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.