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Tips October - December 2007

Combined Stresses in B31 codes

Question: I downloaded and installed the evaluation version and found it "addicting". One question I have relates to combined thermal, operating, and static seismic stresses. I do not seem to find where I can evaluate stresses from both operating and seismic together. Please advise if and how I can accomplish this.

Reply: Thank you for your compliment on CAEPIPE.

The combined stresses one gets from CAEPIPE for something similar to what you want is termed "Occasional Stresses (SLO)" as per most ASME/B31 piping codes. These are the sum of stresses due to Sustained (W+P) and Occasional loads such as Wind or seismic or time history etc.). Under Sorted Stresses, CAEPIPE displays the max. of such combinations at EACH node. But under the Code compliance results screen, you can see occasional stresses (SLO) due to each occasional load (click on the right black arrow in the toolbar to cycle among the different loads).

There are no combined Operating stresses according to B31.1/31.3 design requirements (since sustained and thermal loads cause piping failure by a different mode, former by collapse and the latter by fatigue); hence you will not see an Operating Stress or for that matter, an Operating Stress + Seismic stress. The support loads, however, are combined for these cases; look under Support Load summary in the Results menu.

But, if you MUST get an idea of what that combined stress might be, reanalyze the model after changing the piping code in CAEPIPE to B31.8 or ASME SIII Class 2 NC 1992; in Results, look for "Total Stress" in B31.8 or Eqn. 11 in ASME 1992 results. Also, refer to the Appendices section of CAEPIPE User's Manual for details on the equations, or look up the specific code itself.