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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 errors may be responsible for the error message. Below is a possible way to find the cause of the instability.

First, it should be checked if the modeling of the system is correct. To find modeling problems, use the model controls (menu "Tools" → "Model Check").

Furthermore, you can structure z. For example, it is possible to calculate under pure self-weight in a load case according to the linear static 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 Video 1):

  • Supports are missing or have been defined incorrectly
  • Members can rotate about their own axes because a corresponding support is missing
  • Members are not connected ("Tools" → "Model Check")
  • Nodes are evidently in the same place, but if looked at more closely, they deviate slightly from each other (common cause for CAD Import, "Tools" → "Model Check")
  • Member Hinges / Line Hinges Create a "Hinge Chain"
  • The structure is not sufficiently stiffened
  • Nonlinear structural elements (for example Tension members) fail

Finally, Figure 02 shows an example. It is a pinned frame stiffened by tie rods. 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 (see Figure 02), or have the members removed one after the other in the calculation (see Figure 02).

To obtain a graphical representation of the cause of instability, the RF-STABILITY (RFEM) add-on module can help. The option "Determine mode shape of unstable model ..." (see Figure 03) allows you to calculate unstable systems. In the graphic, the component that leads to the instability is usually recognizable.

If load cases and load combinations can be calculated according to the linear static analysis, the calculation is only broken when calculating according to the second-order analysis or the second analysis. Order, then 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. In order to find the "weak point", the following approach is recommended, which is required by the RSBUCK module (for RSTAB) or RF-STABILITY (for 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 is used as an aid (see Video 2). Then, the buckling or buckling shape can be calculated and graphically displayed on the basis of this load combination in the RSBUCK (RSTAB) or RF-STABILITY (RFEM) add-on module. With the graphical output, it is possible to find the "weak point" in the structure and then specifically optimize it.

Videos

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

Keywords

Unstable Instability Abort calculation critical load Singular

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RFEM Main Program
RFEM 5.xx

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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

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RF-STABILITY 5.xx

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Stability analysis according to the eigenvalue method

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The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions

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RSBUCK 8.xx

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Stability analysis according to the eigenvalue method

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