Push over

[mbopen]

Hi.
Please help me.
I model a 2D X braced steel frame. I used "Steel02" Material for all of elements and I used Fiber section for beams, column and braces.
For buckling of braces, I used an imperfection in middle of element.
Now after pushover analysis, my Pushover curve have one elastic segment and inelastic segment. But the slop of inelastic segment does not fall and it seems to
the frame dose not collapse Even in large displacements.
Please help me