Hi, welcome back, I soon start with my question.
I noticed that in sustained cases, allowables undergo variations (they diminish) at the points with SIF (stress intensification factor). Though I can understand why, I cannot find in Caesar the setting that has to do with it. I think it has to do with weld joint efficiency and it seems strange because in B31.3 since 1980 this parameter shouldn't be taken into account. I have read Caesar technical documentation but still I couldn't focus.
Could you help me?

It is not "eff" that is affecting the allowable, but "w". See Paragraph 3.2.3.5c. Note that "w" has been removed from this paragraph for the 2008 Edition, which CAESAR II does not yet address.

Version 5.20 gives you the option of ignoring "w" (in the Configuration).

Ok, many thanks, I understood!!
Is this option available only on 5.20? I am still using 5.10.

Yes, this is only available in Version 5.20. We added this option because the 2008 Edition of B31.3 made the use of "w" optional.

Getting back to the problem I read that for B31.3 the "w" factor applies (in the SUS cases) when there are weldings over a temperature of more than 510 degrees.
In Caesar II I see it on bendings and tees, what if I want to apply it on straight run pipe (which however has weldings along the run)?

Assuming you know where the welds are, you would have to define a node at that location, and then check the "SIF&Tee" checkbox, indicate a butt-weld, and specify the "w" factor.

It is paragraph 302.3.5c . B31.3 2008 edition. pp.15.