Offshore wind turbines are highly exposed to timevarying loads. For support structures, estimation of the fatigue damage during the lifetime of the structure is an essential design aspect. This already applies for the preliminary design stage. In determining the dynamic amplification in the frequency domain, modal analysis is a common tool. This paper describes a main drawback of the application of modal analysis in preliminary support structure design. Exact mode shapes are not available, due to the concentrated inertia of the rotor nacelle assembly (RNA). Generally the mode shapes of a cantilever beam are applied, in which the RNA mass is neglected. In analyzing three turbine types (V90-3.0MW, SWT-3.6-107 and NREL 5-MW), an overestimation of dynamic amplification due to sea level loading is observed. Estimation of the structural response at the second natural frequency turns out to be poor. Within the range of considered RNA masses, the actual magnitude of the tower top mass does affect the relative error of the modal analysis much. Besides, by increasing the tower and support structure stiffness, the relative error diminishes.