Difference between two Eigenfrequency analysis?

[ridbay]

Hi,

i have a question about the eigenfrequency analysis. What is indeed the difference between the two eigenfrequency analyses;

1- structure is clamped,
2- structure is not clamped.

At first case, i get the first 6 modes equal to 0.0 (rigid body modes). But the 7. mode of analysis for unclamped structure does not agree to first mode of analysis for clamped structure.

How can i interprete the analysis results?

Thanks in advance

 

[GregLocock]

Your observation is correct. The frequency of the first flexural mode of a structure will depend on the boundary conditions.

Sometimes it will increase in frequency as you add constraints. And sometimes it will decrease.

The answer is in the maths.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[electricpete]

There was a discussion here
thread384-155945
it was mentioned that the frequency of the first few non-zero modes of a free free beam are the same as the first few non-zero modes of a fixed-fixed beam.

It seems paradoxical because we expect the clamped system to have higher natural frequency.

The answer as Greg explained is that the free free beam had several zero-frequency modes. I don't think the clamped-clamped has any. So the first natural frequency of free free (zero) is indeed lower than the first natural frequency of the fixed/fixed.

You say you got zero frequency modes for the first case (clamped). That doesn't sound right. Maybe you got the zero frequency results for the second case (not clamped)?


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[electricpete]

Just to clarify why I think your six zero-frequency modes should apply to the unrestrained case:

The six zero-frequency modes for the unrestrained body:
Translation parallel to each of the three axes.
Rotation about each of three axes.

Once you set the unrestrained body in motion, it continues in the same direction (zero frequency) due to it's inertia (or rotating inertia).


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[ridbay]

Thanks to all for sincere helps.

And electricpete, you are right. i was confused in explanation. I got the zero frequency results for the second case (not clamped).