Pushover Analysis

[KenjiSakuragi]

Dear All,

I have a problem with Pushover analysis of a simple Frame (3-Story by 6-Bay).
Pushover analysis is conducted without problems.
But, I try to draw the relationship between story drift ratio and shear force of each floor, the story drift ratio is decreased from the middle.
I can't understand what happened.

I wrote the configurations.

1. Column : nonlinearBeamColumn (Pinching4).
2. the column is assumed shear wall. So, it only resist Vy. P and My is much larger value
3. There is no beam. Rigid floor assumed. So, the nodes of each story is linked by equalDOF 1.
4. geomTransf : PDelta

Anyone could help me please?
thank you