil

structures, including full-scale implementations, actuator types and characteristics, and

trends toward the incorporation of more modern algorithms and technologies

The use of control and automation has proven to be very successful in mitigating the

effects of dynamic forces like earthquake and wind forces

Structure is considered as a system by identifying the equations governing the dynamic

properties of the system. On the basis of mathematical modeling of the system is done

Following is system Analysis terminology

System - Plant or Process

Excitation - Input signal

Response - Output signal


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Mathematical model of structure

A simple building can be converted into a spring mass system as follows:

In the simplified model, the masses correspond to slab masses and stiffness correspond to

column stiffness (i.e, the force required per unit lateral displacement of column)

Uncontrolled system

Controlled system

Open-loop


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Damping Sources

An increase in the effective damping of a structure, accomplished by any of the four major

sources of damping: structural, aerodynamic, soil, and auxiliary, will also lead to decreased

structural motion. Structural damping is limited to the damping already available inherently inthe materials: steel, concrete, or their composite. At times, aerodynamic damping may also

contribute in the along wind direction, depending on the wind velocity, structural shape, and

building dynamic characteristics. However, the contribution in the across wind direction is

negligible and may even become adverse at higher wind speeds, though the presence of

adjacent structures may introduce different effects. Although not marked for high rise

buildings, damping contributions may also be obtained from the soil-foundation interaction,

i.e. soil damping. Unfortunately, these three forms of damping make only limited

contributions. In addition, the damping in the structure cannot be engineered like the mass and

stiffness properties of the structure, nor can it be accurately estimated until the structure is

completed, resulting a certain level of uncertainty (Kareem & Gurley 1996). In cases where

the inherent damping is not sufficient, auxiliary damping devices may be introduced, offeringa somewhat more predictable, adaptable, and reliable method of imparting additional damping

to a system.

Various types of controlled system

Active Structural Control: Active control systems operate by using external energy

supplied by actuators to impart forces on the structure.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 dissipateenergy in the structure. In an active feedback control system, the signals sent to the

control actuators are a function of the response of the system measured with physical

sensors (optical, mechanical, electrical, chemical, and so on.

Active control:

Active control systems are those that employ large actuators that directly input forces into the

structure in an attempt to limit structural deflections.

The goal is to design a control system to keep stresses/strains/displ./accel. (calledoutputs) at certain locations below specified bounds (peak, rms) when disturbances

(wind, earthquake) below specified bound are applied.

Designer decides choice of outputs based on comfort (e.g. accelerations) and safety

(e.g. stresses).

Hybrid Structural Control Systems: A hybrid control system is typically defined as

one which employs a combination of two or more passive or active devices.The

common usage of the term "hybrid control" implies the combined use of active and

passive control systems. For example, a structure equipped with distributed visco-

elastic damping supplemented with an active mass damper on or near the top of the


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structure, or a base isolated structure with actuators actively controlled to enhance

performance.

Hybrid control:

Uses active & passive devices. Advantages of both active and passive systems are present and their limitations are

reduced.

Essentially an active control system

Examples: viscous damping with AMD, base isolation with actuators, TMD+AMD).

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

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

the structure. The energy in a passively controlled structural system, including the

passive devices, cannot be increased by the passive control devices.

Passive control:

No external power is required for these systems

They work on simple mass stiffness relationship

Here a mass is provided with the mass of building.

Passive control device (TMD, Base Isolator) imparts forces that are developed directly

as a result of motion of structure (i.e., no actuator involved).

Total energy (structure + passive device) cannot increase, hence inherently stable. Relatively inexpensive.

Reliable during earthquake

Not as effective as active, hybrid, semi-active control.

Types of passive control devices

Metallic Device:-Basic principle behind Metallic yield damper is that the metallic

device deforms plastically, thus dissipating vibratory energy. Used in earthquake

applications.

Friction devices: here friction between sliding faces is used to dissipate energy. When

used in base isolation systems, the friction coefficient has conflicting requirements. Itshould not be too large otherwise shear forces from ground during a strong earthquake


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will transmit to the structure. Also it should not be too small or the entire structure will

move due to small/medium wind/earthquake loads. These devices can also be fitted

between two storeys to damp their relative motion. Used in earthquake applications.

