It is difficult to say what is the real allowable stress for your structure because you did not tell governing Code, but I understand that my practice will be helpful to you.
The designation of allowable stress for fillet welds are as follows;
1) AISC : Shear = 0.3 x nominal tensile strength of weld metal for shear on effective area, Nominal = same as base metal for tension or compression parallel to axis of weld
2) AWS : Shear = 15.8 ksi for E70XX electrode, Nominal=11200 x weld thickness (lbs/in)
These two standards do not consider the temperature effect. However, as you know, ASME Codes include temperature effect but they do not tell the allowable stresses for each welds clearly. Therefore; in my case for FEA model, I consider the weld strength is same to base metal and check the weld stresses with ASME VIII,Div.2 stress criteria as below;
ASME VIII,Div.2 indicates ;
Primary stress(primary membrane plus bending) should be less than 1.0Sy(or 1.5kSm, or 0.5Su) and Primary plus Secondary stresses should be less than 2Sy(or 3Sm,or Su) at operating temperature,for occasinal loads.(I neglect the first stress criteria,general primary membrane shoul be less than 2/3Sy, since you might checked this stress)
Your structure undertakes pressure and temperature stresses simultaneously. It seems that your structure is for just one time use. This means your structure has primary and secondary stress without cyclic loading. Since you checked weld stress with FEA, I think, your structure is safe if FEA stress results satisfy above two stress equations simultaneously.
For more conservative check, the maximum weld stress(total stress) should be within the yield strength of base metal.
Mr. Ohliger¡¯s topic is very good and also I want to know the more reasonable practice. If anybody who know the more practical or exact practice, please advise that.
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Sun Wee
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