Instability Types That May Occur During Structure Analysis

The structure analysis can issue the following 'instability' messages:

1. type 1 - there is zero value element on the diagonal of stiffness matrix
2. type 2 - there is zero value element on the diagonal of inverted stiffness matrix
3. type 3 - some values of stiffness matrix element are very disproportional.

The first, second and sometimes the third type of the instability is usually caused by mechanical instability of a structure, for example a part of the structure is a mechanism or the structure support parameters (number, type and positions of supports) are not sufficient. The third type can also occur due to the fact that there are big differences in section profiles of some elements.

All types of instabilities can be met for skyline solution method. For frontal method only the first and third type can occur. If the message is displayed for the frontal method it is suggested to recalculate the example with the skyline method selected. In the case of skyline solver the number of nodes and degrees of freedom where the instability has been found is precisely indicated.

Iterative solvers do not report instability messages and the instability of the model may result in slow convergence of analysis. In such a case thorough verification of the model prior to calculations is advised.

The solution method can be changed in the Job Preferences dialog box (Structure Analysis option).