support

Dear friends

I am modeling a
plate subjected to the vertical loading. the problem is, when the analysis goes
to the nonlinear part, the reactions in some nodes get negative value, while the
plate must be free to move upwards. so there must be no negative value
for reaction. i was wondering, if someone help me define the support in a way
that the plate would be free to move upward and constrain it for the downward
movwement.

Best Regards