Dear sir:
When we do static seimic analysis by CAESAR II,we find that we can not define the snubber rightly in model.Our train model is very simply,refer to node10,it's a fix point,there are 5meters(X direction) pipe(OD219*8),node20 is end of the pipe,we add a XSNB at node20,the uniform loads is 0.2g,0.2g,0.2g.Our result as following:
CASE 2 (OCC) W+T1+P1+U1
CASE 4 (OCC) W+P1+U1
10 Displ. Reaction
2 OCC -778937. 407. -1629. 0. 4074. 1018.
4 OCC 343. 407. -1629. 0. 4074. 1018.
20 Flex XSNB
2 OCC 779342. 0. 0. 0. 0. 0.
4 OCC 65. 0. 0. 0. 0. 0.
We can find that in case2,the XSNB is treated as X restraint and the expantion load act on XSNB.But we only want the result of seimic.According to COADE PIPE STRESS ANALYSIS SEMINAR NOTES,when there is non-linear restraint in model,we should use OPE+WIND(or Uniform load) to consider the Gap or frict.But how can we define the snubber to avoid expantion load acting on snubber?
Thanks for your share in advance!
Regards!

By default, snubbers are active in OCC load cases and disabled in all others. So yes, in case 2 the XSNB would be treated as an X restraint.

Snubbers should really be used in the dynamics modules, but if you need to incorporate them in a static analysis, here is the procedure:


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Because CAESAR II can not perform "load stepping", static analysis with snubbers requires a manual "up front" analysis to determine thermal movements. Here is the procedure.

1) Analyze a "hot operating" case, without your occasional loads.

2) Take the displacements from this analysis at the snubbers, and put them back into the input. At the location where the snubbers are defined, define a CNODE and put these displacements on the CNODE.

3) For your "real" analysis, apply these displacements to all load cases. Because they are on the "far side" of the snubber, they won't affect anything unless the load case is called "OCC", which activates the snubber stiffness. So,

- when you run your standard OPE case (W+T1+P1+D1), the snubber node will displace as before. There will be no restraint because the load case is OPE, not OCC.

- when you run the operating + occasional case (W+T1+P1+D1+WIND1), the snubber node will displace along with its CNODE. There will be a difference in these two displacements due to the WIND1 load and the snubber stiffness, because the case is now set as "OCC".
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Dear Richard Ay:
Before I posted my question,I have tried your above solutionm,but it's wrong.My model is so simple as above,when I add a cnode at node 20,and input the thermal displacement of node20 into this cnode,but the Fx of snubber is too big than normal value.Our ceasar is ver4.50 051018,maybe you can try same model.it's only 5meters pipe,X direction,one end is fix point(displacement is zero),another end is XSNB,T1 is 200 deg.C.material is A106B.
Thanks for your kind reply!

A couple of things come to mind here....

1st... you said "I add a cnode at node 20,and input the thermal displacement of node20" My repsonse to this statement is you should be using the operating displacement not the thermal displacement.


2cd.... you said "the uniform loads is 0.2g,0.2g,0.2g" did you set the kaux flag to g loading?

3rd... in this model even the smallest tiniest decimal point error will show up as a huge, huge, anchor restraint number because everything is in the axial direction, you may have a decimal place problem.

Some points for you to ponder on....

John's points are well thought.

A little more on this particular model - an X run with an anchor on one end and a rigid, X restraint on the other. How many digits are you typing when entering those CNode displacements, 5? (You can display more accuracy by dumping your data (e.g. to Excel as text), or building your own custom report.)

Calculate your axial stiffness (AE/L) and multiply by your tolerance on your X displacement to see what your tolerance is on the calculated X load. For example, if the number you type is 2.123 (in.), your tolerance is +-.0005 or a .001 swing. If you type 2.1234, the swing is .0001. Multiplying this small number by such a large (axial) stiffness gives a big range on calculated load. A lot of your big number may be due to this "too big" swing.

Thanks above.
There is a wrong in my case defining,I miss D1 in my case2.After revised it,there is no problem.