Is there a clear statement/interpretation about stress classification due to ground settlement
in B31.1?

We have a steam line with difference displacements at supports, caused by the different
settlement of two pipe racks. The effect is limited to some supports but passes a tee piece.

What are arguments for sustained or expansion stress check and is there room for interpretations?
The difference displacement cause self limiting stresses, is the increase of the thermal expansion
stress range a possible/sufficent stress check?

I don't believe B31.1 says anything about this.

I've always considered ground settlement to cause a sustained effect. Consider:

1) If a support settles and causes an excessive load redistribution, the system will fail due to weight, i.e. sustained.

2) Expansion is for cyclic events - settlement isn't cyclic.

I hope this helps.

Could it not be argued that since the settlement is a single cycle that the following could occur. When the settlement occurs any overstressed portions of the piping will undergo a stress re-distribution, similar to the 'shakedown' that occurs with expansion stresses. This is not a sustained case as the stress diminishes over time. Hence we could regard a settlement as being compared against the expansion stress range rather than the sustained case.

Although there are different schools on this I have to agree with Kenny.

In a current interpretation, B31.3 says thou shalt evaluate sustained stress in all suppport configurations - like Rich says above.

But the nuclear code addresses this issue head on under Thermal Expansion [para. NC-3653.2[b]] with equation [10a]: i[MD]/Z < 3.0Sc where [MD]="resultant moment due to any single nonrepeated anchor movement [e.g. predicted building settlement]".

I would evaluate any displacement-based load against an expansion limit. But some are concerned about the magnitude of a suppoprt displacement as large gaps may allow collapse - certainly a sustained stress issue.

The new Appendix S of the B31.3 code [to be released in the 2004 Edition] will eventually address this issue - perhaps in the 2006 Edition.

Back to the original question "Is there a clear statement/interpretation about stress classification due to ground settlement
in B31.1?"

Nope, No, Nein,

Please refer also to european code EN13480-3, par. 12.3.6: "Stress due to one-time displacement of pipe supports".

Best Regards,

Veit Bockemühl
esn GmbH
vb@esn-gmbh.com

Although ASME B31.1 does not address ground settlement, ASME B31.3 does in section 319.2.1 (c). This section clearly states that this is a displacement strain. It also allows stresses due to settlement to exceed normal displacement stress allowables providing “due consideration if given to avoidance of excessive localised strain and end reactions”.

As for the sustained case with settlement, this is no different to any system with displacement loads including thermal effects. B31.3 requires the sustained stresses to be checked in all operating positions. Supports that lift off due to settlement should be treated the same as supports that lift off due to thermal deflections (ie a “Hot Sustained” case).