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#9084 - 12/18/06 11:54 PM Allowable Stress
oaksdarcy Offline
Member

Registered: 12/17/06
Posts: 14
Loc: Brisbane, Australia
I am only new to CAESAR II, so this my seem a little trivial to some.

I am having trouble generating a code stress check, and stress distributions. I have checked the 'allowable stresses' box when I selected the material in the piping input. The material is 106B, the code B31.3 and the allowables are automatically allocated. When I generate the stress report, it says the allowable stress is zero, hence it does not generate the correct stress plots or code compliance reports.

Many Thanks.

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#9085 - 12/19/06 01:30 AM Re: Allowable Stress [Re: oaksdarcy]
Captain Kenny Offline
Member

Registered: 09/09/05
Posts: 72
Loc: Scotland
Hi
I'm guessing you've fallen into the classic trap of opening the software box without reading the instructions first. By instructions I mean the design code you are supposed to be working to. Before you go any further stop, put the software away and read the code.
If you search this forum you'll find many many people have asked the same questions at one point [I'm sure I have done it too BTW], so do spend an hour browsing the past posts here.
Assuming it's B31.3 read at least paras 300 -> 319 and especially paras 302.3.5, 302.3.6, 304, 319 and App D. The answers to your questions are all in there [if a little hidden perhaps]. What to watch out for, is that the code is not a cookbook and doesn't tell anyone how to design anything, and a lot of very subtle points are contained in the specific meaning of certain words.
If you can get hold of any text by Kellogg [Design of piping systems ~GBP70 on abebooks or other used bookshop if you can find one], Markl, Casti or Becht - buy it know and read read read.
The other thing to be aware of is that the code only sets stress limits. What about forces on supports, local loads where supports are fixed to the piping, acceptable deflections etc etc etc. No software will tell you if these are acceptable or not. That is where the engineering, training and experience comes in.
This is a field where a little knowledge can certainly be dangerous. Ultimately you have to remember that real people's lives are potentially at risk if you mess up.
No body was ever born a stress engineer, so please accept this post as encouragement to learn your chosen trade more fully before pressing the 'run' button blindly on the software.
As a hint - you're problem does not have anything to do with the allowable stresses in the input - they are likely to be correct. Read B31.3 para 302.3.5 again and post back what you think the problem might be.


Edited by Captain Kenny (12/19/06 01:44 AM)
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Kenny Robertson

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#9087 - 12/19/06 03:40 AM Re: Allowable Stress [Re: Captain Kenny]
SUPERPIPER Offline
Member

Registered: 08/13/03
Posts: 405
Loc: Europe
Sticky!!!!!!!!!
_________________________
Best Regards


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#9091 - 12/19/06 09:13 AM Re: Allowable Stress [Re: oaksdarcy]
Richard Ay Offline
Member

Registered: 12/13/99
Posts: 5975
Loc: Houston, Texas, USA

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#9350 - 01/11/07 04:03 PM Re: Allowable Stress [Re: Richard Ay]
oaksdarcy Offline
Member

Registered: 12/17/06
Posts: 14
Loc: Brisbane, Australia
Richard, Kenny,

I have taken onboard your advice, thanks.

Regards,
Michael

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#9392 - 01/16/07 02:57 AM Re: Allowable Stress [Re: Captain Kenny]
nadyesz Offline
Member

Registered: 01/12/07
Posts: 6
Loc: Mexico
I think it is not fare to be rude to somebody that asks a common question (if it is a common question, may be COADE should make an effort to further clarify its "code stress" concept in the manuals). I disagree with the idea that you should not use CAESAR unless you are an expert in code specifications. There are many engineers around that just need understanding materials behaviour or stress analysis, and not necesarily are familiar with code rulings or, worst, with the somehow obscure (for the beginner, of course) "code stress" concept that CAESAR uses everywhere.




