I am analysing a subsea piping system according ASME B31.8 and I have a question regarding SIF in flanges.

ASME B31.8 Chapter VIII states that a SIF shall be used to calculate the longitudinal stress and combined stress.
Different SIF are listed in Appendix E.

How is this SIF generally added when modelling the flange as a rigid?

Will the stiffness of the rigid compensate for the stress concentration in the flange?