# Critical moment of the I-beam for the plate model

**URL:** <https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179>\
**Category:** General Questions\
**Created:** [January 5, 2026, 11:00am UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179 "2026-01-05T11:00:24Z")\
**Posts on this page:** 1\
**Showing post:** 23

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**Author:** ![FEAnalyst](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/feanalyst/32/688_2.png) [@FEAnalyst](https://prepomax.discourse.group/u/FEAnalyst)\
**Post date:** [January 6, 2026, 12:31pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/23 "2026-01-06T12:31:45Z")

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> [@MisiaKu](#):
>
> I did as you wrote. Applying a moment to a reference point, which is a support, and coupling the edges using RIGID BODY does not produce the correct result regardless of the load of 1 N_mm or 1 kN_m. For both cases, I retain the value of 0.9 and 1.0.

Did you apply opposite moments (one positive and one negative) and did you use the same simple supports ? I’ve tried it yesterday and I think it wasn’t so bad.

> [@MisiaKu](#):
>
> I found this description:

Where is this long quote from ? I can’t find it when I search for the exact strings.

> [@MisiaKu](#):
>
> In summary:  
> In my opinion, it is impossible to achieve more precise symmetry in a simple way. My method only approximated one buckling mode because the central cross-section is too stiff. The entire cross-section cannot move in the Y direction (U2). This is not true for free support. A slight flexure occurs.

I would say that symmetry, rigid body constraints, buckling or nonlinearities and shells indeed are problematic combinations, especially in CalculiX due to its significant limitations. It’s often better to avoid rigid bodies, sometimes even symmetry (particularly for buckling problems), but more importantly, it often makes sense to use solid elements (at least for part of the model, e.g. where rigid body constraint is applied).

> [@MisiaKu](#):
>
> The author of the program probably also encountered this problem:

Yes, it’s a major limitation of CalculiX. As I’ve mentioned, its dev decided to prevent the usage of rigid body constraints with shells even in linear analyses in the newest release of CalculiX (not yet provided with PrePoMax), but I asked him if he could allow it for linear cases.

Here’s a list of the most important known CalculiX limitations: [Known CalculiX limitations](https://prepomax.discourse.group/t/known-calculix-limitations/3050)

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