The B31.1/B31.3 (and similar) Codes are setup to evaluate stress based on the recognized failure modes: primary and secondary (for details on this refer to Markl's papers).

The Primary stress state/failure mode is force based, where the load does not change with system displacement. The "Sustained" case is the load case for the evaulation of this state.

The Secondary stress state/failure mode is displacement (thermal) based, where the loading is cyclic. The "Expansion" case is the load case for the evaluation of this state.

So from a "stress point of view" you only need to evaluate these two load cases (ignoring Occasional which is also Primary). The Operating case is necessary for restraint design, limit checks, and as a computational component for the Expansion case. However, the stresses from the Operating case are neither primary or secondary, and therefore this case is not a Code Compliance case.

Read the "sticky topic" at the top of this Forum on this same subject.