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Checking Eigen Vlaues

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benjamin2013
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Checking Eigen Vlaues

Hello,

 

I ran a model with a modal analysis on a structure (see attached). I decided to thoroughly check the results because the acceleration calculated by the spectral analysis were surprising (in the model attached I removed the spectral analysis to make the file smaller). A view of the model is shown below, the lines in red represent lateral elastic supports (this building is attached to another).

Capture.PNG

The data I used is the table "Eigen modes, with the following columns:

Node / Case / Mode / Participation coefficient in X-dir / Participation coefficient in Y-dir / Eigen vector in X-dir / Eigen vector in Y-dir

 

Using the help document on modal analysis (http://docs.autodesk.com/RSAPRO/2014/FRA/index.html), as the Eigen vector is normalised using the mass matrix, we should have: 

Sum[(Eigen Valus)*(Participation Factor)]=1 if all modes are considered,

or Sum[(Eigen Valus)*(Participation Factor)]<=1 if all modes are not considered.

 

In this case, only 71,6% of the total mass in the X-direction and 82,2% in the Y-direction are used by the modes considered (100 modes calculated). Surprisingly, the sum described above is not achieved. the two screenshots below show the results of this sum located on the model geometry, on the X-direction and Y-direction. Every dot in green has a sum lower than 1, all the other are higher (green<1.00<yellow<1.05<orange<1.10<red<1.5<black).

 

Capture-2.PNGCapture-3.PNG

 

Why am I having such results? Is there something wrong with my model or my assumptions?

 

Thanks!

benjamin

 

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Message 2 of 2
benjamin2013
in reply to: benjamin2013

Hello again,

 

Does anyone have any clue on the results I have here? I'm really stuck and double-checked my model and results several times, without finding any any explanation...

I attached the values returned by Robot for one point where the sum (eigen vector)*(participation coefficient) is higher than 1, to illustrate my point. Hope this will help finding the problem...

 

Thanks for your attention,

Benjamin

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