# \*BUCKLE step with prescribed displacements
**URL:** https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404
**Category:** Uncategorized
**Created:** [December 17, 2022, 6:58pm UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404 "2022-12-17T18:58:17Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@Disla](https://calculix.discourse.group/u/Disla)
#### Post date: [December 17, 2022, 6:58pm UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/1 "2022-12-17T18:58:17Z")
According to the manual “The load specified in a \*BUCKLE step should not contain prescribed displacements.”
I have done it and as far as I have tested it provides the correct buckling factor “and buckling shapes” except for the nodes affected by the displacement BC .
Seems like the solver doesn’t apply the overall scaling factor ( or normalization process) to the displacements on nodes affected by the BC.
¿Is this something that could be fixed, or does it comes from ARPACK?.
¿Could I use at least reliably the eigen values?
---
### Author:  [@Calc\_em](https://calculix.discourse.group/u/Calc_em)
#### Post date: [December 17, 2022, 7:33pm UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/2 "2022-12-17T19:33:21Z")
There are a few cases when the ccx documentation mentions that something isn’t possible or shouldn’t be done but it turns out that it works fine.
Here’s what Abaqus documentation says about this approach:
> Nonzero boundary conditions prescribed in an eigenvalue buckling step will contribute to the incremental stress Δσ and, thus, will contribute to the differential initial stress stiffness. When prescribing nonzero boundary conditions, you must interpret the resulting eigenproblem carefully. Nonzero prescribed boundary conditions will be treated as constraints (i.e., as if they were fixed) during the eigenvalue extraction. Therefore, unless the prescribed boundary conditions are removed for the eigenvalue extraction by specifying buckling mode boundary conditions (see the discussion below), the mode shapes may be altered by these boundary conditions.
---
### Author:  [@JuanP74](https://calculix.discourse.group/u/JuanP74)
#### Post date: [December 17, 2022, 11:47pm UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/3 "2022-12-17T23:47:53Z")
you can check the result by running a geometric nonlinear step for that “load level”. A sharp change in the displacements at the critical locations will show that buckling has happened.
I think, as Calc\_em explains, that cannot be correct in general because you introduce constrains that are not there at the onset of the instability.
---
### Author:  [@Disla](https://calculix.discourse.group/u/Disla)
#### Post date: [December 19, 2022, 12:03am UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/4 "2022-12-19T00:03:34Z")
Hi and thank you both.
> [@Calc\_em](#):
>
> thus, will contribute to the differential initial stress stiffness.
Ok. Just in case, I’m using the equivalent force (although it seems there is some small difference in the result).
> [@JuanP74](#):
>
> you can check the result by running a geometric nonlinear step
Yep. That’s a second phase but I’m struggling with it.
As soon as I offset the flanges (Flange1 and Flange2) to start adjusting the model and get a better representation of the geometry the nonlinear convergence fails.
It’s working fine in static and in nonlineargeom too but only without the shells offset. Excluding the flange’s nodes from the rigid body definition doesn’t help. I have also tried different solvers and different versions of ccx. Linear and quadratic elements.
¿I would appreciate if you could you take it a look?. I think everything is properly set.
Maybe I should discard to solve the model with the shell elements.
> **[WeTransfer - Send Large Files & Share Photos Online - Up to 2GB Free](https://wetransfer.com/downloads/5ba83e2ed3b4c72bc7498bab96893e5320221219000152/5b9d45)**
>
> WeTransfer is the simplest way to send your files around the world
---
### Author:  [@JuanP74](https://calculix.discourse.group/u/JuanP74)
#### Post date: [December 20, 2022, 7:56am UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/5 "2022-12-20T07:56:07Z")
I’m looking into it. Offsets are a source of difficulties many times but I didn’t expect this.
---
### Author:  [@JuanP74](https://calculix.discourse.group/u/JuanP74)
#### Post date: [December 23, 2022, 5:57pm UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/6 "2022-12-23T17:57:10Z")
the issue lies in the creation of the rigid body, from the manual:
> If the participating nodes in a rigid body definition lie on a straight line,
> the rigid body rotation about the line is not defined and an error will occur.
> To remove the rotational degree of freedom, specify that the rotation about the
> axis is zero. If a is a unit normal on the axis and uR is the displacement of the
> ROT NODE, this results in a linear MPC of the form a.uR = 0 to be specified
> by the user by means of a \*EQUATION card.
Nevertheless the model seems to be too stiff even using S4R shells, try it with parabolic elements to catch the onset of buckling.
---
### Author:  [@Disla](https://calculix.discourse.group/u/Disla)
#### Post date: [December 24, 2022, 7:45pm UTC](https://calculix.discourse.group/t/buckle-step-with-prescribed-displacements/1404/7 "2022-12-24T19:45:48Z")
Hi, Juan, Calc\_em
Thanks for looking at this. The problem has been puzzling me for some days.
I think Nonlinear analysis has some deeper issue with offset shells. I will pay attention if I find some other examples.
Just in case someone else finds some issues converging offset shells I have been able to solve it just creating three parts and connecting them with tie contact.
Problems suddenly disappear , convergence is then sweet again.
Thanks again for your help and merry Christmas everyone.
