I just ran a B31.3 cantilever in CAESAR II 2017 with two load cases: 1) W(SUS) and 2) W(EXP). The stress at the anchor will be based on the weight bending moment.
With the default setting "All Cases Corroded" FALSE, I show a much larger sustained stress than expansion stress range. This is the expected result since the sustained case uses a corroded section modulus and the expansion case does not.
When I set the configuration switch "All Cases Corroded" to TRUE, the sustained stress and expansion stress range values are identical.
This is the way the program is intended to function.