Rigid body modes in modal analysis | NX Nastran FEA

Rigid Body Modes 

The model or part of the model may be displaced without stress if it is not sufficiently constrained. These stress-free displacements are called rigid-body modes. If your model is unconstrained in the FEA simulatiion, the first six modes of your solution will be the rigid-body modes

We can define the frequency tolerance under which NX Nastran considers a mode to be a rigid-body mode in its damping calculations. For example, if you define this tolerance value at 0.001 Hz, the modes generated up to this frequency are considered to be rigid-body modes and are excluded from damping calculations. 
  1. In the Solution dialog box (when creating or editing the SOL 103 – Response Simulation solution), expand the SOL 103 – Response Simulation group.
  2. In the Rigid Body Modes Below box, enter the frequency below which NX Nastran should consider modes to be rigid-body modes. The solve excludes these rigid-body modes from modal damping calculations. If you leave this value blank, the solver uses a value of 10–3 for the tolerance.
  3. Click OK.