Nonlinear restraint stiffness (not bilinear)

How can I simulate nonlinear stiffness (not bilinear) of a restraint if I have data?

Actually the stiffness value itself is linear. There is a constant stiffness until something happens - like lift-off, which if that occurs the stiffness goes to zero. Since the stiffness curve is not a straight line, we refer to this as a non-linear restraint.

What are you trying to do?