Capacity curve of concrete frame

[GoudaiHHH]

Hi,
I have a question about the capacity curve of a concrete frame building. It's a five-storey building and I modelled the beam and column using infrmFB with default value. The capacity curve I got from the pushover is not quite what I expected. What I got is the curve goes up, reaches the maximum base shear, then it starts to drop. But what I expected is a horizontal line after the maximum base shear based on hand calc. Considering the plastic hinges form at each floor level, when it comes to the top level, there is little increase in base shear but huge increase in displacement, hence the horizontal section. Is there any setting I need to adjust in order to get that shape?
Thank you for your help.

[seismosoft]

There are several reasons for the drop of the capacity in the descending branch of the curve, for instance P-delta effects with large vertical loads on the columns. Note that in the more complicated fibre-modelling the moment-curvature diagrams of the sections are not bilinear, but rather have a more smooth shape.
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