I have modeled a 6-story steel structure (3d) with a combination of base isolators and viscous damper in the base level in OpenSees which corresponds to a MCEER report so as to verify my modeling in this FE software.

The report has an appendix that is included of $2k file of Sap2000. I tried the same earthquake record in my model and sap2000 model to become sure about the accuracy of my model... the response of two models in the elastic zone almost matches but when the plastic behavior begins, two curves that are obtained from analyzing the models (for example time-disp curve of the roof), are not match.

Could anyone help me Which parameters or commands in OpenSees are effective on the non-linear behavior of the structure?