I would like to pose a somewhat philosophical question, i.e. Is it necessary to use only the 1 SIF in equations such as Equation 11 of B31.1 in order to comply with the requirements of the Code?

As food for thought I offer the following 2 tidbits:

1. The Foreword of B31.1 states in part "a designer who is capable of a more rigorous analysis than is specified in the Code may justify a less conservative design, and still satisfy the basic intent of the Code".

2. The December 1992 issue of Mechanical Engineering News states in part "The equation which most follows the intention of the B31 Piping Codes for longitudinal and bending loads is given below, and is implied from B31.1 equation 11
Stress = PD/4t + iaxF/A +(1/Z)[(ioMo)^2+(iiMi)^2+(itMt)^2]^1/2"

So to re-state the question: If the designer has determined the various SIFs from an FE analysis, is it acceptable to use them in a B31.1 analysis?

Your question is directly answered by paragraph 119.7.3 which states "In the absense of more directly applicable data, the flexibility factors and stress-intensification factors shown in Appendix D may be used."