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).