Dear Clyde

Please note that two nuts set are there in both end of tie road. So tie rod will neither allow to compress the bellow nor allow to expand the bellow ( except minor rod thermal expansion effect).

Tied bellow only allow lateral movement of pipe not any axial movement.
So when you use tie rod complete pressure thrust will be absorb by the rod.

using a untied bellow for high pressure & large bore pipe diameter is very dangerous.

Pressure balance bellow is highly expansive, should not be used unless there is real demand for it.

Best Regards

Habib