If the code design is NEN 3650 this don't have anything to do with B31.1.
The NEN 3650 section 10.5.6.b assessment criteria for the stress limit in accordance with the elastic theory is reflected by the following condition.
Sdesign / SCF < Slimit
Where: Sdesign = Calculated design stress - based on design loads.
SCF = Stress correction factor.
Slimit = Stress limit (allowable stress).
You need to check:
Equivalent Stress Sv1 (pm)
Equivalent Stress Sv2 (pm + pb)
Equivalent Stress Sv3 (pm + sm)
Equivalent Stress Sv4 (pm+pb+sm+sb)
As per NEN 3650, par. 10.5.6.b, the calculated stresses are to be combined into the so called equivalent stresses Sv1, Sv2, Sv3 and Sv4. The equivalent stresses for the different stress combinations are determined in acc. with table 9 of mentioned paragraph, and are checked against the stress limit Re(ө), taking into account the stress correction factor. This method considers a possible redistribution of stresses due to Elastic Shakedown (relaxation).
Loads case (just an example):
Sv1 pm 1.5 P1 compared against 1 x Re(Φ)
Sv1 pm 1.5 P1+ 1.5 W + H + 1.2 WAV +1.2WIN compared against 1 x Re(Φ)
Sv3 pm + sm 1.5 P1+ 1.25 T1 + 1.5 W + H + 1.2 WAV+1.2WIN compared against 1.5 x Re(Φ)
Just an example I offer , loads case depends by many factors and external loads. You have all description in standard.
Conclusion: Read the standard
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Marian