The analysis can issue the following ‘instability’ messages:
· type 1 (or 0 for sparse solver in versions older than v.21.0) - there is zero value element on the diagonal of stiffness matrix,
· type 2 - there is zero value element on the diagonal of inverted stiffness matrix,
· type 3 – the ratio between maximum and minimum values on the diagonal of inverted stiffness matrix is suspiciously high
The first and second and sometimes the third type is usually caused by mechanical instability of a structure, e.g. a part of the structure is a mechanism (for instance too many releases or incorrect definition of releases) or the number, type and positions of supports are not sufficient. The third type can be also due to big differences in section profiles of some elements or existence of abnormally short elements.
For skyline solver and starting from v.21.0 for Sparse solver all of the above types can be reported. For frontal method only the first and third type can occur. For sparse solver in versions earlier than v.21.0 only the first type can occur. If the message is displayed it is suggested to recalculate the example with skyline method. In the case of skyline and sparse solver the number of node and degree of freedom where the instability has been found is precisely indicated.
Sparse solver can also report instabilities during Gauss elimination - they correspond to type 3 instabilities.
Iterative solvers do not report instability messages and the instability of the model may result in slow convergence of analysis so it this case thorough verification of the model prior to calculations is advised.
The solution method (solver) can be changed in: Job Preferences \ Structure Analysis