A joint with nuts on only the outside of the flanges (allowing bellows compression) can be modeled using a <em>directional restraint</em> on the CNODE of the tie-bar.

For example, if your joint is along the positive "X" direction, the CNODE at the "TO" end of the tie-bar would normally be assigned an "X" restraint (to the flange node). This would simulate a nut on both sides of the flange, preventing axial movement of the bellows. If a "+X" was used instead, the tie bar could move in the "+X" direction relative to the flange node, thereby allowing compression.