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Member
greg.parent
Posts: 5
Registered: ‎06-26-2012

Eigen Values / Natural Frequencies

1214 Views, 11 Replies
06-26-2012 03:05 PM

I am reviewing the design of a guyed transmission line tower and I need to calculate the periods/natural frequency of this structure.  I can model it in Robot but I was wondering how to analyze the structure to obtain the 1st and 2nd mode of the natural frequency of this structure. 

 

I have searched this on the Robot user manual and found related topics under: Table Columns/Eigenvalue.  However I am having no luck getting the Eigenvalues tab to appear.  I need some direction on how to do this.

 

Any advice?

 

-Greg  

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Valued Mentor
tony.ridley
Posts: 653
Registered: ‎09-07-2011

Re: Eigen Values / Natural Frequencies

06-26-2012 08:14 PM in reply to: greg.parent

Hi mate,

 

You need to run a modal analysis. 

Set up number of modes, and add load to mass conversion if necessary. 

 

After analysis go to results----------->advanced------------->modal analysis.  This will give you the frequencies and associated eigenvalues etc.

 

Tony

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Product Support
pp2008
Posts: 529
Registered: ‎06-23-2008

Re: Eigen Values / Natural Frequencies

06-27-2012 12:47 AM in reply to: tony.ridley

Greg,

If you run this modal analysis for a guyed transmission line tower with cable elements pay attention to loads in static load case directly preceding the modal load case.

Modal load case will use cable stiffness from this case. It means that changing static loads in preceding load case (and thus changing stiffness of cables) you will receive different frequencies of vibrations.

 

More details in:

1/ Rafal's answer in this post: http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Barre-de-treillis-traction-compression-vs-s%...

2/ my answer in this post: http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Seismic-load-combinations/m-p/3495432/highli...

 

Regards,

 



Pawel Pulak
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Member
greg.parent
Posts: 5
Registered: ‎06-26-2012

Re: Eigen Values / Natural Frequencies

06-27-2012 06:58 AM in reply to: greg.parent

Thank you for your reply, it definitely got me going in the right direction. 

 

Had another question however,  I set up the modal analysis to check 5 modes.  The first two modes had the exact same natural frequency and period.  The third mode was much higher and the fourth and fifth modes did not calculate a natural frequency or period it just gave a value of “N/A” for everything.

 

It seemed odd to me that the first two modes would have the exact same frequency and period.  Shouldn’t the second mode have a much higher natural frequency?  (See attachment)

 

Also my structure is not supported symmetrically, there are guy wires supporting the lateral load in only one direction.  Does this modal analysis analyze natural frequencies and period in different directions?

 

Thank you

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*Expert Elite* | International Moderator
Romanich
Posts: 584
Registered: ‎09-29-2009

Re: Eigen Values / Natural Frequencies

06-27-2012 07:13 AM in reply to: greg.parent

First natural frequency is always lower than second, second lower than third and etc.

   
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Rafal.Gaweda
Posts: 5,604
Registered: ‎04-26-2010

Re: Eigen Values / Natural Frequencies

06-27-2012 07:32 AM in reply to: greg.parent

Send us model to check



Rafal Gaweda
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Valued Mentor
tony.ridley
Posts: 653
Registered: ‎09-07-2011

Re: Eigen Values / Natural Frequencies

06-27-2012 07:43 AM in reply to: greg.parent

The first two modes had the exact same natural frequency and period.  The third mode was much higher and the fourth and fifth modes did not calculate a natural frequency or period it just gave a value of “N/A” for everything. 

It seemed odd to me that the first two modes would have the exact same frequency and period.  Shouldn’t the second mode have a much higher natural frequency? 

 

Just be careful with how you understand modal analysis.  The term "natural" frequency is not applied to every mode.  Every mode has a frequency associated with it.  That may not be the "natural" one for the structure.  If you only get three modes this could be OK for a simple structure, depends on a lot of things.  

 

 

Also my structure is not supported symmetrically, there are guy wires supporting the lateral load in only one direction.  Does this modal analysis analyze natural frequencies and period in different directions?

 

Check Pawels post from earlier.  If you are using cables as guy wires they may not activate in the modal analysis depending.  Frequency and period are essentially the same thing reported in a different way, but neither depend on X, Y, or Z global directions.  The mode shape is what it is regardless of xyz direction.

 

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Member
greg.parent
Posts: 5
Registered: ‎06-26-2012

Re: Eigen Values / Natural Frequencies

07-03-2012 09:07 AM in reply to: Rafal.Gaweda

I am having an issue attaching the Robot Structural analysis Professional 2011 file.  It gives me the following warning:

 

Please correct the highlighted errors and try again.

  • The file does not have a valid extension for an attachment. doc,pdf,dwf,dwg,dxf,dst,gz,gzip,zip,bm,bmp,gif,jfif,jpe,jpeg,jpg,psd,tif,tiff,png,htm,html,xml,xls,lsp,svf,3ds,vcf,png,text,txt,log,env,dcl,rvt,rfa,ipt,iam,ipn,dwfx,idw,csv,nwf,nwc,nwd,sdf,cfz,vtfx,vtf are the valid extensions.

I believe that the Robot extension is: ".rtd" but that is not one of the valid extension types listed above.

 

Please let me know how I should attach the Robot Structural analysis Professional 2011 file that I have created.  Thank you,

 

-Greg

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*Expert Elite* | International Moderator
Romanich
Posts: 584
Registered: ‎09-29-2009

Re: Eigen Values / Natural Frequencies

07-03-2012 09:47 AM in reply to: greg.parent

Use *.zip format file

   
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Member
greg.parent
Posts: 5
Registered: ‎06-26-2012

Re: Eigen Values / Natural Frequencies

07-05-2012 12:49 PM in reply to: greg.parent

One thing I have not been able to model is the material of the conductor or of the guy wire.  For example the guy wire material is extra high strength steel and has a yield strength close to 175ksi.  I can't seem to input this material into robot.  Right now the guy wires are modeled as a 70ksi steel that was already in the material drop down list.  However, this may have no effect on the structures natural frequency or period.

 

I have noticed that if I apply the wind loads in different directions I get different values in the frequency and period.  I believe the best way to account for wind blowing in any direction is to modify the load case before the modal case.  I can modify this load case to represent the wind blowing in, say 8, different directions.  I can then run a modal analysis when load is applied in each different direction.

 

I am no longer getting the same Structural Frequencies for the 1st and 2nd modes, however they are very close.  For the short structure:

 

Mode 1: F1 = 0.92Hz

Mode 2: F2 = 0.98Hz

 

I would have expected a larger gap in Frequencies between the 1st and 2nd Mode.  Does anyone know why they are so close?

 

Also the guy wires were installed with an initial pre-tension.  Is thre any way I can model that?  Likewise the conductor was strung with a particular tension.  Is there any way I can model the cable elements to have these initial tensions?

 

I would appreciate it if someone take a look at these models and see if I am on the right track.  Thanks.

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