Following an example on your CaesarII Applications Guide on pp.5-20, the Effective ID box is empty.

The axial travel of 2" (extension) could cause overstress in the connected pipe if the Effective ID is considered.

Why in this example the effect of pressure thrust is neglected?

I agree, the "effective ID" value should be specified. When this entry is defined, CAESAR II will compute a <em>pressure thrust</em> force and apply it at the end of the bellows (nodes 15 and 16), away from the joint.

In this model, once the gap closes, the slotted bars will contain this pressure thrust force and keep the sytem from blowing apart.