Blocking the displacement along Z<0 only

[AG1980]

How can i block the displacement of a surface along Z<0 only and tolerate it along Z>0?
As i know, this cant be done in boundary conditions...
is self contact the only way or there is an easier way?

Thank u all

 

[rstupplebeen]

I would make a rigid plane and apply contact.

Rob Stupplebeen

 

[AG1980]

Is there no easier way?????????????