I am trying to model the forces acting on the inside of a grain silo. The tank of the silo is composed of a thin cylindrical shell on top of a thin conical shell. The forces are composed of a component normal to each face and a component "parallel" to each face caused by the friction of the grain.

I have been able to model the normal components on both the cylindrical and conical face without any problem. I have also been able to model the parallel componenent on the cylindrical face using the reference geometry option in the pressure dialog. However, I am unable to model the parallel component on the conical face. When attempting to do so, I receive an error message telling me to select a plane, edge, or cylindrical face.

Is there any way to apply pressure along a face that is not plane or cylindrical?