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What's the world's longest suspension bridge?


The Akashi Kaikyo Bridge: Done, but Never Finished
The motorway leading to the Akashi Kaikyo Bridge. The bridge transformed a 45-minute ferry ride into a four-minute car trip.
The motorway leading to the Akashi Kaikyo Bridge. The bridge transformed a 45-minute ferry ride into a four-minute car trip.
© pasmal/amanaimages/Corbis

Six years into the construction process, and in 1995 the Akashi Kaikyo Bridge had survived its first earthquake. Surely, the going would get easier, right?

Wrong. The bridge's support cables became the next point of contention. Because the longest suspended portion of the bridge measured more than a mile, ordinary steel cables were just too heavy. Instead, Japanese engineers had to invent a solution: a new type of steel wire that was flexible, incredibly strong and composed of a new blend of alloys and silicone. At just 5 millimeters (1/5 inch) thick, a single strand of this ingenious steel could support the weight of three rhinoceri. Thousands of these strands were compacted into a single cord of steel wire more than 300,000 kilometers (190,000 miles) long and installed by helicopter to hold up the Akashi Kaikyo's roadways [source: Curiosity Aroused].

To lift the 290 sections of roadway deck into place, cranes were built just for this project. The deck's trusses included a strategic triangular design to allow airflow, but also incorporated vertical arms to act as rudders, stabilizing the deck during high winds [source: National Geographic].

Finally, in 1998, 10 years after construction began on the Akashi Kaikyo Bridge, the final piece of deck was in place. The six-lane suspension bridge, which transformed a 45-minute ferry ride into a four-minute journey by car, was complete. But not finished.

Although designed to last 200 years, the Akashi Kaikyo Bridge requires daily maintenance. The Honshu-Shikoku Bridge Authority, charged with the bridge's upkeep, operates a control center that monitors everything from the bridge's horizontal and vertical movement to the air conditioning system designed to keep its steel trusses from corroding. Travelers pay a toll of 2,300 yen (about U.S.$25) to cross the bridge by car [source: Honshu-Shikoku Bridge Authority].

Japanese engineers may have designed and built a suspension bridge that holds world records in triplicate, but they won't be able to rest on their laurels. Emerging technology, including lightweight carbon-fiber cables, could someday make it possible to construct suspension bridges with a reach 10 times that of the Akashi Kaikyo [sources: Curiosity Aroused, National Geographic, Ryan].