Regarding Maximum allowable stress & Design Stress Intensity

[aa1122]

I have a query regarding Maximum allowable stress values , S , (From Table 1A) and Design stress intensity, Sm , (From Table 2A).

For A106 Gr B (-20 to 100)---- Design stress intensity= 20.0 Ksi
For A106 Gr B (-20 to 100)---- Maximum allowable stress values , S = 17.1 Ksi

My doubt is why this difference is coming?

I understand that 1A is based on : Maximum Principal stress Failure Theory
I understand that 2A is based on : Maximum Shear stress Failure Theory

Now as per Maximum Principal stress Failure Theory(Rankine Theory)---Plastic deformation occurs in a 3-Dimensional stress state whenever the Maximum Principal stress exceeds Syield.

And

As per Maximum Shear stress Failure Theory(Tresca Theory)---Plastic deformation occurs in a 3-Dimensional stress state whenever the Maximum Shear stress exceeds Syield/2.


But how we can find the clue about difference in Design stress intensity & Maximum allowable stress values?

Kindly share your opinion.

 

[TGS4]

The difference comes about because of the different design basis, as described in II-D Appendix 1 and 2. Don't over think this - it's a matter of design margin.

The issue that you present has some merit as a controversy in VIII-2, because the fundamental formula for calculating wall thickness is based on maximum principal stress, yet combined loading is based on von Mises.