how to define spring stiffness of spring element, which connected between concrete slab and steel beam?
in what unit it would be?
what about the force vs displacement curve of the spring, how does it work?
The units will be consistent with whatever unit system you are working in. So, if you have defined the model with force in Newtons and dimensions in mm, then the spring stiffness will be in N/mm.
Not sure I understand what you mean by;
what about the force vs displacement curve of the spring, how does it work?