The paper studies the problem of factoring geometric and physical nonlinearities in seismic analysis of cranes. Experimental study of factoring nonlinear characteristics of dynamics model was conducted. A series of mathematic 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 a factor in the seismic analysis of cranes. Herewith contacts between wheel and rail and one-sided stiffness of wire ropes are required to be taken into account. Any liberalization of dynamic model entails deviation of stresses, displacements and accelerations as results of the analysis.