Cables are one of the main components to inhibit the extension of suspension bridge spans.
As spans become progressively longer and dead load increases, the steel cables become longer and heavier. The relationship between center span length and dead load is shown in Fig. 15.27, for a three-span catenary suspension bridge with a stiffening truss girder. What this indicates is that as the center span length increases, the cable weight increases at a faster rate than the dead weight of the suspended structure. Stated another way, as the span increases, there is a decreasing percentage capability of the cable to carry live load, Fig. 15.28.
This results from the increase of the ratio of cable weight to weight supported with increasing span. By analogy, the same is true for the cable-stays of cable-stay bridges. This means cables for spans much larger than the Akashi Kaikyo and Tatare Bridges will become increasingly difficult to install and tension, become less efficient with respect to load carrying capacity, and become more costly to erect. Higher strength and lighter cables will be required for future spans exceeding todays technology.
(Yoshida, I. V., Fujiwara, M., and Yokoyama, K., Future Projects for Highway Construction Across Straits in Japan, and Technical Considerations.