This may be a strange question, but;

From a stress, and code stress, standpoint; what is the physical significance of the node at the junction of the branch and header legs?

I understand that the piping model is made up of basically structural members. In that sense, it is easy to visualize the junction. But with a tee, the intersection of the centerlines at the junction node is in the middle of tee with no real "physical" connection.

The nodes at the connecting welds at each end of the header runs and the branch run, in the case of forged welding tee, are obvious; but if the node at the junction exceeds the allowable what is the physical significance of this? What part of the tee is over stresses? Or, if the junction node exceeds the allowable, but the weld nodes do not, for whatever reason, is that a problem?

B31.3, fig 319.4.4B seems to be concerned with the forces, in-plane and out-of-plane moments at the weld faces of the fitting, and not at the junction.

Also, the junction node seems to have two different sets of forces and moments. One set from the pipe run through the header at the junction and another set from the branch at the junction. It would seem that these two sets of forces and moments would be additive, or the same.???

Any help in clarifying my confusion would be greatly appreciated.