Period of a portal frame

[kavir]

Hello
I made a portal frame with elastic beam column element and concentrated plastic hinges at the ends of elements. then I analysed it with and without "equaldof" for tow nodes on the roof under earthquake record. when I use "equalDOF", period of the structure is about 0.6 sec and without it the period is 0.45 sec but the velocity history of the structure is same in tow ways. can any one explain me why?
I need to understand it because decreasing in period affects on viscus damping energy.

[selimgunay]

This period difference seems to be big and it is unexpected. How do you model the concentrated plastic hinges and how do you make sure there is axial stiffness at the plastic hinges for the case without equaldof