Dear All

I am analysing an existing 18” pipe system that has a number of points along the pipe run where the pipe has grown off its supports and hence when it is at ambient temp it will still have a displacement at these points in the +Z direction. When analysing the system at next heat up to B31.3, I have included the displacement D1 at these points in the SUS case. Can you please confirm this is correct? Also if I have a displacement in the +Z direction in the cold condition but still want it to be free to grow from this position in the +Z direction, can I model this in Caesar without having to calculate what the expansion would be and input it directly as a D2 displacement at these points.

Hi Adam ,

First you need to ensure that your pipe that has "grown off its supports will still have a displacement when it is at ambient".It might sit back when it is at ambient.
Additionally you might want to try using tha "Gap" field in the "Restraint" auxillary.

You can't do this with "predefined displacements", since this will "fix" the pipe at that point, preventing further movement in that direction.

You are heading down the path refered to as "load stepping", where the analysis of load case "n" begins at the position where load case "n-1" completed. This is something CAESAR II can't do.

As stated above, you first need to confirm that the pipe will still have a displacment (off of the support) when it cools back down. Then you have two modeling alternatives, neither of which will be 100% correct.

Option A) If the pipe doesn't sit back down, then that section of pipe is probably not stressed, but something someplace else yielded to result in this condition. So this section of pipe could just be initially coded "up off of" the support. The problem with this alternative is that you don't know what yielded or what the state of stress is in other areas of the model.

Option B) You could code the pipe as you have now, on the support. However, at the locations where the pipe lifted off, you could define a vertical "cold spring element" (with an anchor at the top) whose length is equal to the "lift off" amount. Then when you put CS in the load case, CAESAR II will pull the pipe up. The problem with this method is that the CS will stress the pipe, which probably isn't true at that instant.

The issue here is in knowing and defining the state of the pipe after it has been cycling.

Thanks for your replies. Just to clarify, the pipe is horizontal and the growth of the pipe in the +Z direction over many temperature cycles has resulted in the saddle support slipping off the actual support column so that it now acts as a limit stop in the -Z direction when the system cools down. I have modelled the system with a D1 displacement of 250mm at this point in its ambient state and then calculated the expansion during the next cycle and input this as D2 at this point so that my loads cases are as follows:
OPE W+P1+D1+D2+T1
SUS W+D1+P1
EXP L1-L2

Is this an acceptable method of analysing this problem or is there a better way I should be doing this?

I am unsure of your intent with that D1=250mm. Your approach might report the proper position of the this node but I'm not confident that the loads and displacements elsewhere are correct. If you model results (throughout) match the field observations, you are OK in terms of structural response. I would still question any code-defined stress evluations for this system.

Here's how I would approach it... The pipe and restraint change their relative positions. You are moving the pipe, I would move the restaint instead. Place a CNode on that +Z restraint and set a +Z displacement (D1) at that new node equal to the distance that the pipe had to move to fall off the column. Include D1 in all analyses.

The problem I have with all this is the pipe apparently "walked" off the support through several thermal cycles. If so, the "ratcheting" has changed the structural state of this system - something somewhere has yielded. We work with an elastic model and cannot accommodate this plastic response. Of course, you can model plastic hinges in CAESAR II (with bilinear rotational restraints) but it would be difficult (for me) to have high confidence in the results. You may gain this confidence by manipulating your CAESAR II model so that it matches the observed response in the field (loads & displacements).

I don't agree. I had a senior guy explain very simply to me how what you see in the field can be explained. In my case it was a limit stop that than closed the 'wrong way'. He pointed out that if a system would expand equally either side of it's center of friction, but then if it hit a stop, all the expansion would be forced one way, away from the stop. When the system cooled, it would contract around it's center of friction and the stop could appear to be closed the wrong way. Nothing plastic invloved, all fully elastic. He also pointed out that simple beam models do not allow for thermal cycling history like this, so what we see in the field is not allways the same as the model predicts. This does not mean the model is wrong or worthless through.
I remeber seeing an article about this by a Czech engineer. I think he called it the 'crawling cat' effect.

OK, yielding is not necessarily a player here when other nonlinear response such as friction is considered.

Regardless, I believe the displaced CNode is the way to go.

Kenny,

Assuming the limit stop has a common gap of 1/8" on either side, you're looking at a maximum difference of 1/4". If someone is designing piping systems so their shoes can't handle 1/4" extra movement w/o falling off the steel, they need to be slapped.

Back to the original question - one thing that needs to be considered with your shoe that overtraveled and fell off: when that system cools down, it's going to pull like hell on that beam that was supporting it. As it probably was not braced to be an anchor location, you are liable to have torqued the beam something nasty.

I would be extremely leary of doing any kind of work involving Caesar and B31.3 to justify the continued use of that shoe and steel. Especially if you know that system has already gone through a cool down cycle. If the system heated up for the first time and the shoe has fallen off, but not yet contracted, you will still have a pretty good chance to jack up the line and modify the steel so that the shoe sits properly again.

Ed
Fair point about the 1/4" gap and shoes falling off. They should have been designed to allow for the predicted range of movement plus a few inches!
My point was that things don't always look like the predictions of the model in the field, but neither is 'wrong' and that yielding isn't always nessecary for it to happen.
I found my seniors explanation of what I had observed in the field illuminating.

I know this is very old case but I am reply to express my understanding and bring some clarity.

I believe this is the case of cold pull and two separate stress analysis to be carried out to mitigate load stepping.
For first cycle:
1) Piping system with support acting normally.
For rest of the cycle:
2) Piping system with cold pull by 250 mm and limit stop which is really large value. This will generate tremendous end reaction.

Is it correct approach?