FAQ 003045 | My model is unstable. What could be the reason?

Question:
My model is unstable. What could be the reason?

Answer:
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. Check of Modeling

First, you should check if the modeling of the structural system is correct. We recommend using the model check tools provided by RFEM/RSTAB (Tools → Model Check). For example, these options allow you to find identical nodes and overlapping members, so you can delete them, if necessary.

Furthermore, you can calculate the structure subjected to pure dead load in a load case according to the linear static analysis, for example. If results are displayed subsequently, the structure regarding the modeling is stable. If this is not the case, the most common causes are listed below (see also the "Model Check" video under "Downloads"):

Incorrect Definition of Supports / Lack of Supports
This can lead to instabilities, as the structure is not supported in all directions. Therefore, the support conditions must be in equilibrium with the structural system as well as with the external boundary conditions. Statically underdetermined systems can also lead to calculation aborts due to a lack of boundary conditions.

Torsion of Members About Their Own Axes
If members rotate about their own axes - that is, a member is not supported about its own axis - this can lead to instabilities. This is often caused by the settings of member hinges. Thus, it may happen that the torsional releases are entered at both the start node and the end node. However, you should pay attention to the warning that appears when starting the calculation.

Missing Connection of Members
In the case of large and complex models in particular, it may quickly happen that some members are not connected to each other, thus they "float in the air". Also, if you forget about crossing members that should intersect with each other, this can lead to instabilities as well. A solution provides the model check of "Crossing Unconnected Members", which searches for members that cross each other, but do not have a common node at the intersection point.

No common node
The nodes seem to be at the same location, but on closer inspection, they deviate slightly from each other. This is often caused by CAD imports, and you can correct it using the model check.

Formation of Hinge Chain
Too many member end hinges on a node can cause a hinge chain that leads to a calculation abort. For each node, only n‑1 hinges with the same degree of freedom relative to the global coordinate system may be defined, where "n" is the number of connected members. The same applies to line releases.

2. Check of Stiffening

If the stiffening is missing, it may al...

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