What does 'Stiffness matrix not positive definite' mean?
One of the unique properties of a stiffness matrix in both matrix structural analysis and the finite element method is that the entries on the main diagonal are positive and non-zero, or positive definite.
Frame produces this error when it encounters a negative or zero number on the diagonal of the stiffness matrix. This is usually caused by one of the following:
- Instability, eg. neglecting to restrain the rotational degree of freedom at the fixed end of a cantilever
- Parts of the model are not connected
- Numerical instability, eg. connecting a 5m deep beam to a 5mm deep beam (resulting in an ill-conditioned matrix)