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[Cheta Rathod]

This could be due to the bonded contacts with large gaps present in the Modal analysis. Gap/Penetration at bonded contact adds extra stiffness to the model in Modal Analysis when the contact formulation is set to program controlled (penalty method). 

Background: With large gaps, Bonded contacts using penalty methods can have accuracy issues. There is a warning issued for the user to either clean-up the model using cnch.adjust or morph or switch to MPC. 

Warning message from the output is copied below. 

**WARNING*: The initial penetration/gap is relatively large. Bonded/no separation option may cause an accuracy issue. Switch to MPC algorithm or you may use the CNCHECK,ADJUST/MORPH command to move the contact nodes towards the target surface.   

Resolution: Change the bonded contact formulation with these warnings to MPC formulation. This should give zero frequency modes for the unconstrained DOFs. As a check, compare higher frequency modes using both MPC and penalty contact formulations. There should be minor changes in the higher frequencies for the same mode shapes.

[Aniket]

Thanks, Cheta for sharing this!
-Aniket
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[dlooman]

This issue only affects the rigid body rotational modes. The penalty based contact across an open gap acts like a rotational constraint.