Hi PTPS,

That is a common question. All of us have wondered the same thing at one time or another. It can be extended to cover "base elbow" supports and "dummy legs".

Truth is the closest we can come to an answer is to do a finite element analysis. Even then, since the loads to be applied to the FEA model will come from our beam theory analysis (which will have inexact flexibilities at elbows), the FEA model will return "approximate" calculated data.

The SIF will vary as the elbow size varies and as the welded appurtenance size varies. Since you cannot model a "branch" at an elbow, you just have to be "creative" and be conservative with your acceptance criteria. Everyone here will tell you that any kind of an "outlet" on an elbow is a bad idea EVEN IF YOU COULD MODEL IT. So, just be careful and conservative.

Sorry about the "non-answer".

Best regards, John.