Since a pipe under internal pressure or vacuum will have its wall subjected to circumferential and longitudinal loading, introducing a discontinuous member such as a Bellows will transfer the longitudinal-pressure forces to the main anchors.
Pressure thrust is the product of internal pressure times the Bellows effective area.
The effective area / pressure thrust area of a Bellows is based on the mean diameter of the Bellows convolutions.
If adequate anchors cannot be provided to absorb the pressure thrust or if connected equipment cannot accept the pressure thrust loading then the joint must be equipped with tie rods. However a tie arrangement equipped Bellows cannot absorb the axial movement.
If proper anchors and guides are not present, the Bellows will deform due to pressure loading leading to total failure.