# Critical moment of the I-beam for the plate model **URL:** \ **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:** 20\ **Page:** 2
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 6, 2026, 12:05pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/21 "2026-01-06T12:05:58Z")
Thank you for your patience and such great commitment in deciphering my text with errors. ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 6, 2026, 12:17pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/22 "2026-01-06T12:17:14Z")
The author of the program probably also encountered this problem: > **[Rigid-body constraint convergance problems](https://calculix.discourse.group/t/rigid-body-constraint-convergance-problems/369)** > > I was solving a larger shell model using a rigid-body constraint and could not solve the problem successfully when I used the NLGEOM setting. I was able to determine that the rigid-body constraint was causing convergence problems. I was able to... ---
**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")
> [@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) ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 6, 2026, 12:36pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/24 "2026-01-06T12:36:48Z")
I applied only one moment, at the end of the beam. Where it theoretically should be. At the point of symmetry, only BC. ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [January 7, 2026, 12:29am UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/25 "2026-01-07T00:29:57Z")
What Poisson value are you using? ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [January 7, 2026, 10:29pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/26 "2026-01-07T22:29:39Z")
Validation Poisson Ratio is 0.3 Your deviation is : 2.15% With Solid Elements C3D8I, 0.3 Poisson and Moment applied directly as Moment to RIGID BODY REF node deviation is 0.14% (4334 KN\*cm). Quite different Stress Distribution. ![imagen](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/f/f5e89cc381fde06a3479bbb48e81cfc71836198d.png) ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 5:00am UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/27 "2026-01-08T05:00:55Z")
Thanks for your help. I wanted to compare PrePoMax to a commercial program using shell elements. Just like in the PDF file. The result is very good. ---
**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 8, 2026, 8:15am UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/28 "2026-01-08T08:15:54Z")
Submitting your initial model (the same input deck) in Abaqus (on which CalculiX is based, but Abaqus has true shell elements) yields the following results for the full model: ![dwuteownik oryginalny inp](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/e/eb6d3aa72aca7d70f6640f668dac78b27134d095.png) And for the model with symmetry: ![dwuteownik oryginalny inp symetria](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/7/797dfa4149d59f2ccc42c29a5b8d5008c9f56cd1.png) Later, I can check how different types of elements (including solids) perform in this solver for that benchmark case. ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 5:41pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/29 "2026-01-08T17:41:17Z")
Thank you for your involvement. I admire your perseverance in running this forum. In this case, you misunderstood me. I used data from a simple engineering program as a comparison. There are few companies that use ANSYS or ABAQUS. Even fewer people can handle them. In your results, I only see deformation. Probable for buckling mode 1. That’s also good. I can’t read the forces or critical moment. This type of problem is fundamental in the design of steel structures in construction and the verification of engineering programs. I’m sure there is a large group of people interested in these results. I apologize for the confusion, but I don’t always find the time to respond and try again. ---
**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 8, 2026, 5:51pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/30 "2026-01-08T17:51:57Z")
I often share results from Abaqus mostly because CalculiX is directly based on it (it’s pretty much its open-source version). To the point that the keyword syntax is pretty much the same with small differences (Abaqus has more options). Of course, Abaqus has more robust algorithms and better element formulations. In this case, its main advantage is that shell elements equivalent to CalculiX ones are not internally expanded to solids. Also, you can often exclude solver error this way - CalculiX has some major limitations and bugs (also related to unusual shell formulation - the screenshots from Abaqus show e.g. that symmetry works equally well when there are no such issues). The screenshots indeed show the deformed shape of the model (the displacement values are irrelevant), but more importantly - at the bottom of each screenshot there’s “eigenvalue” - this is the buckling factor you also get from PrePoMax. Here, it’s 1036. The model definition is the same as in CalculiX - the input file was used directly in Abaqus due to shared syntax. ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 9:41pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/31 "2026-01-08T21:41:39Z")
![obraz](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/b/bf8566a2955c6fcaf431ceb7f92271ac43d1069a.png) ---
**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 8, 2026, 9:55pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/32 "2026-01-08T21:55:21Z")
This is from a frequency step. Are you evaluating natural vibrations of this model now ? ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 10:13pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/33 "2026-01-08T22:13:51Z")
I’m sorry, I didn’t understand you. ![obraz](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/f/f4ae8b5649c4af4e8d3e986ea75245615a15b408.png) ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 10:15pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/34 "2026-01-08T22:15:29Z")
The results are different 1036 vs 993.1227 ? ---
**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 8, 2026, 10:23pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/35 "2026-01-08T22:23:36Z")
Yeah, eigenvalues are common for both linear buckling and natural vibration (eigenfrequency) calculations. But what I got, is a linear buckling eigenvalue = buckling factor. I obtained 1036 instead of around 993 by only changing the solver to Abaqus (btw. PrePoMax even can even serve directly as its preprocessor) and thus using true S4 shells instead of expanded ones (those in CalculiX become a single layer of C3D8I elements). ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 10:25pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/36 "2026-01-08T22:25:07Z")
![obraz](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/9/9f4aff5d3eeefc6dd29e5c5abae8666a2383bd73.png) ![obraz](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/8/84bfa6d9e28e6d5430f19bee9f28202817341d92.png) ---
**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 8, 2026, 10:36pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/37 "2026-01-08T22:36:23Z")
It’s a free LTBeamN software, right ? Mentioned on this blog: [Zagadnienia związane z "Belka stalowa stężona i nie zabezpieczona przed zwichrzeniem w klasycznym podejściu normowym"](https://chodor-projekt.net/encyclopedia/analiza-belka-stalowa/) (there’s an interesting statement about the practical usage of this software and classic formulas at the end of the article) I wonder if it’s possible to refine the mesh in that software and see how this affects the results. ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 8, 2026, 11:00pm UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/38 "2026-01-08T23:00:14Z")
Yes, I use it sometimes. ---
**Author:** ![ANYS](https://yyz2.discourse-cdn.com/free1/user_avatar/prepomax.discourse.group/anys/32/7737_2.png) [@ANYS](https://prepomax.discourse.group/u/ANYS)\ **Post date:** [January 9, 2026, 12:33am UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/39 "2026-01-09T00:33:38Z")
Distributing the load linearly across the whole section and not only the flanges. 4346 KN cm. Pressure of 1\*z/7.99898743560001 KPa (z units in mm) Signed Von Misses for the Static Solution. ![imagen](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/e/e896952a075d5813aea0a9f4632a7952386242d5.png) ![imagen](https://global.discourse-cdn.com/free1/uploads/prepomax/original/2X/a/ad04aac0ca22d8f706fc1ced206f98d33c1a8ffb.png) ---
**Author:** ![MisiaKu](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@MisiaKu](https://prepomax.discourse.group/u/MisiaKu)\ **Post date:** [January 9, 2026, 5:39am UTC](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179/41 "2026-01-09T05:39:56Z")
What does the mesh look like? Is this result from PrePoMax? Was the load applied to the nodes as concentrated forces? [Previous page](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179.md?page=1) [Next page](https://prepomax.discourse.group/t/critical-moment-of-the-i-beam-for-the-plate-model/3179.md?page=3)