Rigid Wall / Basement Mechanical Constraint

Hello,

it´s probably a ridiculous question, but I need a mechanical constraint just like an ideal rigid wall or basement which the DUT is unable to penetrate but is able to lift off it. A roller constraint where a displacement in z is possible and -z=0 would be perfect. Imagine a flat metal sheet which experiences a force just in the middle and is able to deform like a bowl. I know such a constraint from other CAD (FEM) programs like SolidWorks as "rigid wall" but did'nt found a handy solution in COMSOL. I hope it´s possible without writing own eq´s. Currently I a built a 3d model.

Thanks for your help,

BR Sven