The Sustained Stress equation used in CAESAR II is:

Slp + Fax / A + Sb < Sh

where: Slp is the longitudinal pressue stress
Fax / A is the axial stress
and Sb is the bending stress

Slp is computed as: PDi^2 / (Do^2 - Di^2)
Sb is computed as sqrt[(iiMi)^2 + (ioMo)^2] / Z

These equations are also listed in the Quick Reference Guide, which is available on-line if you don't have a hard-copy.

To ignore the SIF for the B31.3 Sustained case, you are correct in modifying the value of the B31.3_SUSTAINED_CASE_SIF_FACTOR. However, I would prefer a value such as 0.0001 instead of zero. Zero works, but the .0001 is more obvious as to intent. (People don't notice zeros, they will notice 0.0001.)

What actually happens when you use this 0.0001 (or zero)? Instead of considering the bending terms above as (i * M), imagine ((f * i) * M), where "f" is the B31.3_SUSTAINED_CASE_SIF_FACTOR. The program first determines the product of (f * i), where the minimum value allowed by the Code is 1.0. So whether you use 0.0001 or 0.0, the product (f * i) will be very small, and CAESAR II will over-ride this and use 1.0 when determining the bending stress.

Note however, the SIF value reported in the output is the value computed according to Appendix D. CAESAR II does not report the value of (f * i)!

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Regards,
Richard Ay (COADE, Inc.)