Answer:
It is quite probable that the high deformations are caused by the consideration of shrinkage and the horizontal support in the model.
The shrinkage is considered internally on the load side as a strain, but in this context, failure due to the shrinkage strain is also possible. If the shrinkage strain is prevented by a rigidly fixed horizontal support, forces arise which may lead to the concrete failure, and thus to a significant increase of the deformations or even to the instability of the model.
In this context, it is important to model the boundary conditions of the model as realistically as possible when using the nonlinear deformation analysis.
