HITSUISHIJIMA AND IWAKUROJIMA BRIDGE MAPS
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.
HITSUISHIJIMA AND IWAKUROJIMA BRIDGE MAPS Photo Gallery
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.
DAVID J. BROWN, BRIDGE MAPS, 1993
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.