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FAQ 003045 | My model is unstable. What could be the reason?
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First Steps with RFEM
Question
My model is unstable. What could be the reason?Answer
The calculation can be terminated due to an unstable structural system for various reasons. On the one hand, it can indicate a "real" instability due to overloading the structural system, but on the other hand, the modeling inaccuracies may also be responsible for this error message. In the following, you can find a possible procedure of how to find the instability cause.
1. Checking the modeling
First, you should check if the modeling of the structural system is correct. It is recommended to use 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 use the structure z. B. under pure self -weight in a load case according to theory 1. Calculate order. 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 video "Model Check" 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. Structurally underdetermined systems can also lead to calculation aborts due to insufficient boundary conditions.
Image 02 - Kinematic System - Single-Span Beam Without Rigid Support - Torsion of members about their own axis
If members rotate about their own axis, that is, a member is not supported about its own axis, it can lead to instabilities. This is often caused by the settings of member hinges. Thus, it may happen that there are the torsional releases entered at both the start node and the end node. However, you should pay attention to the warning that appears when starting the calculation.
Image 03 - Entering Torsional Releases on Start and End Nodes - Missing connection of members
Especially in the case of large and complex models, it may quickly happen that some members are not connected to each other, and thus they "float in the air." Also, if you forget about crossing members that should intersect with each other, it can lead to instabilities as well. A solution provides the model check of "Crossing Unconnected Members," which searches for the members that cross each other, but do not have a common node at the intersection point.
Image 04 - Result of Model Check for Crossing Members - No common node
The nodes rest apparently 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 by using the model check.
Image 05 - Result of Model Check for Identical Nodes - 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.
Image 06 - Kinematic System due to Hinge Chain
2. Checking the stiffening
If the stiffening is missing, it may also lead to the calculation aborts due to instabilities. Therefore, you should always check whether the structure is stiffened sufficiently in all directions.
3. Numerical problems
An example of this is shown in Figure 08. It is a hinged frame that is stiffened by tension members. Because of the column contractions due to vertical loads, the tension members receive small compressive forces in the first calculation step. They are removed from the structure (since only tension can be absorbed). In the second calculation step, the model without these tension members is unstable. There are several ways to solve this problem. You can apply a prestress (member load) to the tension members in order to "eliminate" the small compressive forces, assign small stiffness to the members, or remove the members one by one in the calculation (see Figure 08).
4. Detecting Causes of Instability
- Automatic model check with graphical result display
The RF-STABILITY (RFEM) add-on module can help you to obtain the graphical display of the instability cause. Select the "Calculate eigenvector for unstable model…" (see Figure 09), it is possible to calculate the unstable structure. The eigenvalue analysis is performed on the basis of the structural data so that the instability of the affected structural component is displayed graphically as a result.
Image 09 - Graphical Display of Instability - Critical load problem
Can load cases or load combinations according to theory 1. Calculate order and the calculation only increases from theory 2. OK, there is a stability problem (critical load factor less than 1.00). The critical load factor indicated which coefficient must be used to multiply the load so that the model subjected to a specific load becomes unstable (for example, buckling). It follows: A critical load factor smaller than 1.00 means that the structure is unstable. Only a positive critical load factor greater than 1.00 means that the load due to the predefined axial forces multiplied by this factor leads to the buckling failure of the stable structure. In order to find the "weak point," the following approach is recommended, which requires the RF‑STABILITY (RFEM) or RSBUCK (RSTAB) add-on module (see also the video "Critical Load Problem" under "Downloads").
First, it is necessary to reduce the load of the affected load combination until the load combination becomes stable. The load factor in the calculation parameters of the load combination can help. This also corresponds to the manual determination of the critical load factor if the RF-STABILITY or RSBUCK add-on module is not available. In the case of purely linear structural elements, it may be sufficient that the load combination according to Theory 1. Order and select it directly in the add -on module. Then, the buckling curve or shape can be calculated and displayed graphically on the basis of this load combination in the corresponding add-on module. The graphical result display allows you to find the "weak point" in the structure and then optimize it specifically. By default, the RF-STABILITY or RSBUCK add-on modules only determine global mode shapes. In order to also determine the local mode shapes, it is necessary to activate the member division (RF‑STABILITY), or to increase the division for trusses to "2" at least (RSBUCK).
Image 10 - Activating Division for Members in RF-STABILITY
Image 11 - Member Division in RSBUCK
Keywords
Dlubal FAQ Instability System unstable Structure unstable Singularity Instability cause Frequently Asked Question FAQ about Dlubal Question and Answer about Dlubal
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