Question: My model is unstable. What can be the cause?
Answer:
The calculation can be terminated due to an unstable structural system for various reasons. On the one hand, it may indicate a "real" instability due to an overload of the system, but on the other hand, modeling inaccuracies may be responsible for this error message. The following is a possible procedure to find the cause of the instability.
1. Check of modeling
First, it should be checked if the structure is correct by the modeling. For this, it is recommended to use the model controls provided by RSTAB/RFEM [Tools → Model Check]. Using these options, you can, for example, find and delete identical nodes and overlapping members.
Furthermore, you can structure z. For example, it can be calculated under pure dead load in a load case according to the first-order analysis. If results are displayed subsequently, the structure is stable with regard to the modeling. If this is not the case, the most common causes are listed below (see also the video "Model Check" in the "Downloads" area):
Incorrect definition of supports/lack of supports
This may lead to instabilities because the system is not supported in all directions. Therefore, it is necessary for the support conditions to be in equilibrium with the system as well as with the external boundary conditions. Statically overdetermined or kinematic systems also lead to calculation breaks due to insufficient boundary conditions.
Torsion of members about their own axes
If members twist about their own axis, that is, the member is not supported about its own axis, it may lead to instabilities. The cause is often due to the settings of the member hinges. Thus, it is possible that torsionally steered members were introduced both at the start node and at the end node. However, a warning message when starting the calculations alerts the user.
Missing connection of members
Especially for larger and more complex models, it can quickly happen that some members are not connected to each other and thus "float freely in the air." Forgetting intersecting members that should actually intersect can also lead to instabilities. The model check "Crossing, Not Connected Members" provides a workaround that searches for members that cross each other but do not have a common node at the point of intersection.
No common node
The nodes are obviously on the same location, but on closer inspection, they deviate slightly from each other. Common causes include CAD imports, which you can use to clear the model.
Creates a hinge chain
Too many member hinges and a node may cause a hinge chain, which may lead to a calculation abort. Only n-1 hinges with the same degree of freedom relative to the global coordinate system may be defined for each node, where "n" is the number of connected members. The same applies to line hinges.
2. Check of the stiffening
A missing stiffening also leads to calculation aborts due to instabilities. Therefore, it should always be checked if the structure is sufficiently stiffened in all directions.
3. Numerical Problems
For an example, see Figure 08. It is a pinned frame that is stiffened by tension members. Because of post shortenings due to vertical loads, the ties receive small compressive forces in the first calculation run. They are removed from the system (because only tension can be absorbed). In the second calculation run, the model is unstable without these ties. There are several ways to solve this problem. You can apply a prestress (member load) to the tension members to "eliminate" the small compression forces, assign a small stiffness to the members, or have the members removed one after the other in the calculation (see Figure 08).
4. Determining the cause of an instability
Automatic model check with graphical output
To obtain a graphical representation of the cause of instability, the RF-STABILITY (RFEM) add-on module can help. With the option "Determine mode shape of unstable model ..." (see Figure 09), it is possible to calculate supposedly unstable systems. An eigenvalue analysis is performed based on the structural data so that the instability of the affected structural component is displayed graphically as the result.
Critical Load Principle
If load cases/load combinations can be calculated according to the first-order analysis and the calculation only increases from the second-order analysis, there is a stability problem (critical load factor smaller than 1.00). The critical load factor indicates the factor with which the loading must be multiplied so that the model becomes unstable under the corresponding load (for example buckles). It follows from this: A critical load factor smaller than 1.00 means that the structure is unstable. Only a positive critical load factor greater than 1.00 allows for the statement that the loading due to the given axial forces multiplied by this factor leads to buckling failure of the stable system. To find...
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