analysis of column with eccentric loading

[forf]

i am doing the analysis of column with eccentric loading with vertical load applied at some distance from the column, so that the column fails by compression at the side of bending. The reinforcing steel goes on yielding at the compression side at the the end of linear phase and after that the reinforcing steel undergoes plastic deformation .after that the curve in bending moment vs deflection , instead of being horizontal , it is increasing linearly .i could not understand the reason

 

[forf]

I came to conclusion that once the reinforcing steel reaches its yield strength , all the reinforcing bar one by one reaches to yield strength and has constant stress(yield stress) in reinforcing bar due to consideration of perfectly plastic model. but however in experiment once the reinforcing bar reaches to yield strength it undergoes buckling.so due to this reason only model overestimates the strength of the structure. Can I consider the post-buckling model of reinforcing bar in Abaqus