SUSTAINED CASE STRESSES - LIFT OFF!!!

[psapiensone]

I know exist a very long discussion about these, but can somebody explain to me detailedly this?

For example:
Can I temporarily delete the supports in the sus case when I have lift off?

Thanks.

Pablo

 

[nickelkid]

You are on the right path, typically I make a seperate lift off run and keep it in the file. See the attached from Peng and Coade, also review Appendix S in B31.3

 

[nickelkid]

Here is the Peng paper

 

[DSB123]

If you have a lift off situation on supports then these supports should not be active in the sustained run since in operation they will not be supporting the piping and remember the code says the sustained stress limits need to be met in the operating condition - so if the pipe lifts off then do not include for sustained stress check.

 

[richay]

"If you have a lift off situation on supports then these supports should not be active in the sustained run"

... which is why you should review the documentation on the "hot sustained" condition discussed in the CAESAR II documentation.

Removing supports that lift off is a bad idea since the Expansion case will be wrong. Similarly if part of the piping system is alternately "spared", then supports may be on or off depending on the state of the local piping.

This is a complex issue - you'll find a lot of discussion on this subject on the COADE website.

Richard Ay
COADE, Inc.

 

[DSB123]

Richay,
Not sure how Caesar handles lift off but still maintain what I have said is correct. If Caesar does this sustained check in the hot condition so be it but is does not go to say that my comment is incorrect.

 

[nickelkid]

Caeser does not identify Lift Off; the user does by reviewing the output. When Lift Off is found the user makes a separate run without the supports (that have pipe Lift Off) to check the sustained stress. The separate run should not be interpreted as removing the supports from the Design.

I see nothing wrong with what DSB123 posted.

 

[DSB123]

RCHandy,
Cheers for the vote of confidence. I have used this approach with many pipe stress programs but alas have never had to use Caesar so do not know its nuances except that it does not report the highest stress automatically for bends.

 

[4Pipes]

I would also fully agree with DSB123. And Peng. Folk argue that Peng covers creep temperatures. One of his papers does but lift-off creates other problems including possible code non-compliance referred to.

I haven't worked on a reasonable size project in 20 years without seeing problems with the default Caesar load cases and lift off. (On paper and on site). Normally I would blame blame the inputter rather than the software. However, I would make an exception with Casesar because the explanation of lift off is included in the manual and presented as implied fact. (or it used to be).

 

[StressGuy]

I agree that Caesar's implementation (or rather, lack thereof) of addressing "true" sustained stresses considering supports lifting off. It's a struggle dealing with new engineers and offices that lack the technical sophistication to understand that when you get a +1000lb load on that Y, your sustained stress report isn't right.


DSB123 - regarding stresses in bends - Coade finally fixed that, I think with the first Windows version. You did have to specify the bend differently in order to get initial and midbend nodes for stresses and it was not the default behavior. These days, clicking the bend checkbox will give you node for three points on the bend as the default.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[DSB123]

Stressguy,
Thanks for the update on the way Caesar calculates the stresses around a bend and I had noted that this option was there BUT the location of the maximum stress in a bend, unless you are very lucky, will not coincide with 0, 45 or 90 degrees. Typically the position of the maximum bend stress could be at any angle around the bend. Using initial, midpoint and far end nodes is not enough to capture the highest stress position but is better than what Caesar used to do by just checking at either end of the bend.

I agree with your first note. I have come across many stress reports where the sustained run indicated pipe lift-off. It's a by-product of having pipe stress software which is easy to use by users whom do not know what to look for in the results. Even had a recent project where the results from a user of Caesar had the vertical seismic loads on several supports greater than the pipe weight, so the pipe would lift off in reality, but it took me some time to convince the analyst that he needed to do something about the situation. Not the software's problem - just the user.

 

[psapiensone]

I found this good article...

http://paulus-sapiens.blogspot.com/2009/03/problem-with-piping-lift-off.html