Boundary Conditions for Z-Axis in a 2D Simulation (Solid Mechanics)

Hi,

I'm having difficulty figuring out how to set desired boundary conditions for the Z-axis in a 2D solid mechanics simulation. I would like my model to be infinite along the Z-axis with a fixed strain, but when I simulate such a structure, I get no strain along the Z-axis. I believe this is because the simulator treats the Z-axis as a free surface, but instead, I'd like to impose Neumann boundary conditions such that strain is maintained along the Z-axis.

Essentially, I'm modeling a substrate with 2.5% compressive strain along the X and Z directions, and I want to see the impact of this strain upon a thin rod on the top of the substrate. The substrate and rod are infinite in the Z direction, so a 2D simulation is used.

Note: I've been able to get the correct results with cylindrical coordinates using 2D axisymmetric geometry (a pseudo 3D simulation), but I would like to model the device using 2D geometry instead of 3D due to computation and memory considerations.

Attached is the model file.

Thanks for your help!
Jamie