Strange forces on restraints 1

[scalabres]

Hi all,

I am using CaesarII software to analyze stress on a steam piping system.

I am astonished because the results show forces on nodes where I am not putting restraints, and it also show displacements at this same nodes (and I'm not using gaps).

This is the first time I see something like this.

I have to say that I'm using some kind of template file prepared by a ex-colleague for this project.

Does anyone know what can be going on? I'm thinking in a special parameter of the program configuration file, but reading the documentation I don't see anything like this.

Advanced thanks

 

[superpiper]

sounds like C-nodes
which are placed in the pipework to emulate an pipe/pipe anchor.

C-nodes are coded on the restraints panel

 

[scalabres]

Superpiper,

I am not using C-nodes at the moment, so the answer has to go by other way.

The reactions are lower than the real one, but thet exist. For example, in a node restrained in X-Y appear the 3 reactions (X, Y and Z), but the Z component is much lower than the X one (10% of the other more or less).

 

[superpiper]

Give your Coade dealer a ring or contact coade directly.
they will spot the problem very quickly and for free.

 

[ApC2Kp]

scalabres,

"the results show forces on nodes where I am not putting restraints" - The local and global elelment forces reports will have the internal forces between elements. The restraint report will show forces at only the nodes with restraints or displacement reactions.

"it also show displacements at this same nodes (and I'm not using gaps)" - Each node in a piping system will have deflection or movement due to gravity or thermal effects, unless it is restrained at that node against those effects by a support or anchor. Are the movements inputted as the displacements of anchor nodes, corresponding to the thermal movements of equipment connections?

If the node with X-Y restraint also has a reaction in Z axis, could it be result from friction at the restraint?

 

[scalabres]

ApC2Kp,

> The restraint report will show forces at only the nodes with restraints or displacement reactions.

This is not what appear in my line as I have forces on not restrained directions. But I think you are right and this is a question of friction. In fact, I'm introducing a 0.3 frictional coefficient, but I understood that the friction forces only appear when I have a gap and the pipe can move in the direction specified. Now, I've read the documentation of the program regarding friction forces:

Ideally, if there is motion at the node in question, the friction force is equal to (Mu * Normal force). However, since we have a non-rigid stiffness at that location to resist the initial motion, the node can experience displacements. The force at the node will be the product of the displacement and the stiffness. If this resultant force is less than the maximum friction force (Mu * Normal force), the node is assumed to be "not sliding," even though we see displacements in the output report.

More interesting still:

For dry friction, the friction force magnitude is a step function of displacement. This discontinuity determines the problem as intrinsically nonlinear and eliminates the possibility of using the superposition principle.

I think this is the reason of the reactions in directions not restricted.

> Are the movements inputted as the displacements of anchor nodes, corresponding to the thermal movements of equipment connections?

No, I am not using displacements nor C-nodes.

Thank you for your answer, you gave me the way

 

[NozzleTwister]

Have you done a input graphics plot to see what is shown that looks unusual. I like the ALT+F12 to get the two line plot, the volume of the hoops graphics sometimes hides the info inside the pipe. Check all of the buttons for Forces, Springs, Displacements and Restraints. Make sure the shift is on so the nodes are displayed for the activated items.

Also, check the input file individual reports. Supports for a node can be called out on ANY suceeding node.

Good Luck,

NozzleTwister
Houston, Texas

 

[NozzleTwister]

On the above post I meant SHIFT+F12 for the two line plot.

NozzleTwister
Houston, Texas

 

[PhilEduard]

Scalabres,

For the benifit of other who might encounter this same problem in the future, could you post this thread on COADE Caesar forum? This issue is more suited in that forum. Thanks.

Phil

 

[NozzleTwister]

scalabres,

Did you ever find out what the problem was? Please give us an update.

NozzleTwister
Houston, Texas

 

[scalabres]

The point was that if you consider a friction coefficient (0.3 in my case), frictional forces arise in both horizontal axes (X and Z) independent of the restraints you consider.

So, if I have a non restrained direction, i.g. X, I get as result a displacement and a reaction. The force at the node will be the product of the displacement and the stiffness. If this resultant force is less than the maximum friction force (Mu * Normal force), the node is assumed to be "not sliding," even though we see displacements in the output report. But it appear a force in that direction.

I hope this can help.

How can I post the whole thread in other group?

 

[PhilEduard]

Scalabres,

You need to join or you can be a visitor I think in the forum at COADE:

http://www.coade.com/support_discussion.asp.

And, simply copy and paste your problem in there.

Phil

 

[scalabres]

Thank you, PhilEduard. Do you think that there is another answer to my problem?

I made this question to COADE staff few days ago and they replied me in this way of friction forces.

Anyway, I didn't know there were forums at COADE site, I have just now get registered so in the future I will also post my problems there.