alues depending on
% whether elements approach or depart from each other.
% The unloading stiffness is always greater than the loading stiffness.
% 
% The energy dissipation in this model is due to the spring hysteresis and
% it is able to simulate the plastic deformation of elements during
% collisions.
%
% $$\left \{ F_{n} \right \} = -F_{n}^{e} \{-\hat{n}\}$$
%
% * *Loading* ($\dot{\delta_{n}}>0$):
%
% $$F_{n}^{e} = K_{n}^{L}\delta_{n}$$
%
% * *Unloading (before detachment)* ($\dot{\delta_{n}}<0, \delta_