Takayuki HORIE

Ultimate strength of composite cable-stayed bridges depending on the ratio of cable axial stiffness to girder flexural stiffness

Masatsugu NAGAI and Eiji IWASAKI

In Japan, the safety factor of the cable being used for cable-stayed bridges has been stipulated to be 2.5 against breaking strength. However, when designing the extradosed PC cable-stayed bridges, the lower safety factor of 1.7 has been employed. Since the vehicle load is mainly carried by flexural rigidity of the main girder, repeated stress in the cable is small and, hence the possibility of fatigue damage is small. It has been reported that no possibility of fatigue damage is the reason of the reduced factor.
This paper examines ultimate strength of cable-stayed bridges selecting a ratio of cable axial stiffness which depends on the safety factor to flexural rigidity of the main girder as design parameter. The center span length of the cable-stayed bridge model is set to be 150 meters and the height of the tower from the deck level is one tenth of the span. Four different flexural stiffness of the girder and five safety factors of the cable were dealt with. After applying the dead load with pre-stressing in the cable and taking into account of creep, shrinkage effect, uniformly distributed load was applied throughout the whole bridge length. From this study, under the condition that the same load factor at ultimate state is obtained, it was found, even though a relatively higher safety factor of the cable has to be employed, that the lower flexural rigidity of the main girder gives an economical solution.