I've had a lot of discussions on using BS 7159, but I'm still trying to get it all straight.

Once and for all, can someone please clarify how to properly set up basic load cases in CAESAR II for a BS 7159 analysis? Here's what I specifically want to know:

1) When setting up load cases, should I select "OPE" stress type for operating (weight, temp., pressure) and "SUS" stress type for sustained case (weight, pressure)?
2) What do I select as the stress type for the expansion case? The recommended load cases suggest "EXP." Should I be selecting "OPE?" If so, why?
3) I have been told there are no expansion "code" cases in BS 7149. Is this true? Why? (I have ideas, but I want a clear, definitive answer on this one.)
4) If the expansion case is truly NOT a "code" case (thereby not appearing on the "code compliance report"), then are the stresses that the program reports for this case in the "stresses" report bogus?
5) I guess it all boils down to, "How do I read the results? Which stress reports are the ones I should compare to my allowables?"

Thanks in advance.

Kelly

Sir,

I suggest you study the code in question for # "3) I have been told there are no expansion "code" cases in BS 7149. Is this true? Why? (I have ideas, but I want a clear, definitive answer on this one.)"

As well as your #5 question.... CAESAR is a program which requires the user to be knowledgeable in the code be used for the analysis.

Mr. Luf,

Unfortunately I do not have access to this code. I have requested a copy because I am fully aware that understanding the code is absolutely essential to performing an analysis. However, I am being forced to run this analysis using this code--even though I made everyone aware this would be a problem (aka "in-one-ear-out-the-other" syndrome)--so I have no choice but to seek out as much information as possible from the people who understand it best.

So, I ask once again, is there anyone out there who can give me a "BS 7159 for Dummies" version?

Thanks.

Rather than looking at different stress types and limiting those values based upon the appropriate failure modes, BS 7159 simply (well, actually not so simply -- I'll touch on that in a minute) limits the amount of absolute strain to which the piping system should be subjected in any of its states. We don't know where the maximum strain might occur -- in the installation or an operating state, so I suggest that you evaluate all of these potential load cases (which CAESAR II normally calls SUS and OPE) to find the worst case. For this reason, whether you label a load case SUS or OPE, CAESAR II evaluates the stresses (or more appropriately design strain times modulus of elasticity) in the same manner, as per BS 7159 requirements.

EXP stresses are typically not absolute stresses, but rather ranges between stress states. Since fiberglass doesn't go through shakedown like steel pipe does, and BS 7159 only limits absolute strain (as opposed to strain range), the code doesn't require a stress check for EXP stresses (so CAESAR II does no stress check). But these stresses aren't really bogus -- BS 7159 uses them to evaluate the influence of fatigue in the loading condition. Where cyclic loadings (i.e., the EXP case) are significant, BS 7159 suggests that the original project design strain be reduced, by dividing it by Kn, where:

Kn = 1 + 0.25(As/Sn)(Log10(n) - 3)

where: As = the stress range during the fatigue cycle (i.e., from the EXP case)
Sn = maximum stress during the fatigue cycle (i.e., the greater of the SUS or the OPE)
n = number of stress cycles during the range (note, for fewer than 1000, this evaluation is irrelevant)

In other words, the greater the percentage of the operating stress that is cyclic, the lower the allowable strain/stress will be for the OPE cases.

So what you want to do is set up your normal SUS, OPE, and EXP cases, and run the analysis. For any EXP case where you expect to have more than 1000 cycles, you should calculate the Kn value as shown above, then go back and enter that value into Kn slot of the CAESAR II Allowable Stress screen corresponding to each temperature, and then run the analysis again (yes, two analyses -- I apologize to any limited run customers out there).

So specific answers to your questions:

1) When setting up load cases, should I select "OPE" stress type for operating (weight, temp., pressure) and "SUS" stress type for sustained case (weight, pressure)? Yes, although selecting either OPE or SUS stress type for all of those cases will give the same results.

2) What do I select as the stress type for the expansion case? The recommended load cases suggest "EXP." Should I be selecting "OPE?" If so, why? Select EXP -- no stress check should be done for these cases.

3) I have been told there are no expansion "code" cases in BS 7149. Is this true? Why? (I have ideas, but I want a clear, definitive answer on this one.) This is true -- BS 7159 is concerned with absolute stress, plus fiberglass doesn't behave the same way under cyclic load as metals do.

4) If the expansion case is truly NOT a "code" case (thereby not appearing on the "code compliance report"), then are the stresses that the program reports for this case in the "stresses" report bogus? No, EXP stresses should be used to go back and adjust the allowables for your OPE cases if necessary.

5) I guess it all boils down to, "How do I read the results? Which stress reports are the ones I should compare to my allowables?" I hope I've answered this question.

Thank you, Mr. Van Laan. Once again you have been most helpful. Hopefully my superiors will obtain that copy of BS 7159 soon. Then, I will read it myself.

Thanks again for contributing to this young engineer's learning library.