Nuts with gaps do not control the axial movement. They only limit it to specific amount provided the unit is installed correctly. Unfortunately that doesn't happen always. Computer gives an answer but the reality is different. Even the heat expansion is unknow. Pipe metal temperature used in the CAESAR II analysis is not what it is during the operation. It is unknown.

I design expansion joints since over 20 years. I also give training and it includes most common mistakes made by the pipe designers

Very common mistake: Pipe designer specifies expansion joint axial movement to be -25/25 mm (-1/1") on 450 C (840F) on a hot line. What is the (bad) mistake?