Question:
I have modeled a structure made of outer and inner cables as well as membranes, and get a message about instability or the error message "Jacobi matrix is not positive definite" in the RF‑FORM‑FINDING add-on module. What should I do?
Answer:
RF‑FORM‑FINDING displays this message because no stable shape of the cable and membrane structure can be found.
In the case of a structure consisting of outer (main) cables and inner cables, we recommend specifying a cable force within the cable parameters for form-finding. The force in the outer main cables should be greater than the force of the inner cables.
Example
Average force in cable:
Main cables: 20 kN
Inner cables: 5 kN
The setting can be made in the "Edit Parameters for Member of Type 'Cable'" dialog box; see the image.