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Error during calculation: “Structure is instable. instabillity found.”

If you get the following error message during a calculation ...

... it means that there is a problem with the degrees of freedom in the structure.
The node or member where the (first) instability is found, is indicated.

Let’s take a look at the example below. The problem is found in node N1, direction fiX (rotation around the X axis).

The supports in the end nodes of the beam are both fully hinged (Rx, Ry, Rz = Free). This means that a rotation of the member around its own axis is not prevented: the slightest torsion in the member will result in an instability. Putting Rx = Rigid in one of the supports in the end nodes will solve the unstability in this case.

TIP: In a large project, it won’t always be easy to find the indicated member or node. The ‘selection’ command can prove to be extremely useful there:

  • Type in SCIA Engineer’s Command line the command ‘SEL’ followed by an interspace and the full name of the requested member or node (e.g. SEL N1).
  • By means of the icon  ‘Zoom all – selection’ you can zoom in on the selected entity, to find out the cause of the instability.