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#9395 - 01/16/07 06:21 AM Re: Allowable Stress [Re: nadyesz]
John C. Luf Offline
Member

Registered: 03/25/02
Posts: 1110
Loc: U.S.A.
I disagree with "I disagree with the idea that you should not use CAESAR unless you are an expert in code specifications. "

This software, its approach, and its results are all dictated by the code the work is performed under. If you don't understand the code you will not model, set up load cases or simply understand your results. CAESAR II does not provide thinking you do!!!!! (or should????) Thats why your pay rate is what it is. Otherwise maybe we should all be replaced by some highly trained chimpanzees!

I wonder if you even look at your restraint reports for things like +Y liftoffs....
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Best Regards,

John C. Luf

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#9415 - 01/17/07 07:02 AM Re: Allowable Stress [Re: John C. Luf]
Captain Kenny Offline
Member

Registered: 09/09/05
Posts: 72
Loc: Scotland
Nadyesz
when I replied to the original post I took great care not to be rude to Oaksdarcy, but tried to frame the response in such a way that my opinion was made along with encouragment to learn more before using fairly specialised software. I think that Oaksdarcys reply suggest that he [hopefully] did not think I was being rude and I hope that he is much better informed now after doing some research and reading.
There is a fine line to be trod sometimes on a forum like this between 'flaming' someone and passing on advice and experience we ourselves have gained from others [no one was ever born a stress engineer], but you must realise that most engineers who reply to posts here are doing so:-
1) out of some sense of community and pride in their field of work
2) to freely pass on accumulated advice they have gained to others in the enginering community in that spirit.

Some [like myself] will often answer posts as a way of stretching our own thinking, testing our understanding and probing the assumptions we use everyday. Nobody [well perhaps a couple, but certainly not me] know ALL the answers and often the person answering the question will learn more in the process of compling a thoughtfull answer than the person asking it.

Regarding your point. The software is completely subservient to the design code being used. It is only a means of doing thousands of calculations quickly. If you do not have fundamental knowledge of what your design code requires, you are not in a position to make any comments on the validity of the results. I do seem to remember a post where the esteemed Mr Luf [or Breen] made the point that 99.9% of code allowable is not nessecarily OK and 100.1% is not nessecarily un-acceptable. These judgements should be made with a full knowledge of the code requirements and intentions, the research that led to those rules and the engineering fundamentals that back all those up. Miss any one of these out and you are at best guessing if the results are safe or not. And by the way, by safe I do mean whether poeple could die because of your actions.
Caesar is a only a tool, and like all tools it still requires intelligent oversight by a thoughtfull user to achieve satisfactory results, and that user must have the nouse to know when to use another tool because he does not think the results are valid. That requires time, application and learning.

People are engineers and engineers wrote the design codes- not computers.
_________________________
Kenny Robertson

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#9457 - 01/22/07 05:20 AM Re: Allowable Stress [Re: Captain Kenny]
Paul B Offline
Member

Registered: 01/23/03
Posts: 28
Loc: UK
I would have thought that if people were compliant with 301.1 of B31.3 then they would have a reasonable understanding of the code they are working to?

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#9464 - 01/22/07 10:53 AM Re: Allowable Stress [Re: nadyesz]
SLH Offline
Member

Registered: 06/04/04
Posts: 79
Loc: Edmonton
Hey, better a little rudeness than wrong information.

I really have to disagree. I have had the discussion of "we don't need engineers to do stress anymore, we have Caesar". The point is that someone who doesn't understand the code should not be doing stress ON HIS OR HER OWN. So the problem isn't so much people need to start somewhere as they need to start working with people who have a clue.

The hard part of stress for most people is not running Ceasar, the hard part is doing it correctly.

Shannon
who didn't used to think she was old and bitter, but is beginning to wonder

Originally Posted By: nadyesz
I think it is not fare to be rude to somebody that asks a common question (if it is a common question, may be COADE should make an effort to further clarify its "code stress" concept in the manuals). I disagree with the idea that you should not use CAESAR unless you are an expert in code specifications. There are many engineers around that just need understanding materials behaviour or stress analysis, and not necesarily are familiar with code rulings or, worst, with the somehow obscure (for the beginner, of course) "code stress" concept that CAESAR uses everywhere.



_________________________
-SLH

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