Hello everyone,
I’ve modeled a tower for a coursework assignment and am still quite unsure about how to define the boundary conditions. I don’t know whether I’ve implemented them correctly.
The program also tells me that torsion is being neglected. However, I can’t really imagine that being the case in my system, so I’m not sure whether I’ve made a mistake or whether the result is plausible.
Could someone take a look and help me out? I’ve attached the file for reference:
Stahlbau-Beleg.rs9 (1.3 MB)
Thanks in advance!
Hello Elisa,
The WA5001 message is only a warning, not a modeling error. Stability checks according to EN 1993-1-1, 6.3 cannot account for torsion in principle. It is still included in the cross-section checks.
At the location shown, T_Ed is only −0.03 kNm and is therefore negligible. This is plausible for a tower: the overall system’s rotation is resisted by axial forces in the bracing, not by torsion in the individual members. If torsion is intentionally significant, I recommend the Warping Torsion add-on with a second-order analysis.
Regarding the boundary conditions: please check the support at the base, the member end releases (release φx at only one end of a member), and the buckling lengths in the steel design.
Thank you very much for the prompt clarification. I’m still unsure about the boundary conditions. In particular, I don’t quite understand how the base supports, member-end hinges, and buckling lengths should be defined correctly in my model.
Currently, the columns are hinged at both the top and bottom, so the governing internal forces at the member ends include no bending moments (My = Mz = 0). However, I’m not sure whether the hinges and supports have been modeled appropriately for the tower’s structural behavior.
Could you please explain in a little more detail:
- how you think the base support should be defined,
- what exactly is meant by “release φx at only one end of the member,”
- and how I can reasonably specify or check the buckling lengths for the columns in this case?
Thank you very much!
Hello Elisa,
-
Foot support means that pinned supports are assumed.
-
Releasing φx prevents torsion caused by restraint. A more realistic alternative for avoiding torsion is to provide bracing in both directions (global X and Y).
- The general method does not use buckling lengths, but rather critical load factors for the planar model.
- For stability design of columns alone (doubly symmetric cross-section), the conventional equivalent member method is generally sufficient.
- When using the general method, the entire system is considered, and care should be taken to define realistic boundary conditions and supports.
- Checking the dynamic graphic view (to the right of the settings) is helpful.
Here is also a useful KB article with additional information: