In CAESAR II, if FAC=1, operating and expansion stress will be calculated and limits to these stresses will be provided. FAC=1 indicates that the piping is "fully restrained". There will be no calculated stress for sustained and occasional conditions.
In CAESAR II, if FAC < 1, there will be no allowable limit for operating stress but there will be allowable limits for sustained, expansion and occasional stresses. Operating stress is dependant upon FAC and the axial load (tension vs. compression). Expansion stress range will be dependent on FAC.
Why? Because that's how we read the Code.
Fully-restrainted pipe ("locked in", buried pipe, FAC=1) does not have force-based failure criteria. The operating state of stress must not exceed yield.
Pipe that is not fully-restrained follows the standard expansion, sustained, occasional checks but with a twist. These B31.4 systems are more likely to have a compressive longitudinal stress (as opposed to the typical B31.3 system in tension). With a compressive component, the simple maximum shear stress calculation no longer applies. That's why tension/compression is an issue.
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Dave Diehl