Fiber model in Non linear time history analysis

Question:

Hi all,

please consider the simple model attached.

It deals with a one span beam subjected to periodic axial load of 650 kN with this time law:



The current rebar of the element is (4P20):




The axial load is lower then the tensile strength of the element. Since concrete has not tensile strength:

Nrd = 524.56 MPa * 4 *314mmq = 659 kN.

Nevertheless Ned = 650 kN < Nrd = 659 kN, Midas gives this result, in terms of axial displacement:



You can note it seems that element yields after 6 seconds.
We can not understand why.

It happens also changing model for concrete, or changing damping, or for axial force lower then 650kN. The deformation are stable under 610 KN more or less.

How does Midas calculate the strength? Why this strange behaviour?
We hope you can help us.

Thank you very much,
kind regards