I am actually implementing a 4-storey RC building in opensees navigator and try to verify the results with the same structure modeled in Seismostruct. The modal analysis shows me exactly the same period (compared to Seismosruct and Sap2000) for the first mode (which is torsional), while the second one is much lesser than the aforesaid software. The model is based on rigid Diap.
Could you help me out for understanding the issue?
Thanks in advance