Analysis Wizard : Model Stability

A model stability analysis is set up identify problems arising from modelling.

Definition

Low Stiffness Modes

In most cases a handful of modes (say 5) should suffice. This will identify area of the model that have zero (or low) stiffness. This may be due to lack of proper connectivity. The 'correct' number of modes will depend on how ill conditioned the model is and how many errors there are. As a rule of thumb, if the result from running the analysis returns all eigenvalues as small, double the number of modes requested.

The shift option allows highly ill conditioned models to be solved successfully. If the default value does not work, the shift can be increased by moving the slider to the right.

High Stiffness Modes

The high stiffness modes are usually less of an issue, but these enlarge the condition number. In particular short elements with high section properties can give rise to ill conditioning.

See stability analysis for details about the Model stability analysis.