What causes large inter-storey drift at the first floor?

[wcq19811982]

Hello everybody：
I' m sorry to disturb you with a question.
I am analyzing a reinforced concrete frame structure with OpenSees program,
and the top displacement and base shear force obtained from OS matched
well with the experimental result. But the inter-storey drift of the first floor
is greater larger than that obtained from experiment. The maximum inter-storey drift is listed as follows:
Number of stories ---------------------Inter-storey drift (mm)-------------
--------Floor 6---------------------------------8---------------------------------
--------Floor 5---------------------------------10---------------------------------
--------Floor 4---------------------------------12---------------------------------
--------Floor 3---------------------------------15---------------------------------
--------Floor 2---------------------------------20---------------------------------
--------Floor 1---------------------------------42---------------------------------

In the serious elastic-plastic stage, the structural response causes large residual
deformation, but the constraint nodes at the bottom of the structure have no
displacement at all. Is this the main reason which causes large inter-storey drift
at the first floor? Who can give me a hand?
I hope get a reply as soon as possible.
Best regards

[wangbin_tongji]

Your mean that other story drifts match well with the experimental result, is only the 1st greater larger than that obtained from experiment ?

[wcq19811982]

yes, other floors match well except for the first floor