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Chapter 1Chapter 1

Objectives D i ( ib ti ) l i

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Dynamic (vibration) analysis and control

1.1 IMPORTANCE OF VIBRATION ANALYSIS

Vibrations may cause:

I Large displacements and severe stresses in a structureI. Large displacements and severe stresses in a structure.

II. Material failure through fatigue.

III. severe discomfort to people who stay in the vibrating structure.

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Therefore, the dynamic response of the structure has to be predictedand the analytical solution has to be applied for proper design ofstructure.

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1.2 STRUCTURES AND THEIR VIBRATIONS

Al ll bj d i f i i- Almost all structures are subjected to various types of excitations(wind, earthquake, impact, explosion, etc.) So, dynamic analysis isneeded in all engineering projects.

- Some selected and important examples of vibrating structures are as

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- Some selected and important examples of vibrating structures are asfollows.

Typical wind-sensitive bridges

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5Cable-stayed bridge (Tokyo, Japan)

6Suspension bridge (Tokyo, Japan)

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Local vibrationGlobal vibration

8High-rise building

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9High-rise building & Elevated railway (Tokyo, Japan)

1.3 DEFINITION OF VIBRATION

-The term dynamic may be defined as time varying and dynamic loadis any load of which the magnitude, direction or position varies withtimetime.

-Vibration is a type of dynamic behavior in which system oscillatesabout a certain equilibrium positionabout a certain equilibrium position.

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1.4 TYPES OF VIBRATION

I. Time dependent external forces such as

- earthquake or underground explosion- wind or water waves- machine- vehicle motion- human

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In fluid dynamics:

II. Motion dependent force

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1.5 CONTROL OF VIBRATION

The modern structures tends to be higher, larger, more flexible andcomplex, so they are prone to vibrate.

Various disturbance due to vibrations:- Collapse- human discomfort- noise- non-serviceability

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The need for passive or active control systems to suppress vibration ofstructure is required.

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Some techniques of vibration reduction:

I. Passive control- base isolation- damper/ dynamic absorber- TMD, TLD- etc.

II. Active control- active mass control

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- active tendon control- etc.

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The first building to be seismically isolated

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Conventional structure Base-isolated structure

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Tuned Liquid Damper (TLD) system

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Tuned Mass Damper (TMD) system

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Visco-elastic Damper

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Active Tuned Mass Damper

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Actuated Mass Damper

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Active Tendon

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1.6 USE OF VIBRATIONVibrations usually have negative impact on a structure, and they cancause damage through large stress, displacements and fatigue.

In several cases, vibration can be used as a mean. For example:p1. Measurement or test

- identification of dynamic parameters of structure (by imposingcertain dynamic excitation and analyzing a response)- checking cable tension- mechanical vibration meters

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2. Vibration suppression- vibration absorbers / dampers

3. Construction or demolition- drills- explosions