Bending stress calculation difference between OPE and OCC load cases

Question

Bending stress calculation difference between OPE and OCC load cases

Emslief (Mechanical)

(OP)

6 Sep 17 06:30

Good day,

I have found an interesting problem related to a wind loading on a B31.3 pipe system. The occasional Load Case read into Caesar LC Editor is W+P1+WIN1 with the OCC stress type being selected (i.e the combination of SUS and wind). When run the analysis results in a bending stress of 399 MPa. This did not look right as the sustained load case only results in a bending stress of 43 MPa. Wind loading is calculated as per AS1170 and is input as pressure (abs) vs elevation (abs) - at ground level: 39 m/s or 1123 Pa which are typical values for most plants. Here's the strange thing, when I change stress type from OCC to OPE the bending stress drastically reduces to 107 MPa which appears to be more in line with the situation.

So at the risk of not understanding/missed something in the code and/or software my question is: does Caesar use different formulae to determine bending stress for OCC and OPE loads? If so why would the OCC bending stress be so significantly higher?

I look forward to the forum's feedback.

Emslief

DSB123 (Mechanical) · Answer 1 · 2017-09-06 11:49:32

RE: Bending stress calculation difference between OPE and OCC load cases

DSB123 (Mechanical) 6 Sep 17 11:49

REad and understand how Caesar works !!!

Emslief (Mechanical) · Answer 2 · 2017-09-07 00:09:47

Hi DSB123 - yes I understand your frustration with my question but believe me I have read and am no nearer to an answer. To try and figure out what the software is doing I have built a simplified model and am getting "unexpected" results for axial stress. I can’ t seem to find the explicit equation of how Caesar calculates axial stress for the SUS case for a B31.3 system. The problem is that the axial stress worked out using the code equation (23d) is 3754.7 kPa yet Caesar gives me 9759 – so it is adding 6000 kPa for some reason. The effects of bending and other loads on longitudinal stress should be negligible as I have modelled a test case of a DN50 SCH 80 A106B pipe only 200mm long. Restrained at an anchor on one end, a +Y (0 friction) support in the middle and a frictionless Guide and Y support at the end. Pressure is 1875kPa.
When I run OPE the axial stress matches equation 23d. When I run the same scenario using ASME B31.1 criteria the SUS axial stress matches 23d. What am I missing here? I have spent many hours going through manuals, reference guides, the code and various forums and am no nearer to an answer.

Thanks for your help and apologies if the question may seem trivial.

Emslief

richay (Mechanical) · Answer 3 · 2018-01-04 05:43:30

The equations used by CAESAR II to compute stresses can be found in the Quick Reference Guide. There could also be other items in play - like corrosion. If you can't resolve this, please open an SR (support request) on Smart Support and send in the job files.

Richard Ay
Intergraph CAS

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Bending stress calculation difference between OPE and OCC load cases