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A suspension bridge's level of technical sophistication is indicated by the length of its central span (see figure). The United States, as the world leader in suspension bridges, approached the 500-meter level in 1883 with the construction of the Brooklyn Bridge, which boasts a 486-meter central span; 50 years later, technology had advanced to the point that the country was capable of erecting a 1,300 meter-class bridge. In Japan, Kyushu's Wakato Bridge, with its 367-meter span, was a long time coming, being completed in 1962; compared with the contemporary level of American bridge-building, it was about 80 years behind.
Among the hindrances to bridge construction in Japan are extremely severe natural conditions. Four major challenges are: (1) withstanding the forces of the country's numerous earthquakes, (2) securing bridge piers in deep waters with strong tidal currents, (3) resisting strong winds such as typhoons, and (4) developing strong yet lightweight materials. By solving these problems one by one, Japan's bridge technology has risen to a position of world leadership only 30 years after the completion of Wakato Bridge. The following pages introduce some of the technological developments accomplished during the Akashi Kaikyo Bridge project.
Among the new technologies employed in building the Akashi Kaikyo Bridge, for which a lengthy span was the only way to straddle the powerful tidal current and deep waters, a caisson construction method was adopted to place the structures on-site, from the water. It was a radical departure from the common approach of assembling most of the structure on land. Work began in dry-docks with the assembly of doughnut-shaped cylinders of steel, each measuring 80 meters in diameter, 70 meters in height, and weighing 15,000 tons. These enormous caissons were then towed to their respective sites and sunk to the seabed.
If the caissons were placed even slightly off level, the towers would not stand vertically; so the foundations had to be set on the seabed at a depth of 60 meters, horizontal to less than a 10-centimeter margin of error. An official of the Honshu-Shikoku Bridge Authority, recalling the early stages of construction, said, "We were under a lot of mental stress dealing with the rigorous precision. And at one point when we were towing the caissons, we encountered an unexpectedly strong current and had to wait it out for a day and night in the Kii Suido channel. The footings may be out of sight once the bridge is built, but they're the part that gave us the most trouble." |
