Saturday, July 26, 2008

Akashi-Kaikyo Bridge , Kobe, Japan

The graceful Akashi-Kaikyo Bridge, linking Kobe City and Awajishima Island across the deep straits at the entrance to Osaka Bay, was opened to traffic on 5 April 1998. Exploiting state-of-the-art technology, it formed the longest part of the bridge route between Kobe and Naruto in the Tokushima Prefecture, completing the expressway that connects the islands of Honshu and Shikoku. With a main span of 1.25 miles (1.99 kilometers) and a total length of nearly 2.5 miles (3.91 kilometers), it was then the longest suspension bridge ever built.

With the growing demand for faster land travel, more convenient links over water obstacles become necessary. If long-span—say, over 1,100 yards (1,000 meters)—bridges are to be politically, economically, and structurally viable, design must be optimized. Because a bridge’s selfweight increases in direct proportion to its span, the structure must be as light as possible while achieving minimum deformation and maximum stiffness under combined dead, wind, and traffic loads. A cable-supported suspension bridge is an ideal way to achieve that.

Alternative designs were developed for the Akashi-Kaikyo Bridge, considering a range of main span lengths. The most economical length was between 6,500 and 6,830 feet (1,950 and 2,050 meters); the final choice of 6,633 feet (1,990 meters) was constrained by geological and topographical factors. The length of the side spans was fixed at 3,200 feet (960 meters), enabling the cable anchorages to be located near the original shorelines. The clients insisted that, because of its immense span, the form of bridge had to assure the public that it would withstand all kinds of loads, including typhoons and earthquakes. Also, it had to express the essential beauty of the Seto-Inland Sea region and evoke a bright future for the Hyogo Prefecture. The Akashi-Kaikyo Bridge would be painted green-gray because it was redolent of the forests of Japan.

Construction began in May 1988. The reinforced concrete anchorages for the cables on the respective shores are of different sizes, because of different soil conditions. As an indication, the one at the Kobe end has a diameter of 283 feet (85 meters) and is 203 feet (61 meters) deep. It is the largest bridge foundation in the world.

Huge cylindrical steel chambers (caissons) form the foundation of the main towers. Fabricated off-site, they are 217 feet (65 meters) high—more than a 30-story building—and 267 feet (80 meters) in diameter; each weighs 15,000 tons (15,240 tonnes). To provide a level base, an area of seabed about as big as a baseball field was excavated under each of them. They were floated into position, and their exterior compartments were flooded to carefully sink them in 200 feet (60 meters) of water. This was achieved to within a 1-inch (2.54-centimeter) tolerance. Each was then filled with 350,000 cubic yards (270,000 cubic meters) of submarine concrete. The foundations of the bridge were seismically designed to withstand an earthquake of Richter magnitude 8.5, with an epicenter 95 miles (150 kilometers) away. On 17 January 1995 the Great Hanshin Earthquake (magnitude 7.2) devastated nearby Kobe; its epicenter was just 2.5 miles (4 kilometers) from the unfinished bridge. A careful postquake investigation showed that, although the quake had lengthened the bridge by about 3.25 feet (1 meter), neither the foundations nor the anchorages were damaged. As the builders boasted, it was “a testament to the project’s advanced design and construction techniques.”

The towers rise to 990 feet (297 meters) above the waters of the bay (for comparison, those on the Golden Gate Bridge are 750 feet [230 meters] high). They have steel shafts, each assembled in thirty tiers, generally made up of three prefabricated blocks that were hoisted into place and fixed with high-tensile bolts. The shafts are cruciform in cross section, designed to resist oscillation induced by the wind. The main cables, fixed in the massive anchorages and passing through the tops of towers, were spun from 290 strands of galvanized steel wire—a newly developed technology—each containing 127 filaments about 0.2 inch (5 millimeters) in diameter. Their high strength does away with the need for double cables, and because they achieve a sag:span ratio of 1:10, the height of the main towers could be reduced. To prevent corrosion of the cables in the salt atmosphere, dehumidified air flows through a hollow inside them, removing moisture. The towers and the suspended structure are all finished with high-performance anticorrosive coatings to suit the demanding marine environment.
From the main cables, polyethylene-encased, parallel-wire-strand suspension cables support the truss-stiffened girder that carries a six-lane highway with a traffic speed of 60 mph (100 kph). The preassembled truss members were hoisted to the deck level at the main towers, carried to their location by a travel crane, and connected; then the suspension cables were attached. This construction technique was chosen because it did not disrupt activity on the water, where 1,400 ships daily pass through the straits.

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