Equivalent damping ratio

Hello everyone

I’m searching for some validations how Sofistik calculate the critical damping (and then the damping ratio) for a frame structure. In the manuals I didn’t find any indication about numerical methods used by the program to estimate the critical damping for each mode of the structure.

Particularly, I’m modeling a simple frame building and I placed rotational springs between each beam-to-column connection. For each of this springs I set in the module SOFISMSHA not only a certain rotational stiffness, but also a certain viscous damping coefficient (FEDE DM). Suddenly I perform an eigenvalue analysis (EIGE ETYP LANC) and I noticed that, varying the damping coefficient, the equivalent damping ratio of the first mode varies more or less in a proportional way with the damping coefficient. But I need to know excatly how this equivalent damping ratio is calculated by your software.

Did Sofistik maybe build a damping matrix inserting directly the damping coefficient for each DOF where is defined?

Thank you for your help!