If you are using the CAESAR II expansion joint modeler to build the tied bellows model, then the necessary elements (spreadsheets) are defined for you, including the CNODES connecting the appropriate degrees of freedom for the tie-bars.

If on the rotational CNODES, you specify a gap, then that degree of freedom (DOF) will be free until the gap closes. Once the gap closes, that DOF association will be rigid (assuming no flexible stiffness was specified). Note, this does not mean that the joint will become rigid in that DOF, only the tie-bar connection.



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Regards,
Richard Ay (COADE, Inc.)