So I'm just now starting to work with the new B31J converter implemented in CAESAR II v9.00. I've taken a model I created that was failing at a branch connection using a reinforced tee, and run it through the FEA Translator. After I converted the model and opened it up I noticed that some of my reducing tees I had modeled were now changed.

The conversion made the reduced branch size the same as the run pipe size, but then kept the pipe after that like it was originally. I ran this conversion on another model and the same thing happened. It appears to affect branch connections where a reducing tee goes immediately into a reducer.

Has anyone else had problems similar to this?

B31J specifies a branch flexibility for tees. The B31J processor will place this flexibility (actually, stiffness) at the surface of the header pipe. This requires a node in your model. I believe you are seeing that header-sized pipe running from the header/branch centerline intersection to the header wall along the branch.
Is there more to it?

That's correct, it is placing a node at the OD of the header pipe. So this is happening because I didn't have a node there already?

My concern is that I'm not sure if it's calculating the new SIFs based on my original reducing tee or if it's calculating it based on the straight tee its creating.

There is a spreadsheet with B31J Essentials that will show you the SIFs for a tee that you describe. Open that processor and check the tee SIFs for the full size outlet and also for the reduced outlet. That should confirm your model numbers.

Where is this spreadsheet located? I have searched the PRG folder where B31J essentials is installed and could only find a PTP Catalog spreadsheet.

What Dave meant was that there is a tool that is part of "B31J Essentials" that pulls up a (spreadsheet) dialog that shows the SIFs and K factors for your tee. Click on the "SIF/K" button.

As to your question, as Dave stated above, that straight pipe is added to connect the center of the tee to the surface of the header where the branch element connects. The SIFs at this location are based on the branch dimensions (as well as the header dimensions).

There is a separate spreadsheet but here's the same thing in B31J_Essentials: