Structural resonance

[fiveworlds]

How do I calculate the resonant frequency of a structure?

[studiot]

That's a big question that needs a heap more information to begin to answer.

[fiveworlds]

http://m.youtube.com/watch?v=j-zczJXSxnw

[Enthalpy]

Sorry, YouTube consumes too much bandwidth.

How is your structure? A truss with few beams, a simple shape?

 

A few cases can be computed by hand, but often it's impossible or just too heavy, so people normally seek a software for that.

[studiot]

Wind induced vibrations are often non resonant, for example the sound of singing in telephone wires is nowhere near the resonant frequency of the wires.

 

This is a big subject and quite interesting but,

 

You really do need to ask a proper question to get a proper answer.

[fiveworlds]

Sorry, YouTube consumes too much bandwidth.

 

http://r3---sn-35153iuxa-5a5e.googlevideo.com/videoplayback?source=youtube&mm=31&expire=1408337787&sver=3&mt=1408316150&itag=18&initcwndbps=1790000&id=o-AAtFS-lfeMnQjXGpYUBfRL279DdD-htnz2Zon_PXfBEM&upn=bhS1_ugCejU&signature=27D750A6E0307FCD3023C1699C9D57569C22F0C3.88427B75CD277707C5428371D98C666CA37D8059&sparams=id%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Cratebypass%2Csource%2Cupn%2Cexpire&fexp=900225%2C902408%2C908570%2C916633%2C919120%2C927622%2C929203%2C931983%2C934024%2C934030%2C936109%2C945122%2C945544%2C946023&mws=yes&ipbits=0&ratebypass=yes&ip=89.218.47.226&key=yt5&ms=au&mv=m&title=Tacoma+Narrows+Bridge+Collapse+%27Gallopin%27+Gertie%27

Edited August 17, 2014 by fiveworlds

[studiot]

Have you checked your post for ??????????

[fiveworlds]

Wind induced vibrations are often non resonant

 

http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance we shorten it to just resonance

 

the sound of singing in telephone wires is nowhere near the resonant frequency of the wires.

 

Temperature can change the resonant frequency of the nuclei also the tension the nuclei are under.

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/string.html

Edited August 17, 2014 by fiveworlds

[studiot]

http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance we shorten it to just resonance

 

Temperature can change the resonant frequency of the nuclei also the tension the nuclei are under.

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/string.html

 

What does any of this have to do with your question?

[fiveworlds]

What does any of this have to do with your question?

 

I learned the basics I want to know how to calculate the resonant frequency of a large structure. All I learned was small scale.

[studiot]

This is the fifth time I have asked you provide some details of what you are asking.

 

Otherwise you might as well ask

 

"How do I do Structural Analysis?"

 

Do you actually know what resonance means and why I said that wind induced oscillations are not usually resonant?

[fiveworlds]

http://video.mit.edu/watch/tuning-forks-resonance-a-beat-frequency-11447/

http://www.math.harvard.edu/archive/21b_fall_03/tacoma/index.html

 

I found some wrong methods http://www.strand7.com/html/naturalfrequency.htm http://peer.berkeley.edu/education/files/worksheets_friendship/Natural%20Frequency%20Worksheet.pdf

but none I think are right. Do you know why wind induced oscillations are resonant? Imagine I have a string. It is resonant 100% of the time. Now when the wind causes the string to oscillate it causes the tension on the string to fluctuate within some level of tolerance say +-1 Newton. By changing the tension on the string I have also changed the fundamental frequency of that same string and it resonates between that range of values.

 

There are things I don't think are right though assume I have ⁵²Fe and ⁵²Cu http://upload.wikimedia.org/math/7/3/c/73cb6f0121da6b745df4141181907357.png

says that they should have an identical resonant frequency since they have the same mass however we know they are not the same.

Edited August 18, 2014 by fiveworlds

[Enthalpy]

This thread looks like a way to waste our time.

[studiot]

I am not sure where you are coming from here since you ask specifically about large structures, then want to discuss individual atoms?

 

I will address these separately.

 

Looking back I realise that I have not been very clear.

 

 

 

Studiot

Wind induced vibrations are often non resonant, for example the sound of singing in telephone wires is nowhere near the resonant frequency of the wires.

 

 

The sound you hear is in the air not the wires!

 

Resonance is about the transfer of periodic energy from one system to another and can occur when the control of this transfer is by the driving system. However resonance only happens when the driving system frequency exactly matches the natural frequency (or a whole number multiple) of the driven system. At this time there is maximum amplitude or velocity of vibration.

 

Control can be by the driving (also called forcing) system or by the driven system.

 

When control is exerted by the driven system we call the oscillation a relaxation oscillation.

 

If the telephone wire was twanged like a musical instrument string it would vibrate at its natural frequency according to Rayleigh’s equation, or the simplified harmonic version may suffice.

The frequency is governed by the wire.

 

 

http://en.wikipedia.org/wiki/Rayleigh%E2%80%93Plesset_equation

 

 

But this is not the mechanism by which vibration is generated when a steady wind passes over a wire.

This occurs by vortex shedding and the frequency of this is controlled by the fluid mechanics of the wind.

 

http://en.wikipedia.org/wiki/Vortex_shedding#Governing_equation

 

The strouhal function has wiggle in its graph that sets the frequency of what you hear.

 

 

http://en.wikipedia.org/wiki/Strouhal_number

 

The frequency of vibration is set by the driving system (the speed of the wind). For most frequencies this does not match the natural frequency of vibration of the wires so resonance does not occur, although the wires still vibrate.

 

In the odd case where it does large amplitude oscillations can occur and rip the wires out.

 

Note that Wiki gives the impression that all periodic transfers of energy are resonant. They are not.

 

A simple basic test is to ask what happens if the drive is removed.

If the driven system reverts to its own natural frequency then the drive was not resonant

If the driven system carries on as if nothing had stopped (except for any slow decay of the oscillation) then the transfer is resonaant.

 

 

 

With regard to your comments on micro systems look here

 

 

http://www.damtp.cam.ac.uk/user/gold/pdfs/quantray.pdf

Edited August 18, 2014 by studiot

[studiot]

fiveworlds

 

Do I take it you have no further interest in this subject, but can't be bothered to tell me?