The cable-stayed bridges having the new structural system such as the Annacis Bridge, stiffened by the plate girder and with precast concrete slabs, have been interested from a viewpoint of economy. It will be more necessary for the Annacis type bridges as well as the ordinary type bridges to estimate the natural frequencies in the initial stage of design in order to check the aerodynamic stability.
In this paper, the effect of cable stiffness on the natural frequencies of the cablestayed bridges is investigated with respect to the relation between a cable-stayed bridge and a beam on the elastic support for their dynamic behaviour. And the simplified method of estimating the natural frequencies of the continuous type cable-stayed bridges with the various girder and cable stiffness is newly proposed for the aerodynamic resistant design.