The cable stayed bridges represent key points in transport networks and their seis mic behaviour need to be fully understood. This type of bridge, however is light and flexiable and has a low level of inherent damping. Consequenly, thery are susceptible to ambient excitation from seis mic loads. Since the geometric and dynamic properities of the bridges as well as the characteristics of the excitations are complex, it is necessary to fully understand the mechanism of the interaction among the structural componenets with reasonable bridge shapes. This paper discuss the dynamic
response of a cable stayed bridge under seismic loadings. All possible sources of nonlinearity, such cable sag, axial-force-bending moment interaction in bridge towers and girders and change of geometery of the whole bridge due to large displacement are based on the utilization of the tangent stiffness matrix of the bridge at the dead-load deformed state which is obtained from the geometry of the bridge under gravity load conditions ,iterative procedure is utilized to capture the non-linear
seismic response and different step by step integration schemes are used for the integration of motion equations. In this study, three spans cable-stayed bridge with different cable systems has been analyzed by three dimensional nonlinearity finite element method. The three dimensional
bridge model is prepared on SAP 2000 ver.14 software and time history analyses were performed to assess the conditions of the bridge structure under a postulated design earthquake of 0.5g. The results are demonstrated to fully understand the mechanism of the deck-stay interaction with the
appropriate shapes of a cable stayed bridges.