# Stability proof of top flange with singly symmetric composite cross-section **URL:** \ **Category:** Product Support\ **Tags:** addon-steel-design, rsection-1, rstab-9\ **Created:** [April 2, 2026, 2:41pm UTC](https://community.dlubal.com/t/stabilitatsnachweis-zusammengesetzter-querschnitt/3235 "2026-04-02T14:41:10Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![IPE300](https://community.dlubal.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@IPE300](https://community.dlubal.com/u/IPE300)\ **Post date:** [April 2, 2026, 2:41pm UTC](https://community.dlubal.com/t/stabilitatsnachweis-zusammengesetzter-querschnitt/3235/1 "2026-04-02T14:41:11Z")
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? ![Top flange cross-section](https://community.dlubal.com/uploads/default/original/2X/3/3265f19c35e9c2e69bfdca0f213c56de058aa41a.png) ---
**Author:** ![marco.nikoleizig](https://community.dlubal.com/user_avatar/community.dlubal.com/marco.nikoleizig/32/102_2.png) [@marco.nikoleizig](https://community.dlubal.com/u/marco.nikoleizig)\ **Post date:** [April 7, 2026, 7:18am UTC](https://community.dlubal.com/t/stabilitatsnachweis-zusammengesetzter-querschnitt/3235/3 "2026-04-07T07:18:07Z")
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](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/005615) [https://www.dlubal.com/en/support-and-training/support/faq/005760](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](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](https://www.dlubal.com/en/downloads-and-info/documents/online-manuals/rfem-6-steel-design/006072) * * * 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: ![image](https://community.dlubal.com/uploads/default/original/1X/8b1a4d47ea10737b559f12b01985d0ad3b4a8096.png) ➡ 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 ---
**Author:** ![IPE300](https://community.dlubal.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@IPE300](https://community.dlubal.com/u/IPE300)\ **Post date:** [April 7, 2026, 9:21am UTC](https://community.dlubal.com/t/stabilitatsnachweis-zusammengesetzter-querschnitt/3235/4 "2026-04-07T09:21:54Z")
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](https://community.dlubal.com/uploads/short-url/885HW1q6QFGOJnOsvVtoXb23kEP.rs9) (1.2 MB) [Obergurt.rsc](https://community.dlubal.com/uploads/short-url/6dJ0SLN07XvvkKM34r8Gx1v2uFb.rsc) (882.7 KB) ---
**Author:** ![marco.nikoleizig](https://community.dlubal.com/user_avatar/community.dlubal.com/marco.nikoleizig/32/102_2.png) [@marco.nikoleizig](https://community.dlubal.com/u/marco.nikoleizig)\ **Post date:** [April 8, 2026, 11:12am UTC](https://community.dlubal.com/t/stabilitatsnachweis-zusammengesetzter-querschnitt/3235/5 "2026-04-08T11:12:03Z")
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: ![image](https://community.dlubal.com/uploads/default/original/2X/5/55ed9fb89771bc762ac2b4f89ddadd0b66116520.png) ![image](https://community.dlubal.com/uploads/default/original/2X/b/b174492299cec3ebe5634b3119496fa15f780e57.jpeg) ![image](https://community.dlubal.com/uploads/default/original/2X/b/ba48a2b570add04ef5eb690d019691a70c5ed2a5.jpeg) ![image](https://community.dlubal.com/uploads/default/original/2X/9/9668b12288e941ffc1f2422b3a46f553d4478636.png) ![image](https://community.dlubal.com/uploads/default/original/2X/8/8fb564a37f241dd1347012ff9ea0e49585537515.jpeg) Please watch the above-mentioned webinar for this. Best regards Marco Nikoleizig ---
**Author:** ![IPE300](https://community.dlubal.com/letter_avatar_proxy/v4/letter/i/bb73d2/32.png) [@IPE300](https://community.dlubal.com/u/IPE300)\ **Post date:** [April 8, 2026, 3:00pm UTC](https://community.dlubal.com/t/stabilitatsnachweis-zusammengesetzter-querschnitt/3235/6 "2026-04-08T15:00:35Z")
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!