The structural analysis software RFEM 6 is the basis of a modular software system. The main program RFEM 6 is used to define structures, materials, and loads of planar and spatial structural systems consisting of plates, walls, shells, and members. The program also allows you to create combined structures as well as to model solid and contact elements.
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There can be many reasons for unstable structural systems. The best way to determine the reason for this message is to use the Structure Stability add-on.
Structure Stability Add-on
This add-on allows you to calculate your structure without loading, and thus to perform an instability analysis using the mode shape.
Therefore, you can display the unstable shape of your structure.
As you can see in our example, the upper steel beams are subjected to lateral deflection.
Upon closer inspection of our modeling, we recognize that we have unconsciously created a hinge chain from couplings of the Rigid-Hinge type. If we remove this hinge chain, we can calculate the load case.
A calculation break‑off due to an unstable system can have different reasons. On one hand, it can indicate a "real" instability due to overloading the structural system; on the other hand, modeling inaccuracies may also be responsible for this error message. In the following text, you can find a possible procedure for finding the cause of the instability.
1. Modeling Check
2. Check of Stiffening
3. Numerical Problems
4. Detecting Causes of Instability
This is not possible in RFEM 5 or the RF‑STAGES add-on module. However, it is possible in the new program generation. In the Construction Stage Analysis add-on for RFEM 6, you can now modify the properties of structural elements.
The warping of a cross-section can be displayed in the "full mode". For this, it is reasonable to increase the display factor for torsional warping in the control panel; see Image 01.
Furthermore, you can select the value of the local deformation ω [1/m] in the Results navigator; see Image 02.
The warping stiffness can be deactivated by cross-section in the "Edit Cross-Section" dialog box; see the image.
Both support forces and loads are assumed for the calculation with warping torsion in the centroid. Accordingly, an asymmetric cross-section would automatically receive torsion; see the image.
After activating Torsional Warping in the Base Data, you can define warping springs and warping restraints. For this, select the Transverse Stiffeners option in the "Edit Member" dialog box; see Image 01.
In the "Transverse Stiffener" tab, you can create several transverse member stiffeners and define the necessary parameters using the "New Transverse Member Stiffener" button. For the "End plate" stiffener type, the resulting warp spring is determined automatically; see Image 02.
In addition to other variants, you can also define a rigid warping restraint or user-defined warping spring stiffness under the "Warping restraint" stiffness type.
As an alternative, you can create member transverse stiffeners using the Data navigator or the menu bar "Insert", "Types for Members", "Member Transverse Stiffeners". In this case, you can use the select function in the "New Member Transverse Stiffness" dialog box to assign them to the corresponding members.
Releases for warping are at each member end by default. Splitting members leads to a warping release.
If you do not want to have a warping release there, but rather continuous warping, you need to define a member set. When activating the "Torsional Warping" add-on, the warping is transferred automatically. If this is not desired for the member set, select the "Discontinuous torsional warping" option; see the image.
First of all, it would be reasonable to check the boundary conditions for the design again. This includes, among other things, the selected load introduction, checking the transverse stiffeners, and the transitions between the members. It is also useful to check the analysis method without the second-order analysis due to large rotations.
However, it is also particularly important that RFEM requires an FE mesh distribution for the warping torsion.You can check the FE mesh settings and the graphical display of the member FE mesh.