Stability proof of top flange with singly symmetric composite cross-section

[IPE300]

Good day,

I am trying to design a truss beam using the RSTAB 9 program. The top flange consists of a composite, simply symmetric cross-section (see image in the attachment), which is loaded by distributed loads in the y- and z-directions. When I carry out the design, I get the following error message: "Error: Further design not possible due to invalid cross-section shape for design." I created the composite cross-section in RSECTION and then transferred it to my model in RSTAB.

I have tried both stability checks according to the general method (6.3.4) and according to second-order theory with 7 DOF with local initial curvature in the y and -y directions. In both cases, I received this error message. Is my general approach correct, and can I resolve the program's error message?

[marco.nikoleizig]

Hi IPE300,

For composite cross-sections created in RSECTION, it is important that the individual cross-section parts are connected to each other, otherwise you will receive a corresponding error message in RSTAB.

https://www.dlubal.com/en/support-and-training/support/faq/005391

For such profiles, it can also happen that the moment of inertia Iz is greater than Iy. In this case, the cross-section should possibly be rotated by 90° in RSECTION.

https://www.dlubal.com/en/support-and-training/support/faq/005615
https://www.dlubal.com/en/support-and-training/support/faq/005760

In the Steel Design add-on, the following error message may also occur regarding the stability check:

For combined loading conditions (e.g., axial force and bending moment about the y-axis), the equivalent member method according to EN 1993-1-1, 6.3.3 is only applicable for doubly symmetric cross-sections.

For singly symmetric cross-sections, boundary conditions and the general 2D method according to EN 1993-1-1, 6.3.4 must be used to verify lateral-torsional buckling about the weak axis and lateral-torsional-torsional buckling.

If you use the General Method here in combination with the Extension Methods, the stability check can usually be performed successfully.

General Method: https://www.dlubal.com/en/downloads-and-info/documents/online-manuals/rfem-6-steel-design/000102

Extension Methods: https://www.dlubal.com/en/downloads-and-info/documents/online-manuals/rfem-6-steel-design/006072

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To analyze the problem more precisely, the model file would be very helpful:

Click File → Save As and select the following settings to reduce the file size:

Then upload the file here (e.g., *.rf6, *.rs9) – this way the community can also contribute to the solution.

Would you prefer not to share the file publicly? No problem – send it to me via direct message: click on my profile picture or my username → Message.

If you have any further questions, feel free to contact me – I am always happy to help!

Best regards
Marco Nikoleizig

[IPE300]

Thank you for the quick response! Here is my model and the top chord as a file. I have already tried connecting the elements of the cross-section and have rotated it by 90 degrees so that I_y > I_z applies. Still, the error message in RSTAB continues to appear.
Dachbinder_KOPIE.rs9 (1.2 MB)
Obergurt.rsc (882.7 KB)

[marco.nikoleizig]

Hi IPE300,

the cross-section was not disassembled, the individual cross-section parts were not connected to each other, the cross-section was not rotated. The following images show the necessary steps:

Please watch the above-mentioned webinar for this.

Best regards
Marco Nikoleizig

[IPE300]

I should have re-imported it into RSTAB after changing my top chord. I had forgotten this, and now the program is also calculating. Thank you very much for your help!