For B31.1 / B31.3 and similar codes, the piping system is evaluated, from a stress point of view by considering failure modes. This leads to the following load cases for evaluation:

• Sustained - for primary, force based loads
• Expansion - for secondary, displacement based self limiting loads
• Occasional - for primary extreme (wind, seismic) loads

For each of these conditions, the piping codes provide equations to determine both the code stress and the allowable stress. From a pipe stress point of view, the system passes the code requirements if all of the code stresses are less than the corresponding allowable stresses.

The B31.1 / B31.3 codes do not consider the Operating Condition to be a stress case, therefore, there are no equations provided to compute a code stress or an allowable stress. CAESAR II computes an operating stress using a basic strength of materials equation, but the allowable is set to zero since there is nothing to compare this stress to.

The Operating case is important and it must be evaulated for:

• The determination of maximum displacments.
• The determination of equipment and support loads.
• The computation of the "Expansion Stress Range".

There are other piping codes (B31.4 / B31.8) that do consider the Operating stress condition as a code case. For these codes there is an allowable stress computed and compared to the actual stress in the pipe.

Take a look at the CAESAR II Quick Reference Guide, for a list of the piping codes supported, the load cases addressed, and the equations for the code stress and the allowable stress. Note, these are the load cases that must be run for stress evaluation. There may be other load cases necessary for structural or operational reasons.