At the present time for providing earthquake resistance of buildings and structures, the multilevel design approach is applied. According to this approach seismic loads corresponding to the level of the maximum design earthquake must be perceived by building through the plastic resource of structures, and at the same time the complete collapse of buildings or parts of it must be prevented. In the standards of foreign countries the work of the construction outside the elasticity limit is taken into account by the reduction factor. The load reduction factor selection consists in reducing the seismic loads, depending on the maximum (allowed) level of residual deformations in structures due to earthquakes. The maximum residual deformations are taken into account by ductility factor. In the present research a nonlinear system with single degree of freedom is used for analyzing the behavior of structures under elastoplastic deformation. The author obtained the relationship between plasticity factor and yield strength of structure characterized by the reduction factor K1. The statistical analysis of data was also carried out in order to estimate the values of the reduction factor K1. Such approaches have already been practiced, but such relations were obtained only for simple loadings. In this paper, these dependences were obtained for more complicated, earthquake loadings.