In B31.3 -2012:
Sa=Ia(Fa/Ap) and Fa is axial load due to pressure and weight
In CAESAR II Fa due to pressure is P*Ain and Fa due to weight is Fax, so:
Fa=P*Ain+Fax
Sa, then, is equal to Ia((P*Ain+Fax)/Ap)
with P*Ain/Ap= Slp,
Sa=Ia(Slp+Fax/Ap)

and CAESAR II uses absolute value on axial stress due to weight