Thanks Dave, exactly what I wanted to know.

I have been looking at some calcs where the corrosion allowance is incorporated into the effective Z calculation in the following manner:

Ts = lesser of (nominal thickness of header - C, or i*nominal thickness of branch - C)

and r2 = mean branch C.S.A - corrosion allowance

which gives a much more conservative stress value than the method CAESAR II uses.

But looking at appendix J in B31.3, it is clear the nominal thickness should be used for the terms in calculating the effective section modulus, and hence I don't believe the corrosion allowance should be subtracted in the above manner.

However I also find it hard to get my head around taking the corrosion allowance off the Ts term (ie Ts - C) as the i*tb term is not a 'real' thickness, hence it does not seem correct to take the corrosion allowance off this value either.

I’m thinking about approaching the B31.3 committee for clarification on this one...