Effect of Friction on Natural Frequency

Hi,
Is it a good practice to exclude friction while checking the Natural frequency of the piping system . The inclusion of the Friction factor on the restraints gives a higher Natural frequency for the system than otherwise.
The requirement is to have at least 4 Hz for the piping. It would be useful if some people can share their opinion on this.

Regards
Anand.

I think the "right" answer is somewhere in between.

Excluding friction will show improbable frequencies and mode shapes.

Counting on "too much" friction will eliminate possible modes.