Thin walled section calculation problem

[maksimd9PUZW]

We are using Robot Structural Analysis 2026 with the EN 1993-1:2022 Eurocode module.
When assigning custom Z-profiles created via the Thin-Walled Section Definition or imported from DXF, the steel design module fails to recognize section parameters tw and tf.

The error message is:
“The following parameters of the section Z 20x5x0.2 used in the calculations are incorrect: tw, tf.”

These sections were fully defined with correct geometry and thicknesses, and work structurally, but the verification (EN 1993-1-3) doesn't proceed.

Robot seems unable to classify the relevant wall and flange thicknesses for user-defined or imported thin-walled sections.

How can we explicitly define tw and tf for these sections so that design verification works properly?

Alternatively, what is the correct workflow for verifying thin-walled cold-formed members in RSA according to Eurocode, including lateral-torsional buckling (LTB) effects?
Calculation files attached in this message

[Stephane.kapetanovic · Accepted solution]

HI @maksimd9PUZW 

When assigning custom Z profiles created via the thin-walled section definition or imported from DXF, the steel design module does not recognise the section parameters tw and tf.
Yes, indeed, for this you must directly enter the thin section type and the variables used in the calculation.

‘The following parameters of the Z 20x5x0.2 section used in the calculations are incorrect: tw, tf.’
For a section of type 1013 or 1015, the variables are H, B, EA, RA, P1_L

These sections have been fully defined with correct geometry and thicknesses, and work structurally, but the verification (EN 1993-1-3) does not proceed.
This is because, without entering the parameters, the calculation performed is standard, without taking into account the thin section.

Robot seems unable to classify the relevant wall and base thicknesses for thin-walled sections defined by the user or imported.
This is not automatic for a single section imported from the section definition module. If you have several of this type, it is preferable to work with your own catalogue.

How can we explicitly define tw and tf for these sections so that the design verification works correctly?
In the User Xml file.

Here is an example of a result based on your section ↓

After calculating your section and saving it in the user section catalogue, you must go to the folder C:\Users\__\AppData\Roaming\Autodesk\Structural\Common Data\2026.
You must have an XML file editor such as XML Notepad, which is often used with Autodesk products such as Robot Structural Analysis. 

The user section file is called ruserpro.xml. If you do not wish to work directly on this file, it is best to make a backup copy before making any changes.

Save your file after making the changes.

Then return to your project and delete all references to sections previously used for your beams.

For example, replace your sections with an IPE 100 and delete the unused sections.

Add your newly modified user section from the user database. To ensure compression verification, add an axial load and modify the support.
Recalculate your project and go to the dimensioning module.

Your section will be taken into account !

Best regards

[maksimd9PUZW]

Dear Stephane,

Thank you very much – your input was extremely helpful.

I would like to clarify a couple of points:

The program reports that the critical moment for lateral-torsional buckling (Mcr) was calculated as for bisymmetric sections:

“The members 1 are made from sections for which the EC3 code does not provide the formulas for calculating the critical moment. Mcr for these sections was calculated as for bisymmetric elements.”

This suggests that the design approach from EN 1993-1-1 was used, as also indicated in the report. However, the rest of the note refers to compliance with EN 1993-1-3.

At this stage, I am unable to evaluate the impact of calculating Mcr as for a bisymmetric section instead of an actual Z-section.
Is there any option in RSA to compute the actual Mcr for such shapes?

Secondly, could you clarify what section types 1013 and 1015 represent in Robot?
Is there any manual or documentation explaining how to work with the cold-formed section databases? I would like to add custom C and Z sections with lips. Is this possible, and if so, what is the recommended way to do this?

[Stephane.kapetanovic · Accepted solution]

HI @maksimd9PUZW 

Due to practical considerations and the limited availability of technical documentation on the subject, it is currently not possible to explicitly account for monosymmetry, or any other asymmetry, in the design against lateral-torsional buckling. However, it remains essential to safeguard against both direct and indirect torsional effects, whether through calculation, bracing, steel decking, or similar means.

The use of the buckling resistance of a bisymmetric section is standard practice, which is why informational messages are displayed.

For any non-standard scenarios, users can manually enter the critical moment in the dedicated input field.

Regarding alternative calculation approaches, if you have access to the CTICM journals, you may find particular interest in issue No. 2-2016, Elastic Instability of Flexed U-Section Bars, and issue No. 4-2013, Formula for Torsional Stresses in an Open-Section Beam.
These articles not only examine the influence of loads on beams but also take a step further by reassessing the applicability of existing formulas for specific section types.

They highlight the importance of considering instability effects both in global structural analysis and in local verifications.

These topics present valuable opportunities for development teams to monitor and potentially incorporate the recommendations into future updates.

You can also visit the CTICM applications page to see if there's anything [LTBeamN,  TORSION] that could be useful for your work. 

To work with databases and gain a full understanding of how to create them from start to finish, you can find several informative posts on this forum by using the search function.

The main method involves copying an existing database and then modifying, deleting, or adding section types as needed—while preserving the existing structure.

An alternative approach, for those with programming knowledge and an interest in XML, is to generate the database and perform section calculations in a single step.

I have developed a program that enables you to read, explore, and work with databases, and more.

Although it hasn’t been widely downloaded yet, I encourage you to give it a try. (SectionXmlDatabaseCatalog, forum page)

Please also note that if you want to quickly model a bar with thin-walled section using finite elements for deformation and stress calculations, you can use the ExtrudeLines tool, available on the Autodesk App Store and on its dedicated forum page.

For the classification of thin sections, here is a document. (EC3_thin_walled_sections.pdf)

Best regards