Structure is Unstable

I am designing - for education- a 3D box truss of three similar(but not exactly the same) repeat sections.
If software is giving me the warning “Structure is unstable” .
Is there any systematic method of finding the problem?
The computer screen structure looks “good”.
The software is “Dr. Frame”.


One way, provided that the software can do it, is to run a eigenfrequency analysis. That will give you any rigid body modes that need to be constrained.

Above is a snippet.

Another way; take a look at the structure and determine whether or not it is stable.