Hi

This is my first post in the forum and I hope to get good responses from the designated members as well as Integraph on the following matter.

Today I heard something interesting from a senior stress engineer. He was claiming that the first three modes of natural frequencies can actually be ignored as Caesar II ignores factors like friction, non-linearity, etc.

I tried to search through the explanations made by Mr. David Diehl in an Webinar plus some other resources found in internet. But I really couldn't find anything that supports the statement that we actually can ignore the first three modes of natural frequencies based on the above mentioned reasons.

Can anybody please help me in this?

Thank you very much!

It may be a useful guide in some systems and it is important to recognize the shortcomings of a linearized dynamics model but I do not think you can generalize to the "first three modes" point.
I suggest you take a look at those first modes and see if they are affected by sliding at friction points or perhaps "moving too much" where nonlinear supports are involved. Where more information is needed, I suggest you change those linear assumptions that may be significant and see how that affects the frequencies.

Hi Dave,

Thanks a lot for your response!

I am still not clear which mode of natural frequency in Caesar II dynamic analysis can be considered as the actual first mode of natural frequency in real life. Can you please explain a bit elaborately how to determine this real first mode of natural frequency if the first few modes as analyzed in Caesar II are heavily affected by sliding at friction points or "moving too much" at non-linear supports?

I would tweak the model to determine the modeling significance of the change. For example, run modal calcs. with no throw "Stiffness Factor for Friction" and then run it a second time with a value of, say, 1000. If the numbers do not change much, it is not important - to this model.
If it is an existing system, tune up the model to match what you see moving in the field.
And look at the animated mode shapes.
Our modal analysis is linear and the results are only as good as your model. It is hard to speak in generalities here.