Viscous/ Viscoelastic devices: A typical example of it is fluid in a cylinder with

piston having an orifice. These can also be semi-active (eg., variable orifice, variable

viscosity). Used in earthquake and wind applications. Tuned mass dampers:TMD, usually having mass about 1% that of structure, fitted

to top of building. It is tuned to reduce vibration for given frequency range. It absorber

mass takes up vibratory energy, leaving the main mass (building) almost static. Main

system properties (stiffness-k1, mass-m1) known, absorber system properties

(stiffness-k2, mass-m2) to be designed such that absorber frequency equals excitation

frequency (w2=w). Not very useful for earthquake excitations which occur over wide

frequency range. Main problems is that the size of the mass to be used and its

displacement relative to the structure, in order that damping is effective.

Some other Dampers are liquid sloshing dampers, Impact dampers.

Semi-Active Structural Control Systems: Semi-active control systems need lesser

energy than active control systems. They do not add any mechanical energy to the

system.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 stability is guaranteed. Semiactive control devices are

often viewed as controllable passive devices.

Semi-active control:

Uses devices where input power requirements are orders of magnitude less than fully

active devices. In fact in some cases battery power is sufficient.

These devices usually dont add energy to the system, hence stability ensured.

These devices can be viewed as controllable passive devices (eg., Magneto-

Rheological Fluid damper where voltage input applied to change viscosity depending

on motion measured by sensors, variable orifice damper, controllable friction devices,

Variable stiffness devices).


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Figure 10: Schematic of various auxiliary damping devices utilizing inertial effects.

(Con: controller, a: actuator, Ex: excitation, S: sensor)

Active Dampers

In the quest to control the vibration of structures, passive control had originally been favored

for its simplicity and reliability - the devices remained functional without an external power

source and posed no significant risk of generating an unstable situation. Still, without the use

of control mechanisms, the devices were incapable of adjusting to a variation in any

parameters of the system.

Clearly, more efficient and swifter control could be obtained from a system with the ability to

respond to changes - hence, active control emerged, producing smaller devices that were

capable of controlling the vibration of structural systems. This aim is accomplished through

the use of hydraulic or electro-mechanical actuator systems driven by an appropriate controlalgorithm, such as: closed loop or feedback, in which the control forces are determined by the

feedback response of the structure, open loop or feedforward, in which the control forces are

determined by measured external excitations, or closed-open loop or feedforward-feedback, in

which the control forces are determined by both measured response of the structure and

measured external excitation. Active systems include active mass drivers, active variable

stiffness systems (AVS), active tendon control systems, active gyro stabilizers (AGS), active

aerodynamic appendages, and active pulse control systems.

An active damping method for a stabilized mirror, and a corresponding active damper

apparatus, comprising providing a tachometer measuring speed of a motor driving the mirror,employing compensation electronics receiving input from said tachometer and the motor, and

employing drive electronics providing output to the motor.

Working principle of an active device :

It uses hydraulic or electro-mechanical actuator systems driven by an appropriate control

algorithm. By this algorithm it generates a control force proportional to the velocity. It

makes the moving mass of the actuator move to counteract the motion of the floor. The

system is optimized such that the motor can be driven with a sinusoidal force where the

peak force is at or near the full motor capacity at any frequency without exceeding the

stroke while maintaining a simple and cost effective controller a relationship between thefundamental natural frequency of the floor system and actuator is established to provide

efficient and stable control.


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Applause Tower

Osaka Prefecture (Completed in 1992)

Design: Takenaka Corporation

Total Floor Space:96,793 m2Number of floors: 3 below ground, 34 above ground

Structural control device: AMD, mass weight 480t

(Utilizing a heliport)

Object: Suppression of building vibration in strong winds or

medium/small earthquakes

(Parallel movements in two directions)

AMD using a rooftop heliport

The rooftop heliport, supported on multistage rubber bearings, can be used as weight for

AMD. The size of this weight is the largest in the world.In addition, there is a large-scale experimental frame to ascertain the durability and reliability

of AMD at the Takenaka Research & Development Institute where the effects of AMD can be

experienced.

Pros and cons of active dampers

Advantages over other dampers

It provides efficient and stable control with the change in properties of structure.

