Bad period of isolated structures

[hbma]

Hi,
I have modeled a 3D bridge isolated with different isolators. Ironically, period of isolated structure falls below that of fixed structure; while, the isolated structure most have a larger period. I this due to the opensees performance to calculate the period by using the initially elastic stiffness of Isolators' behavior?

[fmk]

OpenSees calculates period based on current tangent .. it does not try and divine the structure and give periods based on that .. the periods you are getting are for the model you are inputting .. if they do not match what the structure you are modeling is the issue lies in the model.