The paper studies the problem of factoring geometric and physical nonlinearities in seismic analysis of cranes. An experimental study of factoring nonlinear characteristics of dynamics model was conducted. A series of mathematical models were developed to describe different ways of linearization of the experimental model. Comparison of results shows that physical nonlinearity of wire ropes is not required as factor in seismic analysis of cranes. Therefore, contact between wheel and rail and one-sided stiffness of wire ropes are required to be taken into account. Any liberalization of the dynamical model entails deviation of stresses, displacements and accelerations as results of the analysis.