linear and non-linear restraint

Dear all,

what differenc in linear and non-linear restraint?

Linearity and non linearity has to do with whether the stiffness of the support is a function of displacement. Say, for ex: if a pipe lifts off a +Y support, will it act as a restraint? Obviously no.Similarly for a guide with gap, if the gap closes, the guide takes load, else it does not. Regarding friction, it is well explained in the CAESAR II technical reference manual.Support non linearity is also known as contact non linearity. The two other non linearities are material ( for ex: plasticity) and geometric ( like a volter's pole).

Regards

Is CII's solver always non linearity ?

In my opinion, we should consider all problems as non linearity. Is it right ?

BR

Hi VanHa
Caesar will solve the problem as you model.
You can build caesar model as linear or as non linear.
If you
1)remove friction
2)do not use any gap in support
3)always use both directional ( + & -) support

then your caesar model become linear if you ignore material non linearity as Mr Anindya already give a nice explanation.

In Practical aspect we commonly use non linear system. It is the same reason why we do not check occasional stress directly like this
W+P1+F1( Occ) rather than collect only occasional effect then add that with sustain to check occasional stress.

Regards

Habib

Hi shr,

Thank you for explanation.I still have some more questions:

How does CII treat material non linearity case ?
And what situation should it considered (in practice)?

BR

c2 does not address non-linear material properties. The only non-linearities that c2 addresses are boundary condition non-linearities.

To answer the last part of your question i.e. when material non linearity should be considered, the answer is , when the stresses are post yield limit.You can obviously ask the question, "what happens in the secondary stress case when the code allows the stress to go up to twice yield"? The asnwer to that is, twice yield is based on elastic shakedown ie in the next cycle and cycles thereof the stresses are "shaked down" to elastic limit.

The 2007 version of ASME SEC VIII DIV 2 has lot of discussion and guidelines on how you can approach post yield stress analysis.B31 codes are very simple, they don't delve into such complexities.In reality, any engineering analysis is non linear involving all or any of the three form of non linearities : material, geometry and contact.However, in most cases, the non linear effects can be ignored and the problem can be treated as linear.

I hope I have answered your question, if not ask for more clarification.

Regards

Hi anindya,

Thank you for your answer. I have understood this problem.

BR

thanks for all valuable suggestion.....

Please give your valuable advise to improve convergence when modelling friction effects in CII, where in our project large bridge piping model often gives non-converging problem during the analysis.

Thanks
Regards
SA

Click the "step by step" option in the top right of the solver (F2 if I remember correctly), check the information in the window bottom right and click continue in the bottom. Do this a couple of times and see which nodes status changes from open to closed to open to closed etc. Remove the non-linearity on this support and run again. Most of the times the non-convergence problem is solved then.
Check your restraint summary to see if no unwanted effects take place, like a high upward force on a +Y support for example.

There are a number of previous posts dealing with "convergence problems". Search for the word "convergence".

Dear shr, did u mean if i collect the occasional first than add it to operation case than it call as non linier case? But if i add the occ case one by one to the operating case that it call as linier case? Please correct me if i'm wrong.

"Load cases" don't make a system linear or non-linear, your boundary conditions (supports) do.

If you have: directional restraints (+Y), gaps, friction, or large rotation rods (+YROD), then your model is non-linear.

Ok then. But i usually do like this for example i have a seismic case. So if my systen is non linier (with +y, and gap) i will set my load case 4 example like this;
L1 = W+P1+T1
L2 = W1+P1
L3 = U1
L4 = U2
L5 = U3
L6 = U1+U2+U3 (SRSS)
L7 = L1+L6 (ABS)
L8 = L2+L6 (ABS)
L9 = L1-L2 (ALG)
But if my system is linier than i will do it like ths;
L1 = W1+P1+T1
L2 = W1+P1+T1+U1
L3 = W1+P1+T1-U1
L4 = W1+P1+T1+U2
L5 = W1+P1+T1-U2
L6 = W1+P1+T1+U3
L7 = W1+P1+T1-U3
And so on , get the basic of occ by substract it with sus case. So am i doing the right thing? Correct me if i'm wrong. Thanks.

No I wouldn't do either. E-mail techsupport@coade.com, Loren has a document describing how to setup non-linear load cases.

(My advice is to always use the "non-linear" setup, then if you go back and change the model sometime later (and it becomes non-linear), your load cases will be correct. This will keep you out of trouble.)

Dear Richard, what i do in the second way just like i read and understand from Mr. Loren Brown file about CAESAR Load Case.

On page 4 of this (PDF) document notice how cases 10 through 15 acquire the effects of the uniform load. I prefer this method to simply putting the uniform loads in the load case by themselves (as in cases 3 to 5 above).

Dear All,

I have doubt while choosing the support std for Linear analysis..

In the linear analysis can we use resting support?

what kind of support shall we use for Z type direction (resting or shoe or hold down guide with gap). Plz confirm

A "resting support" makes the system non-linear.

A "gap" makes the system non-linear.

hi Richard ay,

Thanks for your confirmation.

Can you suggest, Z type support std and plz send me some of the support std.

thanks richard,

Shabeer,

Is your Z the vertical axis?

Dan,

yes. we are using Z as vertical.

Then you need a hold-down support.

See some examples.


http://www.pipingtech.com/products/ptpcat/pipeshoes/nofw/fig1100.htm

http://www.pipingtech.com/products/ptpcat/support/holddown/index.html


or from Lisega, see lift-off restraints inside this catalog.
http://www.lisega.com/pdf/pg4_us.pdf

Regards,

Dan,

Thank you for your response.

what is Code-defined stresses and their limits

Zubair,

You should have started another topic (thread), your question has nothing to do with non-linear restraints.

"Code-defined" stresses are stresses computed according to a particular (piping) Code equation, for a particular loading condition. Each such stress will have a corresponding "Code-defined" allowable stress value - the limit.