AKASHI KAIKYO BRIDGE MAP
One of seventeen major spans built in Japan in the space of twenty-five years
It has been said that engineers at Japan’s Honshu-Shikoku Bridge Expressway Company (HSBE) “didn’t want to challenge the gods” by adding the 10 meters that would bring the length of the Akashi Kaikyo Bridge’s main span to an even 2 kilometers. Still, the HSBE has managed to defy almost every other aspect of bridge design and construction in building seventeen major crossings on three routes that hop across the Seto Inland Sea. Fourteen are world-class spans, including the massive Tatara Bridge (see here) and the idiosyncratic Hitsuishijima and Iwakurojima Bridges (see here). But the Akashi Kaikyo, with its record-breaking 6,532-foot (1,991-meter) suspended main span, is the group’s crown jewel, aptly known locally as the Pearl Bridge. Opened to traffic in 1998, the $7.6 billion structure was once the most expensive suspended span and remains the longest. There are certain memories of the past that have strong steel springs and, when we who live in the present touch them, they are suddenly stretched taut and then they propel us into the future.
Part of the Kobe-Awaji-Naruto Expressway, the Akashi Kaikyo is the easternmost of the three multibridge crossings, and links the islands of Honshu and Awaji. The route continues across Awaji Island to the Onaruto Bridge (1985), a 2,873-foot (876-meter) suspension bridge, and ends on the island of Shikoku. Construction on the three-span bridge began in 1988. One of the last of the HSBE crossings completed, the 2.4-mile (3.9-kilometer)-long Akashi Kaikyo benefited from the experience accumulated while building the earlier spans. (Incredibly, no major bridges were built in Japan before 1973.) Today, thrill seekers can climb 984 feet (300 meters) to the top.
AKASHI KAIKYO BRIDGE MAP Photo Gallery
Bridge construction also benefited from advances in materials, most notably a high-strength steel wire never before used in a suspension span. The improved tensile strength of the cable wire permitted two, rather than four, main cables to be used. To reduce corrosion, always a concern, particularly in humid climates, the HSBE developed a system that injects dry air into the cables.
Once the steel towers were in place, cable installation began. To avoid disrupting ship trafficsome 1,400 ships pass through the strait each daya helicopter was used to install the pilot suspension rope in 1993. Subsequently, the HSBE prefabricated cablesbundling wires into strands off-site, then transporting them and pulling them from anchorage to anchorageto improve on conventional spinning methods. On other decisions, engineers stuck to the tried and true, just scaled up to fit the Akashi Kaikyo’s colossal dimensions.
Prefabricated circular steel caissons, each with a diameter of 262 feet (80 meters), were towed to the site, submerged, and filled with regular concrete and a new underwater concrete that was developed for this project.
The bridge’s location in a seismic zone influenced its conservative design. A conventional deep truss was used to stabilize the deck, resulting in a heavier profile than the slender, aerodynamic decks of most recent spans.
Engineers designed the bridge to withstand a magnitude 8.5 earthquake, and considered seismic impacts from two distances: within 93-mile (150-kilometer) and 186-mile (300-kilometer) radii, establishing new seismic standards for long spans. Despite this, 1995 opened with a shock. On January 17, the magnitude 6.9 Great Hanshin Earthquake struck, its epicenter only six miles from the span. HSBE’s initial relief that the structure was still standing gave way to anxiety when measurements indicated that the quake had pushed the towers 3 feet (1 meter) apart, requiring that the suspension hangers and deck be redesigned. The month lost to the earthquake was regained when the erection of the deck’s stiffening truss proceeded smoothly.
