Analyzing a piping system with spring hangers where the structure supporting the spring hanger can see a -1" vertical displacement. I've created a "structure" with rigid elements with a -1" vertical displacement, D3, and all other degrees of freedom fixed. Then used a cnode to connect the spring node to the structure. The piping is connected to a nozzle with a -2" vertical displacement during operations, also D3. Output limits the the movement of the spring to the -1" displacement of the structure. My hanger load case is W+D3+T2+P1. Suggestions?

Did you manually specify the spring or tell it to size for the nozzle?

If the system is small enough, you could try holding the spring location constant and have everything else move with respect to it?

Did both manual and auto size.

Thought of the same thing, but system is too large. I've simplified the question to eliminate all the other variables at play.

Thanks for your input.

I've never tried this myself, but some thoughts:

So, assume your pipe is nodes 10-20, and the spring connects at 20.

And assume you have a floating rigid, nodes 110-120, and spring connects at 120.

Verify that without the displacement CAESAR attempts to size your spring and that it indeed moves.

Option 1: Put the spring on the pipe and connect it to the rigid. Move the rigid at the node not connected to the spring.
Option 2: Put the spring on the rigid, attach it to the pipe. Move the opposite end of the rigid.
Option 3: Same as options 1 and 2, except apply an anchor on the rigid, and CNODE it to 130. Apply displacement at 130.
Option 4: Same as options 1 and 2, adding an anchor point, but specifying a temperature and length of rigid element that will result in it growing to the desired displacement.

One or more of those should work.

You indicate that the output limits the movement of the spring to the 1" displacement of the attached structure. How are you observing this? I'm trying to wrap my head around the description of your setup. What movement are you expecting to see at the spring location?

"My hanger load case is W+D3+T2+P1"

Did you included H in your load case?

You state that the pipe node to which the hanger is attached moves down 2 inches while the other end of the hanger moves down 1 inch. If that's the case, then why would you NOT expect the hanger to see a 1 inch downward change?
I'm not saying that actual operation will show a maximum differential of only one inch, but that is what your model is "saying".

I'm trying to determine the total downward displacement of the pipe node to which the hanger is attached and size spring accordingly. The spring is attached to a structure. This structure will have a -1" vertical displacement. The piping is connected to a nozzle 20 ft away. This nozzle has a -2" vertical displacement. Therefore I am trying to size the spring so that the -1" vertical displacement at its supporting structure is not included in the spring's travel range. I know the the spring alone should travel downward less than 1" since the difference between the 2 initial displacements is 1". When I cnode my spring to a "support" node and place a -1" displacement on this cnode my result is always exactly a -1" displacement on the pipe where the spring is attached and thus my spring is sized for the same -1.0" travel. I would expect to see the pipe at this node to be moving downward around 1.75" and the spring to be sized for about a 0.75" movement.

Are you sizing the spring for 1 inch of deflection or 2 inches of deflection? Are these two deflections (nozzle & structure) always in unison?
While setting us up for 1's and 2's, you end up talking about 0.75. Why the difference? Is this expected out-of-balance pull on the pipe in the cold case?
Use your first two (or three) load cases to select the spring. After that, you can add other load cases to see how the spring responds. If those two deflections are not in unison, they would be defined in their own D sets and, using these additional load cases, you check the spring operation.

Went old school with a pic from my white board (attached).

Black lines are original installed piping
Green lines are after displacements

N50 is the beam attachment. Beam will see a -1" Y displacement (D2).
N40 to N50 is a very short rigid element.
N10 is node where spring is located on pipe. On the spring input N10 is cnoded to N40.
N20 is a nozzle which will have a -2" Y displacement (D1).

Blue marks show spring in original installed position, referencing that point as 0" on spring scale. Blue also shows an ESTIMATE of what I think the spring scale will move, 0.75", when both displacements are acting on system. The blue 1.75" is total Y travel pipe sees at N10 and equals the 1" beam displacement + 0.75" estimated displacement due to the additional -1" (-2" total) displacement at N20. That ESTIMATED 0.75" should be what the actual spring travel would be. There is another rigid support off to the left of pic which will cause some bending of the pipe, thus not allowing it to move down the full 2".

You show the spring connected to node 10 on the pipe. But you also show "10" near that element 50-40. It looks like the spring has 10 at the top and 10 at the bottom. The same node can't be in to places.
I suggest you change the top 10 to 40 on your drawing ... and do the same in CAESAR II.

Confirming your CAESAR II input:
In your hanger definition window, at the top, you will see your node 10. Next to that Node cell (in this hanger definition window) you will see a cell labeled Cnode. Confirm that you have typed in your 40 here.

Regarding your blue numbers - I can see that you expecting node 10 to drop 1.75 inches. But what does the -0.75 indicate? Without a node associated with each end of that 0.75 change, you cannot expect CAESAR II to display such a number.

40 drops 1 inch because 50 says so. As 20 drops 2 inches, the spring will be pulled down one inch while the pipe will be pulled up one inch. At this point it's just balancing that one inch pull based on the relative stiffness of the spring and the pipe.

Once again: are you asking CAESAR II to size this spring or are you modeling an existing spring?

Dave,

I'm reading that node 10 is where the spring is defined and it is CNODED to 40, not that node 10 is trying to represent two different points.

He's expecting node 10 to move down 1 3/4" due to deflection of node 20. That will stretch the coil. The 1" downward movement from node 40/50 will try to compress the coil. He's expecting a net of 3/4" stretch to the coil, so the operating load should be higher than the installed load.

RbtMc, now that I can see what you're talking about, I have a question about this from your first post:

"Output limits the the movement of the spring to the -1" displacement of the structure"

Where are you seeing this value? In the Hanger Table report? Or is this the Y movement you are seeing at node 10 in the displacement report.

As noted by danb, I'm also curious why you don't have H in your load case. If Caesar is sizing the spring for you, it generates two cases to figure out the movement and load. If you've manually sized a spring, it needs to show up in your load case as H. I assume without it, your load case is going to generate movements as though there's no spring attached at node 10.