$\newcommand{\Dev}{\operatorname{Dev}}$ $\newcommand{\dev}{\operatorname{dev}}$ $\newcommand{\grad}{\operatorname{grad}}$ $\newcommand{\divg}{\operatorname{div}}$ $\newcommand{\Ei}{\operatorname{Ei}}$ $\newcommand{\tr}{\operatorname{tr}}$

A node set can be attached to a rigid body using the rigid boundary condition. Rigid nodes are not assigned degrees of freedom.

<bc type="rigid" node_set="set1">
  <rb>2</rb>
</bc>

The rb parameter defines the material (which must be a “rigid body” material) that in turn defines the rigid body. The node set must be defined in the Mesh section.