Periods

[nicola27]

I'm doing a nonlinear analysis with accelleregramma of a reinforced concrete frame.
I used non-linear elements and sections in fiber concrete01 and steel02.
When I calculate the proportional damping, calculation periods and frequencies.
The time that I find myself using the nonlinear elements is different from that calculation period with the elastic elements, why?
When I use the nonlinear elements such as OpenSees calculates the period, such as stiffness use it?
Must be the same?