FAQ 002257 EN 1 December 2017

Modeling | Structure

My structure is unstable. What can be the reason?


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. Below is a possible course of action to find the cause of the instability.

First, it should be checked if the system is okay with the modeling. A good tool to find model modeling problems are the model controls (Tools menu> Model control). Furthermore, you can see the structure z. B. calculate under its own weight in a load case according to theory 1st order. If results are subsequently output, the structure is stable in terms of modeling. If this is not the case, the most common causes are listed below (see Video 1):

- supports are missing or have been defined incorrectly
- Rods twist around their own axis (the rod is not held around its own axis)
- Bars are not connected (Tools -> Model Control)
- Nodes are obviously in the same place, on closer inspection, however, these differ slightly from each other (common cause in CAD Import, Tools -> Model Control)
- Rod end joints / line joints cause a "joint chain"
- The structure is not sufficiently stiffened
- Nonlinear structural elements (eg. B. Tension rods) fail

To the last point, an example is shown in Figure 2. It is an articulated frame which is stiffened by tie rods. Because of the stalk shortenings due to the vertical loads, the tension rods in the first calculation pass receive small compressive forces. They are removed from the system (only train can be picked up). In the second rake passage, the model without these tension bars is then unstable. There are several ways to solve this problem. You can give the tension rods a preload (bar load) to "eliminate" the small compressive forces, give the rods a small stiffness (see Figure 2), or have the bars removed one after another in the calculation (see Figure 2).

To get a graphical representation of the cause of instability, the module RF-Stabil (RFEM) can help. With the option "Determine eigenform of the unstable model, ..." (see Figure 3) supposedly unstable systems can be calculated. An eigenvalue analysis is carried out on the basis of the structural data, so that as a result the instability of the affected component is represented graphically.

If load cases / load combinations can be calculated according to theory of 1st order and the calculation only increases from theory of 2nd order, then there is a stability problem (branch load factor less than 1.00). The branch load factor indicates the factor with which the load must be multiplied in order for the model to become unstable under the associated load (eg. B. buckles). It follows: A branch load factor less than 1.00 means that the system is unstable. Only a positive branch load factor greater than 1.00 allows the statement that the load due to the given normal forces multiplied by this factor leads to kink failure of the stable system. In order to find the "weak spot", the following procedure is recommended, which requires the module RS-Knick (RSTAB) or RF-Stabil (RFEM) (see Video 2):

First, the load of the affected load combination should be reduced until the load combination becomes stable. The load factor in the calculation parameters of the load combination serves as an aid (see Video 2). This also corresponds to a manual determination of the branch load factor if the module RS-kink or RF-Stable is not available. Subsequently, the buckling or buckling figure can be calculated and graphically displayed on the basis of this load combination in the module RS-buckling or RF-stable. Through the graphical output, the "weak spot" can be located in the system and then optimized specifically.

Video 1-en.wmv (16.52 MB)
Video 2-en.wmv (23.97 MB)


Unstable Branching Load Singular


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