Because you have not read or do not understand the code you are working to. You do not understand by default there is no required code operating case stresses.

There is an optional method shown in the 2004 book but it is not a required code case.

Buy a copy of B31.3 2004 and read it...

B31.3 2004 requires the following loads to be evaluated... displacements, sustained loads, and occasional loads (combined with sustained loads). Note there is no required analysis for operating loads which are combinations of perhaps all 3 of the above load types. I repeat “There is an optional method for calculating operating loads/stresses shown in the 2004 book but it is not a required code case.”

V5.0 of CAESAR II may have the optional calculation of operating stresses available but again this is not required by the code it is merely an acceptable alternative.