Development of structural countermeasures against instability phenomena of long-span cable-stayd bridges

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The objective of this research is to establish structural countermeasures against instability phenomena in long-span cable-stayd bridges. The instability phenomena, such as buckling due to static loading and wind-induced vibration, are not significant for cable-stayd bridges with conventional range of the span length. For extremely long-span cable-stayd bridges with a span of more than 1000 m, however, the instability phenomena must be paid attention in design stages. In the present research, three span cable-stayd bridges with a multi-fan cable system are considered as a standard cable-stayd bridge.The main results obtained in the present research are as follows :1.It is shown that the modified E_f method is applicable to evaluate the load-carrying capacity of the girders in cable-stayd bridges with spans above 800 m, where the bending stress is small compared with the normal stress due to the axial force.2.Based on the E_f method, a rational design method for the buckling of the girders is proposed.3.To improve the load carrying capacity of the girders, elastic supports of the girders in the longitudinal direction and reinforcement of the girders are considered as recommendable countermeasures.4.A numerical method to find the initial configuration of cable-stayd bridges under dead load is developed. The present method is required for the finite displacement analysis to evaluate the load-carrying capacity of flexible cable-stayd bridges.5.Damping characteristics of stay in cable in cable-stayd bridges are investigated on the basis of experimental data as well as energy analysis. The effects of mechanical countermeasures against wind-induced vibrations, i.e., cable-cross tie and viscous damper on damping is clarified. Furthermore universal estimation curve of damping is proposed for design of viscous damper.