Q:

How to troubleshoot the warning of “Not able to separate the modes”?

R:

Most likely, you have assigned the Retained rotational degrees of freedom.

This can also generate an output with inconsistent period.

Avoid giving the rotational degrees of freedom and check the result.

 

In dynamic analysis, to accurately calculate the effects of a concentrated mass along the length of a member, it is best to include a joint at that location. Also, if a local mode due to the concentrated mass is important to the analysis, then the model should include retained degrees of freedom at the joint at the location of the mass.

In this way, the dynamic analysis will use mass which is distributed in a manner that matches the mass distribution of the model.

If you continue with the warning message, you might encounter incorrect structural frequency. Please cross check the same in dynpac listing file.