FAQ 002257 EN

Modeling | Structure RSTAB RFEM RF-STABILITY RSBUCK

My structure is unstable. What can be the reason?

Answer

There can be different reasons for an unsuccessful calculation due to an instable. On the one hand, this can indicate a “real” instability due to an overloading of the system. On the other hand, the error message can result from inaccuracies in the model. Below you find a possible procedure for discovering the reason for this instability.

First of all, you should check if there are errors in the model. For example, you can calculate the structure only with its self-weight in a load case according to the linear static analysis. If results are displayed afterwards, the structure is stable concerning the model. If this is not the case, the most common cases are the following (see Video 1):

- Supports are missing or have been defined incorrectly
- Members are twisted about their own axis (torsional releases are defined at both member ends)
- Members are not connected with each other (Tools --> Model Check)
- Nodes seem to be in the same place, at a closer look they deviate minimally from each other (common cause at CAD import, Tools --> Model Check)
- Member end releases/line hinges cause a "chain of releases"
- Stiffening of the structure is not sufficient
- Failure of nonlinear structural elements (for example tension members)

Figure 2 shows the latter point. You can see a hinged frame which is stiffened by tension members. Due to the column contractions as a result of the vertical loads the tension members receive minor compressive forces during the first calculation. They are removed from the structure (because only tension can be absorbed). During the second calculation, the model is then unstable without these tension members. There are several ways to solve this problem. You can assign a prestress (member load) to the tension members to "eliminate" the minor compressive forces, allocate a little stiffness to the members (see Figure 2) or have removed the members successively during the calculation (see Figure 2).

The RF-STABILITY add-on module (RFEM) may be useful if you want to display graphically the reason for instability. Use the "Calculate eigenvector for unstable model ..." option (see Figure 3) to calculate supposedly unstable structures. On the basis of the structural data, an eigenvalue analysis is performed so that the instability of the structural component in question is displayed graphically.


If load cases/load combinations can be calculated according to the linear static analysis and the calculation is only cancelled when performing the second-order analysis, it is mostly caused by a "critical load problem" (critical load factor less than 1.0). The critical load factor indicates the number by which the load must be multiplied so that the model under the associated load becomes unstable (buckling). It follows that a critical load factor less than 1.0 leads to an unstable structure. Only a positive critical load factor higher than 1.0 makes it possible that the loading due to specified axial forces multiplied by this factor results in buckling of the stable structure. To be able to determine the "weak point", we recommend the following procedure which requires the RSBUCK (RSTAB) and RF-STABILITY (RFEM) add-on module (see Video 2):

First of all, the load of the concerned load combination should be reduced until the load combination becomes stable. The load factor in the calculation parameters of the load combination is a useful tool here (see Video 2). This also corresponds to manually determining the critical load factor if the RSBUCK and RF-STABILITY add-on module is not available. Based on this load combination, you can then calculate the buckling modes in the RSBUCK and RF-STABILITY add-on module and display the results graphically. By displaying the results, you can detect the "weak point" in the structure and then optimize it systematically.

Attachments
Video 1-en.wmv (16.60 MB)
Video 2-en.wmv (18.98 MB)

Key words

instable, critical load

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