Some problem about using bond_sp

[toy0410]

Hi, dear all. I have some questions about modeling a zero section that including bond_sp material into a pile at fix rotation head. In the model of Sritharan, the zero section was putted at the fix end of a column and only setting the lateral dof of the nodes beside the zero section equal. It's work and the axial force can pass well. In my model, when the zero section is putted at the top of pile and set the lateral dof of the nodes beside the zero section as Sritharan, the pushover analysis has an error. Then I make the dof of lateral and axial equal, the analysis work. My question is in that situation the section with bond_sp material work or not? And other question is why axial force can't pass well in my model?

Best wish.