The empirical formulas in B31J are based on physical testing of (pipe fitting) specimens in a lab AND Finite Element Analysis. The empirical formulas are provided in B31J so that implementation by Engineers and software vendors is possible. These B31J equations f(which are dependent on D, T, d, and t) are more accurate and more realistic that the old Appendix D equations (which were only a function of the D/T of the header).

If you doubt the new B31J equations, use FEATools and the FEA option. We have been recommending this approach since 2013.