Crossing Seto Inland Sea, Kojima-Sakaide, Japan Designer/Engineer Honshu-Shikoku Bridge Expressway Company Completed 1988 Span 1,378 feet (420 meters)

Materials Steel, concrete Type Cable-stayed

Twin cable-stayed bridges, the only such pairing in the world

One of the most ambitious undertakings in the history of civil engineering began in 1970 with the formation of the Honshu-Shikoku Bridge Authority, now the Honshu-Shikoku Bridge Expressway Company (HSBE), a Japanese government agency. HSBE was charged with linking Honshu and Shikoku, the largest and smallest of Japan’s four main islands, by 2000, with a breathtaking number of road and rail bridges, seventeen in all. The bridges cross the island-studded Seto Inland Sea via three routes: the eastern Kobe-Naruto route, which includes the world’s longest suspension bridge (see here); the western Nishi-Seto route, which once boasted the world’s longest cable-stayed bridge (see here); and, on the central Seto-Chuo route, the Hitsuishijima and Iwakurojima Bridges, the world’s first twinned cable-stayed spans. Beating the deadline by a year, the last of the vast collection opened in 1999.


The Seto Inland Sea, a major transportation route and a national park, is located in an earthquake- and typhoon-prone area. These commercial and environmental factors determined the Honshu-Shikoku bridges’ size, style, and color, and spurred the development of several technologies that advanced the frontiers of long-span construction. They include, most notably, the development of aseismic and wind-proofing techniques; a railroad expansion joint system for high-speed train travel on suspended spans; and lightweight, high-strength structural steel and cable wire. All seventeen bridges have a life expectancy of two hundred years, almost twice the international norm, per HSBE policy. “Defensive maintenance” is the rule; when eleven cracks, each measuring less than a quarter of an inch (5 millimeters) were discovered along the 4.2-mile (6.8-kilometer)-long Seto-Chuo Expressway, it made the news. Perhaps the most striking of all the structures on the Kojima-Sakaide Route [are] the exactly-paired Hitsuishijima and Iwakurojima Bridges . Only in Japan would there be towers of this shape, flaring outwards at the top in imitation, perhaps, of the helmets which were worn by medieval Japanese warriors.


A sextet of long-span bridges known as the Seto-Ohashi Bridges, on the 23-mile (37-kilometer) Seto-Chuo route, carry vehicular and rail traffic. They include three suspension bridges: the Shimotsui-Seto, a 3,084-foot (940-meter)-span suspension bridge; and the Kita Bisan-Seto and Minami Bisan-Seto bridges, two nearly identical suspension bridges with spans of 3,248 feet (990 meters) and 3,609 feet (1,100 meters), respectively, that share a common anchorage. The suspension bridges are the first to carry railroad traffic since Roebling’s Niagara Bridge of 1855. The Yoshima Bridge is a continuous truss viaduct with a main span of 804 feet (245 meters).

An aerial view shows, from front to back, the Shimotsui-Seto Bridge,

Hitsuishijima and Iwakurojima bridges, Yoshima Bridge, Kita Bisan-Seto Bridge, and Minami Bisan-Seto Bridge.

The last two of the six are the most unusual: the identical cable-stayed Hitsuishijima and Iwakurojima Bridges, each of which has a main span of 1,378 feet (420 meters) and side spans of 607 feet (185 meters). Although there are larger cable-stayed bridges, the duo is unique both as a pair and for their unprecedented deep-trussed double decks. They were opened to traffic in April 1988.

Leave a Reply

twenty three + = twenty nine