Is it possible to specify different joint properties for tension and compression?

Yes, different stiffness values for tension and compression can be assigned to a joint in RS2.

In the Stress Analysis tab of the Project Settings dialog, there is an option to specify a reduction factor for joint stiffness for tension.  This option can also be coupled with zero tensile strength for the joint.

 Phase2-jointstiffnessfactor-edited.png

Specifying different joint stiffness for tension and compression