Stiffness factor for friction

When you do a modal analysis to check the natural frequency of a piping system, there's an option in Control parameter to use Stiffness factor for Friction.
I'm still unsure how to estimate this number from the static analyis. On the help (F1), it;s said Kfriction= F*mu*Fact

My question is where can I find this Fact(Friction factor) on control spreadsheet?


Thanks

"F" is the load applied to the support, "mu" is the coefficient of friction you defined for that support back in the Static Input. "Fact" is the value you define on the "Control Parameter" dialog.

This factor is actually a "fine tuning" control, to help with the accuracy of the solution. The problem is you can't use this unless you're trying to match field results. For example, you have a harmonic loading induced by equipment and you have measured the field response. Once your model is matching closely with those field results, you can use "Fact" to tweak the results to make things closer.

Thanks Richards.
So it's best to leave that Kfriction = 0 if we want to check the natural frequency of a piping system ?

I've seen a practice to close the restraint gaps on the piping system when you run a natural frequency analysis. Would this be an acceptable approach?

Yes, that is how I would make the run.



I wouldn't change the model. This is what that first input field is for on the "Control Parameters" dialog. You point to a reference Static Load Case so that c2 can linearize the model. If the gap closes in the reference case c2 leaves the restraint but takes out the gap for the dynamic analysis. On the other hand, if the gap doesn't close, c2 removes the restraint for the dynamic analysis.

By not changing the model you can go back to the Dynamic Input and reference a different (static) load case and see if there are any effects on the dynamic results (aside from the frequencies changing).

I'm in the middle regarding this friction stiffness thing.
If you do not consider friction in dynamics, the animated mode shapes may show Y supports slipping freely in the horizontal plane - like an air hockey table. With any sort of normal load, that will not happen. I would go back and rerun the modal analysis with artificial guides or use that friction stiffness factor. (I typically use a value of 1000.) What's nice about the friction stiffness factor is big normal loads get stiffer perpendicular restraints.
Take a look at your output to confirm your input.

Thanks Dave.
What do you mean by artificial guides? If the piping system has guides already, the animated mode shape should not be moving freely in the guide direction, should it?
What I normally do is to use a friction stiffness factor value below 100 and check if the animated mode shape confirms the restraints and supports location

If you know that the pipe will not slip at these friction points and you want to include this "not slipping" in your dynamic model, you could add restraints to the input model to prevent this slipping. You are not installing these, just adding them to your model to product a desired result. These restraints are my "artificial guides".
If 100 works for you keep on using it. Just be sure the results "look" right. It's more qualitative than quanitative at this point. I've looked at a lot of models it is my opinion that the results are not too sensitive to the entered value. I think it's more related to the effective system stiffness and mass around these supports - as the system stiffness increases, this factor (I guess) should be bigger.

Mr. Dave,

I am doing a modal analysis for a two-phase slug flow line, my client requirement is that the piping system natural frequency shall be more than 5 Hz. So, in the dynamic control parameter option, when i enter the stiffness friction factor as 0, the natural frequency of the system is very less (say less than 0.5 Hz), when i enter the stiffness as 100 or 1000, the frequency reaches more than 5 Hz.

As discussed by you in the earlier forum posts, i agree with your suggestions, but the only requirement is, on what basis we can select the stiffness value as 100 or 1000. Since there is no backup for it, our client is not accepting to proceed with 100 or 1000, so i kindly request the Coade to provide a suggestion in the Caesar-II help guide to use the stiffness factor for friction in the range of 0 to 1000 or more.

Thankyou.

Hi

I have experience with modal analysis. I input the stiffness of friction by take the [(vertical load*friction)/horizontal movement] from static data (from only resting support) and input into the dynamic analysis. The result is more reasonable than 0 input.
However, I would note that this stiffness is represent for all support but the data that you taken is from one support. So you should take from several support and find the proper one.

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