B31.1Stress due to sustained loads

[carletes]

Dear all,

As you know, ASME B31.1 states that stresses due to sustained loads are lower than Sh. When calculating longitudinal stress due to sustained loads only moments and pressure effect is taken into account but not stress due to axial loads (for example Bourdon effect). Does anybody know the reason? Or perhaps anybody knows, at least, where I can fing the explanation? (perhaps an ASME paper).

I don't really understand it....

thanks for any help

 

[davefitz]

Not sure, but certainly axial loads must be considered for the effect of gravity loading, espescially if the pressure part is used to support itself or another pressure vessel.

 

[bvi]

It is a simplification in the rules. Remember, the rules were developed when folks were doing hand calculations for these stresses. The justification for the simplificiation is that the effect of axial loads other than pressure is generally insignificant.

 

[JohnBreen]

Hello,

Just my opinion.

BVI is correct, Pure axial stresses "seldom" are significant in real world systems. We did at one time (maybe 25 years ago) have a term in the equation for sustained longitudinal stress (aka -"additive stresses"). That term was simply the axial tensile or compressive force divided by the cross sectional area of the pipe (F/A). It was removed because the committee thought it was a needless complication.

Some of the current stress analysis computer programs either give you the option of including this term or (TRIFLEX) they put the term in regardless.

Thanks for bringing up the topic, it is nice to know folks out there think about this stuff.

Regards, John.

 

[Cockroach]

Longitudinal stresses are often insignificant compared to hoop and radial stresses. The most significant of course, is the hoop stress.

Depending on your exact setup, longitudinal stresses are often constricted, i.e. the component is not free to lengthen due to endcap reaction to bore pressure. This IS NOT the case when external loading induces components of stress in the longitudinal direction, you need to be very aware of this important fact.

Therefore the piping is modelled as a biaxial state of stress and not triaxial. On occasion this is not the case, the ASME B31.1 specification seems to be slightly misleading or at least, confusing, as you have correctly pointed out.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada