Hi,
I have a question related with pressure stiffening effect.
As you know b31.1 code includes a pressure stiffening effect.
And i also know that I can reflect this on Caesar ii with configuration setting to "default".

My ultimate question is "elbow stiffening pressure" tap of load case option on Caesar II.

When I select default option of pressure stiffening effect on configuration, we can see the elbow stiffening pressure tap of the load case option is all "PMAX".

But i think that it shoud be changed to "None" on Expansion Case. is it right?

please advise me about this issue.

B31.1 is not prescriptive in this respect. Appendix D Note (5) simply says: "In large diameter thin-wall elbows and bends, pressure can significantly affect magnitudes of k and i.". There is no criteria established to define "large diameter thin-wall elbows and bends". This is one reason why the setting appears in the program's Configuration and Load Case Options - giving you, the user, ultimate control.
In my opinion, this stiffening would affect all load cases. The expansion range case may be more significant as higher stiffness, here, produces more conservative results. I'm looking at stiffness (k) here but keep in mind that this pressure stiffening will also reduce i and this smaller term will reduce the calculated stress range.

To add to what is stated above, remember that the Expansion case is a range case. There is no [K]{x}={F} solution performed for (where pressure stiffening effects [K]) the Expansion case. So if your Expansion case is "OPE-SUS", then both the OPE and SUS cases should likely consider the Code Default to include pressure stiffening effects. The resulting displacements from these two load cases would be used to determine the (EXP) range. Altering the pressure stiffening setting for an EXP case would only affect the SIF (i), as pointed out above, since "k" would not be used.