I remember several years ago, while describing the nonlinear capabilities in CAESAR II, an engineer said to me that lifting off a support is poor design. To him it was a yes/no, good/bad or black/white issue. I am sure that this is (was?)the opinion of those earlier pipe stress engineers who could handle a key punch machine. They just didn't allow it to happen.

So now we have software that can handle the structural response of piping systems with nonlinear boundary conditions. And yet we do not have clear guidance from the piping codes. The latest B31.3 interpretation of which I am aware states that sustained stresses should be examined in ALL support configurations.

Now I've been working with analysis for many years and, with only an exception or two, I am quite confortable with how CAESAR II does it. It is my opinion that "the Code" calls it sustained stress because it is always there and can lead to failure by collapse. Also, all changes in stress are secondary and these lead to fatigue failure.

I think this subject has a lively set of comments because, since the piping codes have not yet "told us what to do", we are all doing what we believe should be done. But remember, the piping code shouldn't be seen as a guide to good design.

What we should do is all get together at some bar after work and throw out a few conversation starters like lift off, sag, shakedown, hot tap, rachet, hot modulus, or friction. Until then, I think this thread would be of greater benefit if, instead of saying CAESAR II is right or wrong, we offered and supported our own opinions on any of these subjects.

Here is one opinion - I have not found a situation where (Operating)-(Thermal), as offered by Richard Havard's auditor, did not produce the same sustained stresses as the linear "hot support configuration" (with "unused" restraints removed).

Here's another - any yielding that occurs while the system moves to its hot position is shakedown.

And here's my question to the field - if a 1mm liftoff is OK but a 50mm liftoff is not good, what measure of "goodness" are you using? Is it Itchy's "make sure no one sticks something in there" or "make sure no one removes the (maintenance) support" or "boy this stress is getting up there" or what?

I hope I'm not hijacking this thread...

Dave