The purpose of this presentation is to investigate the influence of real boundary conditions on the nominal stress in the critical zone of a thin-walled beam and, also the problem of the selection of the rational regime of dynamic load when planning full-scale tests. The estimation of nominal stress of a beam under its form bended axes was carried out using the result of dynamic analysis of a beam by the method of concentrated weights. There were simulated non-classic boundary conditions that describe real supporting of the beam more correctly. The account of elasticity of internal and external constraints allows us to obtain more exact values of the first two natural frequencies of vibrations as results of experiment. The analysis shows exclusively strong influence of elasticity of supporting of a beam and the elastic constraints with other elements such as dynamic characteristics, in particular, and natural forms and frequencies. It is shown that the nonlinearity of elastic constraints causes nonlinear effects of the structure dynamic deformation.