The equilibrium of the nonlinear model is achieved when the force imbalance is zero or sufficiently small. This equilibrium is obtained by an iterative procedure that consists of steps at which the force imbalance is computed ad tested. If it is not small enough, an incremental displacement related to the imbalance is computed, the displacement is adjusted and the imbalance is again evaluated. If the displacement exceeds a certain level, the stiffness matrix is also updated and the process repeated.