I am in the midst of a failure investigation and Caesar is making predictions that do not make sense to me. I'm thinking that this is a mathematical problem, i.e. a function of the way CAESAR II calculates stresses. I wanted to see what you guys thought if you have a minute.

During a recent rainstorm in this area, runoff water pooled beneath the footings of several overhead pipeway column supports. The water pooled under the supports because the drain culvert that serviced the area was clogged with debris. The soils in this area are collapsible when wet, so when the soil got saturated from the runoff, it collapsed downward about a foot, taking the footings with it. This caused large movements (displacements) resulting in severe bending of the piping attached to the pipeway.

My client asked me to examine the situation and make any recommendations. My analysis to this point has been to model the failure by modeling the piping attached to the supports and then forcing the large displacements at the support nodes using a cnode. The support (restraint) uses the cnode as the displacement node.

Well, I built the model, and the stresses that CAESAR II predicts in the displacement load case are far in excess of the ultimate strength of the pipe. CAESAR II is predicting the max stress as 200+ ksi. This is A106B pipe which has min. specified ultimate strength of 60 ksi.

To me, this says that the pipe should have fractured, but the pipe has not in fact physically fractured, it is just 'all bent up'. I am sure that it has gone plastic althought I have not yet done the calculations to verify this. The pipe also has not locally buckled or 'kinked' from the bending. I could back out the principal stresses and calculate the von Mises' stress to check that criterion too but it still will be far in excess of 60 ksi.

I know that CAESAR II will only do what it is told. I am confident that my model is as close a mathematical representation of the actual physical situation as I can make it. I don't think there's anything else I can do to make it more accurate. My point here is that I'm trying to get confidence in CAESAR II's results. I guess one conclusion to be drawn here is that CAESAR II is best left to the design situation in which the material of the piping system is still in the elastic range, assuming 'failure' is defined as the proportional limit of the material, the allowable displacement stress range of B31.3 notwithstanding. Is this strictly a mathematical manifestation? Or am I forgetting something from Mechanics of Materials 403?

Thanks very much in advance for any help/ideas/suggestions/random thoughts.


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Thanks,
Pete
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Thought for the day:
Good judgment comes from experience;
Experience comes from bad judgment.