Q: When using nonlinear analysis in Advance Design with large displacement enabled, sometimes we get an error message indicating instability at a certain degree of freedom.
A: This message means that the structural system became unstable, it is identical to the message we get when the structural system is poorly supported.
For a structural model with good support conditions, this instability error message during nonlinear analysis with large displacement is caused by generalized buckling occurring in the structure. Buckling causes instability in certain degrees of freedom similar to what insufficient support conditions do.
Advance Design nonlinear solver is equipped with stabilized iterations that will try to overcome local instabilities by adjusting bad pivots in the stiffness matrix.
During each stabilized iteration, Advance Design will adjust bad pivots and reiterate.
For buckling instability with local effects, bad pivots should be fixed with a few stabilize iterations and Advance Design will continue its calculations. For buckling instability affecting the global level, bad pivots will keep reappearing until the maximum number of stabilized iterations is reached. Then, the calculation is stopped and the instability error message is displayed.
Increasing the maximum limit of stabilize iterations may be sufficient for many cases, however, the user should be very careful when doing this. In fact, with every bad pivot adjustment, the stiffness matrix is slightly changed. Doing this excessively could impact results accuracy. For such instability problems, we recommend the user do the following steps:
- Run a generalized buckling analysis for the same loads/combinations that are used in the nonlinear analysis. This is to identify if the buckling problem has local or global effects.
- If these loads/combinations reach or get close to a buckling mode with global effects on the structure, nothing more can be done. The structure is collapsing due to buckling.
- If these load/combinations reach or get close to a buckling mode with local effects, the user is advised to change the load steps number. In this way, he might avoid falling directly into the local instabilities state during load steps.