Accurate predictions of a structure dynamics require precise modeling of its boundary conditions including any nonlinear effects. This paper investigates the behavior of frictional supports that are examples of boundary conditions exhibiting nonlinear effects, such as stiction and slip phenomena, depending on the structure vibration amplitudes. The dependency of restoring forces in a frictional contact to the vibration amplitude level is identified in this study using experimental observations. In an experimental case study measured responses of a beam fixed at one end and frictionally supported at the other end were expanded using corresponding nonlinear normal modes of the structure and a reduced order model of the continuous system containing dominant nonlinear effects of the contact was obtained. The obtained discrete model constitutes bases for identification of restoring forces in the contact interface using force state mapping. The identified nonlinear restoring forces are then employed to specify parameters of a predictive contact model for the boundary support. The obtained contact model is capable of predicting damping and softening effects due to micro/macro-slippage and accurately regenerates the experimental results at various response levels.