I was wondering whether its possible to use boundary conditions in SAP2000 to model a contact surface that restrains joint displacements in the -ve z direction only?
Never used SAP2000, except exporting to a SAP2000 format from Strand7 - that is about the closest I have come to using it .
What you are asking for is a nonlinear contact. In Strand7 we would use a nonlinear element called a point contact, that is only active in compression (when the distance between its nodes is getting smaller than its original length).
In SAP2000 there should be a similar element/link called GAP. This video is very good explaining how to use them for modelling contact situations:
Bearings with uplift may be modeled using nonlinear gap (compression only)link objects. Since these objects demonstrate nonlinear behavior, you must specify nonlinear analysis in the load case.