A calculation method for structures under seismic impacts is proposed taking into account changes in buildings' structural design during deformation, based on the normal forms
method and the presentation of design forms for calculating seismic forces in the form of expansions in the forms of natural vibrations. These formulas are close to the calculated ones given in a number of standard documents. The solution is made in time steps. At each step, the structural design is adopted from the results of the static calculation for seismic forces determined at the previous stage. The solution method is getting built in the form of forms expansions using the software MATHCAD. The seismic forces numerical values are
calculated at each time step, taking into account changes in the structural design in the previous step by static calculation for seismic forces impacts. The proposed approach allows us to evaluate the seismic forces values, taking into account changes in the structural design during oscillations at each time step. A change in the structural design is evaluated by static calculation based on seismic forces determined in the previous step. The coefficient K1 values were estimated as the ratio of seismic forces at the stage preceding
failure to seismic forces at the stage corresponding to the end of the elastic stage.