Abstract: Based on the mechanism of interaction of superstructure, foundation and soil, the development of the finiteness of bending rigidity and shear stiffnes of the superstructure is suggessted, and the upper limit of bending rigidity development is pointed out. The conditions that dominate the structure changing from bending deflection to shear distortion and the law of transformation are also analyzed. Based on that, two stages with corresponding formulas for calculating the internal forces of the foundation are put forward and an example is given.