It absorbs vibration and helps us in designing an earthquake-resistant structure which

suppresses the vibration itself.

Disadvantages

An active control system is a much more complex system than the

passive control system

An active Damper adds energy to the system thus many civil structures (without

control) which are stable may get destabilized with active control.

They are very expensive systems to design and are expensive to operate due to the

large amounts of power they need.

Active device is larger in size in comparison with passive device so it occupies more

power with respect to a passive device


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Semi Active DevicesAbstract Control devices can be used in civil structures to dissipate energy from earthquakes,

reduce structural damage and prevent failure. Semi-active control devices have been shown to

be more energy-efficient than active devices and more effective in reducing seismic structural

vibrations than passive devices.A type of semi-active control device, the magnetorheological (MR) damper, consists of a

hydraulic cylinder containing micron-sized, magnetically polarizable particles suspended in a

liquid such as water, glycol, mineral or synthetic oil. The damping capabilities of this device

can be quickly varied by changing the viscosity of the MR fluid from viscous to semi-solid

through the introduction of a magnetic field. The objective of this research is to develop a

fuzzy controller to regulate the damping properties of the MR damper. Because fuzzy control

uses expert knowledge instead of differential equations, it allows for the development of

simple algorithms. It does not require accurate information on structural and vibration

characteristics of the system and is therefore an attractive alternative for complex and/or

nonlinear systems. Introduction Civil structures have very little damping and are therefore

susceptible to dynamic loading such as those caused by intense winds or earthquakes. Control

devices are often used to reduce structural damage and prevent failure. Passive controllers,

such as tuned mass dampers and base-isolation dissipate incoming energy from the system

and since they require no power to operate, are very reliable systems. Active controllers, such

as active tendons and active tuned mass dampers, are able to dissipate energy and/or exert a

required force to the structure. They do however require a large amount of energy to operate

and since they insert energy into the system, they have the ability to destabilize it. Semi-active

controllers have been shown to be more energy-efficient than active devices and more

effective in reducing seismic structural vibrations than passive devices. They require little

power for operation, being able to run on battery power, and since they do not inject energy

into the system, they guarantee stability:

(b)

a) Kyobashi Siewa Building and (b) its AMD unit: (c)performance of structure under wind.(taken from

Application of vibration control structure system: AMD


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Kajima Corporation

In brief :

Semi-active control falls between passive & active on the control spectrum.

In a semi-active control system actuators operating on external power are used but

they do not add energy to the structure.

The actuators are used to control or assist a passive control device.

Benefits :

The actuator used does not require large amounts of external power.

Semi-active systems are more aggressive than passive systems

Obtain control results close to that of an active control system.

An active variable stiffness system as well as the active variable damping system is

considered semi-active system.

Conclusions

Earthquakes represent the single largest potential cause of casualties and damage from

a natural hazard. Every year we face lot of devastation and loss of infrastructure as

well as lives due to it. Thus earthquake mitigation has become very important

For earthquake mitigation civil engineers apply various ways like: changing rigidity,

mass, damping, shape, or applying passive or active control forces to lesson the

vicious effect of earthquakes. Installation of active dampers in large civil structures is

one of the effective measure to lessen this devastation


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In active mass dampers designer decides choice of outputs based on comfort (e.g.

accelerations) and safety (e.g. stresses)(By providing certain algorithm) we can change

the criteria of output force according to the place and properties of the building.

Although very effective in controlling vibrations in any civil structure, active dampers

are less popular. Major cause of this is its high installation and maintenance cost.

Moreover it needs power supply and its power consumption is very high Passive dampers are not so effective in controlling vibrations but as they are economic

they can be used in the places having less threat of earthquake

Semi active device is the best alternative. It doesnt consume energy and at the same

time it is approximately equally effective in controlling vibrations caused by

earthquake.

References http://www.freepatentsonline.com/6874748.html

http://www.columbia.edu/cu/gsapp/BT/BSI/TMD/tmd.html

http://ieeexplore.ieee.org/iel5/6739/18043/00837479.pdf?arnumber=837479

http://www.takenaka.co.jp/takenaka_e/quake_e/seishin/seishin.htm

Special Thanks to Luv Doshi for his notes

Earthquake Mitigation Using Active Dampers

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