Completion of the Akashi Kaikyo Bridge, the world’s longest suspension bridge, was a triumph of modern engineering.
In 1969, during the administration of Prime Minister Eisaku Sato, the Japanese government set forth a comprehensive development plan to promote industrial growth and commerce in the eastern part of Japan—specifically in eastern Honshū and Shikoku. (Honshū is the largest of Japan’s four main islands, and Shikoku is the smallest, lying directly south of the eastern part of Honshū.) The plan involved the construction of three highway links between the two islands, using the numerous smaller islands of the Inland Sea—the body of water that separates Honshū and Shikoku—as connecting points.
The first of these highways, the central(or Kojima-Sakaide) route running between Kurashiki in Honshū and Sakaide in Shikoku, was completed in 1988. The western (or Onomichi-Imabari) route, between the Hiroshima region in Honshū and the Matsuyama region in Shikoku was completed in 1999. Whereas the construction of these two routes was greatly facilitated by the many small islands located in their respective regions of the Inland Sea, the construction of the third, or easternmost, link—the Kōbe-Naruto route—was made more difficult by the absence of similar islands in that region.
The plan for the third route involved the connection of the Kōbe area in Honshū with the Naruto area in Shikoku by means of the single large island of Awaji. This required the building of two large suspension bridges, one connecting the Kōbe area (at Maiko) with Awaji Island at Iwaya in the north, and the other connecting the southern end of Awaji Island with the northeastern end of Shikoku in the south. The smaller, southern bridge—known as the Ohnaruto Bridge—was completed in 1995; the larger, northern bridge, the Akashi Kaikyo Bridge, was completed three years later. At the time of its opening, on April 5, 1998, the Akashi Kaikyo Bridge was the world’s largest suspension bridge.
In addition to the challenge of constructing a bridge of such great length—3.8 kilometers (almost 2.4 miles) with a central span of 1,991 meters (more than 6,532 feet)—the building of the Akashi Kaikyo Bridge involved numerous other structural and engineering problems. The Akashi Strait, which the bridge would cross (kaikyo is the Japanese word for strait), is an extremely busy shipping lane. The bridge thus had to be tall enough to allow ships to pass under it, and its construction needed to take place in a manner that would not disrupt ongoing shipping. In addition, the waters the bridge was to span were notorious for gale winds, typhoons, tsunamis, earthquakes, and strong tidal currents, all of which had to be taken into account by the bridge designers, the Honshū-Shikoku Bridge Authority.
Construction of the Akashi Kaikyo Bridge began in 1988. Huge anchorages for the bridge cables were constructed at either end of the planned span, and work began on the two massive support towers out in the water. The concrete foundations for the latter were built into the ocean floor, at a depth of approximately 60 meters (about 197 feet), through the use of large steel caissons, and the prefabricated steel tower sections were then fixed into place on them. To allow clearance for ships, the bridge platform would eventually be placed at a height of 65 meters (a little more than 213 feet) above the water surface. The towers themselves were thus built to a height of 282.8 meters (927.8 feet), making the Akashi Kaikyo Bridge the world’s tallest bridge as well as its longest.
An aerial view of the Akashi Kaikyo Bridge, the world’s longest suspension bridge.
The bridge’s design and construction incorporated many unique and innovative aspects. To achieve the overall aerodynamic stability of the structure—of particular concern given the strong winds to which the bridge would be subjected—the engineers built a wind tunnel (the largest of its kind ever built) to test design features. As a result of this testing, twenty tuned mass dampers, or harmonic absorbers, were attached to each tower as a means of adjusting for wind direction and velocity. In addition, a unique cable design was employed in the construction. Instead of using cable made of individual strands twisted together, which could weaken over time as the strands rubbed against each other, the bridge designers used cables in which the strands had been laid side by side and then squeezed together by a specially designed machine.
By prefabricating bridge sections and then transporting them by means of a floating crane to the installation site, the builders allowed shipping in the Akashi Strait to continue without interruption during the various stages of construction. To this same end, a helicopter was used to carry a pilot rope across the strait to assist in bringing the individual cable strands into position.
On January 17, 1995, as the bridge’s construction was in its latter stages, the area surrounding the construction site experienced the devastating Hyōgo-ken Nanbu earthquake (also known as the Kōbe earthquake).
The bridge was completed at a cost of 500 billion yen (the equivalent of approximately $5 billion). The original plans for the bridge had included both railway and road traffic, but the final structure was designed solely for the latter, with three traffic lanes running in each direction. The finished bridge incorporated some 300,000 kilometers (more than 186,400 miles) of cable (sufficient to circle the world 7.5 times), 700,000 bolts used to fasten the bridge towers in place, and 1,737 lights used for nighttime illumination. Special colored light tubes were also mounted on the main cables; these are operated by means of computer to produce light displays during the celebration of national holidays and other events.
The Akashi Kaikyo Bridge stands as a triumph of modern technology. Successful completion of the bridge in 1998 provided continuing evidence of Japan’s economic growth and resurgence during the second half of the twentieth century, following its devastating defeat in World War II. Despite a prolonged economic slump that lasted throughout the 1990’s, Japan remained at the end of the century one of the world’s major economic powers, and the nation’s ability to build the world’s longest suspension bridge provided a powerful symbol of that fact.
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Cooper, James D. “World’s Longest Suspension Bridge Opens in Japan.” Public Roads 62 (July/August, 1998): 32-36. Offers a good summary of the project, including background, information on the bridge’s construction and design, and discussion of the effects of the 1995 earthquake. -
Kashima, Satoshi, and Makoto Kitagawa. “The Longest Suspension Bridge.” Scientific American 277 (December, 1997): 88-93. Presents an excellent summary of the bridge project. Includes photographs, diagrams, and a map of the larger Honshū-Shikoku development project. -
Nagai, Masatsugu, and Shuichi Suzuki. “Long-Span Bridges (Honshū-Shikoku Bridge Project).” In Bridge Engineering Handbook, edited by Wai-fah Chen and Lian Duan. Boca Raton, Fla.: CRC Press, 1999. Provides a brief overview, with map and photographs, of the Honshū-Shikoku development project, including the building of the Akashi Kaikyo Bridge. -
Normile, Dennis, and Frank Vizard. “A Bridge So Far.” Popular Science 252 (March, 1998): 48-53. Presents informative discussion of the Akashi Kaikyo Bridge. In the same issue, related articles appear on U.S. bridges and on the methods used to protect bridges from earthquakes.
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