Seismic risk analysis of modified fan type cable stayed bridges

A seismic risk analysis of modified fan type cable stayed bridges is presented using the concept of damage probability matrix. The cable stayed bridge is modeled as a two dimensional system with the deck idealized as a continuous beam subjected to bending action and axial compression. The response of the bridge is obtained by the frequency domain spectral analysis. A double filtered power spectral density function with seismic intensity parameter taken as the magnitude of an earthquake and a correlation function between the support excitations are considered as seismic inputs. For a given magnitude of earthquake, the damage probability matrix is determined by defining three damage states namely, major, moderate and minor. The seismic risk index is determined by combining the damage probability matrix with the probability of occurrence of different magnitudes of earthquake. As an illustrative example, a three span modified fan type cable stayed bridge is analyzed for an extensive parametric study. The parameters include degree of correlation, angle of incidence of earthquake, ratio of the components of ground motion, and soil condition. Some of the important conclusions of the study indicate that (i) the longitudinal component of ground motion has a considerable effect on the annual probabilities of failure of the bridge deck; (ii) annual probabilities of failure are significantly more for soft soil condition; (iii) fully correlated ground motion between support excitation provides less value of the damage indices as compared to the uncorrelated ground motion; and (iv) annual probabilities of failure are not very sensitive to the variation of angle of incidence of an earthquake.