Hello all, Inspired by MASTAN and it's very useful documentation, i'm programming an FE application myself. I'm pretty pleased that, so far, my application can handle 3d frame elements. Unfortunately i'm stuck trying to code non-lineair behaviour, specifically transforming the element geometric stiffness matrix to the global stiffness. I'm missing some examples/explanation regarding the transformation matrix for the geometric stiffness matrix. Can anyone provide me with some reading tips? All help is appreciated!