Free vibration of structures is investigated by using a multidegree of freedom nonlinear model that allows for nonlinear behavior of the soil underlying the structure. The considered nonlinearity is due to the dependence of the soil shear modulus on vibration levels. The soil-structure interaction is modelled as nonlinear complex stiffness. The analysis is carried out by using mathematical perturbation theory. Modifications in the natural frequencies and existence of harmonics are demonstrated. The results are applied to an application example of massive deep water gravity structure.

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