Initial stiffness of analytical results are always larger?

[wangweiyi]

I am modeling the behavior of a RC column in static reversed cyclic test. I got many details about some tests at nisee.berkeley.edu/spd/ and modeled them.
I compared the experimental and analytical result and the initial stiffness of analytical results are always larger than experimental results so that I can't get the yield displacement exactly and in my research the yield displacement is very important .
I used fiber section ,BARSLIP Materia and Section Aggregator command to take flexural ,slip and shear deformation into account. I try to modify the parameter of material but the result is still not very good.

I read some paper of Filip C.Filippou and find his analytical results also have this problem. I don't know why. I hope you can solve my problem,thanks.