The problem of geometrically and physically nonlinear deformation of flat bar system. For bar elements having homogeneous isotropic layers of variable cross-section, used classical theory of thin rods. The study of the deformation process in the one-parameter proportional loading and the identification of the ultimate load (the stability of the second kind) is performed based on the variation of the dimensionless parameter of loading system. An example of calculation and analysis of the impact of non-linear factors in a metal frame with three-layer bars.