A Seminar On Vibration Analysis And Damping In Structures

Submitted to: Submitted By:Mr Rahul Bhaiji Divya Lattoo

Utkarsh Tiwari

IntroductionStructure –

A structure is a combination of parts fastened together to create a supporting framework, which may be part of a building, ship, machine, space vehicle, engine or some other system.

Vibrations –

THE CAUSES AND EFFECTS OF STRUCTURAL VIBRATION

Cause

Effect

THE REDUCTION OF STRUCTURAL VIBRATION

THE ANALYSIS OF STRUCTURAL VIBRATION

Stage I. Devise a mathematical or physical model of the structure to be analysed.

Stage II. From the model, write the equations of motion.

 Stage III. Evaluate the structure response to

a relevant specific excitation.

The Vibration Of Structures With One Degree Of FreedomFREE UNDAMPED VIBRATION

Translation vibration

Torsional vibration

Energy methods for analysis• For undamped free vibration the total energy in the

vibrating system is constantthroughout the cycle. Therefore the maximum potential energy V(max), is equal to the maximum kinetic energy T(max) , although these maxima occur at different times during the cycle of vibration. Furthermore, since the total energy is constant,

 • T + V = constant, • d(T + V)/dt = 0

• ω = (k/m)1/2 • Condition of stability –

.

FREE DAMPED VIBRATIONThe most common types of damping are

Viscousdry frictionhysteretic

Vibration with viscous Damping• Case 1 ζ less than 1, that is, damping less than critical• The motion of the body is therefore an exponentially

decaying harmonic oscillation 

• Case 2 ζ = 1; that is, critical damping• Critical damping represents the limit of periodic motion;

hence the displaced body isrestored to equilibrium in the shortest possible time, and without oscillation or overshoot.

• Case 3 ζ greater than critical,• Since both values of s are negative the motion is the

sum of two exponential decays

Vibration with Coulomb (dry friction) damping

Equation Of Motion - mẍ = Fd – kx

Vibration with hysteretic dampingExperiments on the damping that occurs in

solid materials and structures that have been subjected to cyclic stressing have shown the damping force to be independent of frequency internal, or material, damping is referred to as hysteretic damping.

the induced stress is σ = σ0sin (Vt+ α)

FORCED VIBRATIONThe equation of motion is

 

X/Xs is known as the dynamic magnification factor

ResonanceThe phenomenon known as resonance occurs

when the forcing frequency is equal to the natural frequency, that is when v/ω = 1.

The maximum value of X/Xs actually occurs at values of v/ω less than unity:

Response of a viscous damped Structure supported on a foundation subjected to harmonic vibration

Equation Off Motion

Vibration IsolationThe force transmitted to the foundation is the

sum of the spring force and the damperforce. Thus the transmitted force = kx + cẋ

and Fx the amplitude of the transmitted force is given by –

Response of a Coulomb damped structure to a simple harmonic exciting force with constant amplitudeThe equation of motion is non-linear because

the constant friction force F, always opposes the motion:

Response of a structure to a suddenly applied ForceThe equation of motion can be written

If the structure possesses viscous damping of coefficient c, the solution to the equation of motion is

Shock excitationSome structures are subjected to shock or

impulse loads arising from suddenly applied, non-periodic, short-duration exciting forces.

X(t) =

Wind- or current-excited oscillationA structure exposed to a fluid stream is

subjected to a harmonically varying force in a direction perpendicular to the stream. This is because of eddy, or vortex, shedding on alternate sides of the structure on the leeward side.

Damping In Structures• Sources of damping

– Inherent damping– Hysteretic or material damping– Damping in structural joints– Acoustic radiation damping– Air pumping– Aerodynamic damping

• Added damping – High damping alloys– Composite materials– Viscoelastic materials– Constrained layer damping– Vibration dampers and absorbers

Vibration IsolationThe force transmitted to the foundation is the

sum of the spring force and the damper force. Motion Transmission TR = X/A =

BIBLIOGRAPHY•  

Structural Vibration and DampingBy C. E Beards

 www.howstuffwork.com

 Theory Of Machine

By R.S.Khurmi

Questions And Query Are Welcome