Hello Fellow Stressers,

Why does the Code do not consider the maximum stress intensity from the combined (Hoop, Longitudinal & Torsional) Stress and compare it to the uniaxial state of stress?

It separates hoop & longitudinal stress evaluation. Hoop stress is used to establish the thickness. While combined bending and torsion is used for longitudinal stress evaluation (in which 2 principal stress is created S1-S2
I believe there is a separate output in stress software considering the hoop, longitudinal and torsion stress states combined (Max Stress Intensity SI).

Thanks,

I have always looked at this question from the "failure" point of view. B31.1 and B31.3 consider primary (force driven) failure as the primary mode of failure. Then Markle published his papers in the early 1950s detailing another mode of failure - displacement driven. He referred to this mode of failure as secondary.

B31.1 and B31.3 (and similar codes) address these two failure modes as primary (what CAESAR II calls "sustained" and secondary (what CAESAR II calls "expansion").

I'm sure you can find better more detailed explanations of this topic in the various texts focusing on Pipe Stress Analysis as well as the B31 Code.

Thanks Richard for that very nice explanation.

For expansion stress, yes, the pressure terms (Hoop Stress & Radial Stress) will drop down from the equation of Maximum Stress Intensity (considering pressure as the primary load).

For Sustained Stress, the equation considers pressure terms which can include radial pressure stress (0,-P,-P,0) and can be the minimum principal stress S3.

Anyway, it's still a good idea to include in the output of the software the Maximum Stress Intensity (S1-S3). Normally, this is higher than the Code Stress, and I observed that the Max. SI is somewhat near to the Hoop Stress Value, which makes sense because S1 is a function of both hoop and longitudinal stress. This is a useful tool for the evaluation of fatigue and Creep.

Thanks,