When subjected to sufficient internal pressure, the bellows (having no resistance to longitudinal load) would straighten out to a straight tube, at which point the pipe and bellows would be capable of sustaining a longitudinal tensile load. A tension system would once again be established; however the bellows would no longer be capable of accommodating axial motion, and might rupture since normally its wall thickness would be only about 1/4 or less of that of the pipe.

Hence it is apparent that in order to maintain the bellows configuration, external (main) anchors are necessary to carry the pressure generated "PA" load. Such systems are called "Compression Systems" since an external compressive force must be supplied to the piping system to prevent the bellows from elongating.

Reference1: For General Information see the "Standards of the Expansion Joint Manufacturer's Association Inc.", 25 North Broadway, Tarrytown, New York 10591