I am a layman of piping stres computation. now because a design project involved sveral pipes, I bgan to learn CAESAR II, now I can use this software, but what confused me was the concept of "Code Stress", In the expanion of a strait pipe (anchored in both end), the Code stress is zero, but actually from cold to a high temp, a strong force produced because of expansion, so why code stress is zero, what is the mean of this concept? thank you very much for your kind help?

Hello,

Yours is an interesting question. This is a topic that is addressed in seminars because it points out that although you are using the best software for piping stress analysis, you also MUST have an understanding of the Code and of basic Piping system design.

"Code stresses" are those that are specifically addressed by the Code. The B31 Pressure Piping Codes address the BENDING stresses (and torsional stresses) due to thermal expansion/contraction and they also address the longitudinal BENDING stress (due to weight) added to the longitudinal pressure stress (the sum of these stresses is called the "sustained stresses" or the "additive stresses").

As you point out, If a designer were to analyze a system that had a straight piece of pipe between two rigid anchors, the Code equations would not address the resulting stress in the pipe because it is not a bending stress - it is a compressive buckling stress. The Code is not intended to be a design "handbook", it is assumed that the designer understands piping systems and that he/she will not place a straight piece of pipe between two anchors.

Many years ago, the Codes had an equation that was intended to address pure longitudinal compression and tension in pipe (this would be addressed together with sustained stresses). The equation calculated the stress as the compressive or tensile FORCE divided by the AREA of metal in the pipe wall: Stress (psi) = F (in pounds) / A (in square inches). However, it was recognized that the compressive force could easily buckle a long slender structural member (before it would approach the strength of the material) and there was no rule in the Code to account for the "slenderness ratio" of the pipe. So the equation was dropped - it was assumed that engineers knew better.

So, as you have discovered, piping systems are more complex than might be assumed by a inexperienced (in piping design) engineer. May I suggest to you that you look into a piping design seminar and that you consider obtaining the EXCELLENT books by Woods and Baguley and by Dr. Chuck Becht - go here: http://www.asme.org/pubs/ and go here: http://www.normas.com/CASTI/B31.3.html.

Please do not think I am "picking on" you. You asked a good question and I hope you will continue to ask questions as you gain experience. Good luck with your projects.

Best regards, John.

Peter,

To answer your <em>"specific"</em> question:

The proper analysis of a piping system involves analyzing a number of different load cases (for <em>primary, secondary,</em> and <em>occasional</em> loads). The piping codes define how the stresses are to be computed for each of these load cases. In the CAESAR II output report, the column labeled <em>code stress</em> is the stress obtained using the particular code's equation for the particular load case in question.

You can get a list of all of the code equations used in CAESAR II by looking at the <em>Quick Reference Guide</em>, starting on page Q-9. If you don't have the hard copy of this small manual, you can find it on-line, from the <em>"help\documentation"</em> menu.

If this is your first endeavor into the world of <em>Pipe Stress Analysis</em>, make sure you discuss your model and the results with someone who has performed this sort of work before. There are assumptions (as John noted above) and limitations you need to be aware of.

The biggest limitation is that piping programs utilize the <em>3D Beam Element</em>. This means the computer sees your model as a series of infinitely thin sticks. In addition, the behavior of the piping system is assumed to be dominated by bending, limited to small deflections and rotations. You can't ask (or expect) the program to give you something that can't be described within these limitations. Hence your results for the beam between two anchors. There was no movement of the nodes, therefore there were no forces or moments, and consequently no stress. (Buckling and other 2nd order effects are beyond the scope of the 3D Beam element, and therefore beyond the scope of typical Pipe Stress Analysis).

I hope this helps.

Peter, Jim Wilcox in Calgary here - representing COADE in Canada. in case it is of interest, we have two pipe stress analysis seminars (Calgary & Edmonton) in October. If you would like more details, here's a link to the seminar page at our website:
http://www.codecad.com/seminarpage.htm