The Akashi Kaikyo Bridge in Japan, the world's longest suspension bridge opened on 5 April 1998, 10 years after construction began. The salient features of this bridge are that it withstands typhoons, occasional catastrophic earthquakes, does not obstruct the international shipping lane and is strong enough to support an expressway for vehicles.
Also known as the Pearl Bridge, it has a main span of 1,991 meters. Costing an estimated 500 billion Japanese yen, its construction began in 1988 and involved more than 100 contractors.
The eastern-most route of 3 north-south traffic routes approved in the 1969 comprehensive Japan national development plan to stimulate local commerce and facilitate the industrial development of the region, the bridge forms a vital link in the Kobe-Naruto highway route, connecting the main island of Honshu to the southern island of Shikoku. A central route connects Kurasiki on Honshu Island with Sakaide on Shikoku Island and a third route, further to the west – the Onomichi-Imabari Route, connects the Hiroshima Greater Metropolitan area with the Matsuyama area.
Totalling 4 kilometres in length, the bridge connects the Kobe area of Honshu with Awaji Island. The geology of the site includes layered alluvium and diluvium deposits over what is known as the hard Akashi or Kobe layer and granite protrudes at the Awaji anchorage. The bridge, constructed in 110 meter-deep water, faces tidal currents of 4.5 mps. Wind speeds of 80 mps and a potential 8.5 Richter magnitude earthquake 150 kilometres from the site had to also be taken into consideration by the bridge owners, the Honshu-Shikoku Bridge Authority.
The main span was designed to be 1,990 meters with 2 side spans of 960 meters each. The bridge roadway surface was constructed atop a 14 meter deep, 35.5 meter wide truss girder system, suspended from main cables passing over 2 steel towers rising 298 meters above main sea level. A 65-meter clearance was maintained over the shipping lane. The 1.12-meter diameter main cables were erected using full-length, prefabricated strands. Approximately, 181,400 metric tons of steel was used in the superstructure and 1.42 million cubic meters of concrete in the substructure.
The anchorages, measuring 63 x 84 meters in plan, extend into the Kobe and granite layers at the site; this required a special foundation construction technology. The Honshu anchorage was embedded at 61 meters below sea level, and the anchorage excavation had to be performed in open air. An 85-meter diameter circular slurry wall, 2.2 meters thick, was constructed and subsequently used as a retaining wall. Excavation within the slurry wall was followed by the placement of roller-compacted concrete to complete anchorage foundation construction.
The Awaji anchorage foundation was constructed using steel pipes and earth anchors to support the surrounding soil. The excavated foundation was filled with specially designed flowing-mass concrete. Both anchorages were completed with the construction of a huge steel supporting frame used to anchor the main suspension cable strands.
The foundations of the main tower piers constructed in the Akashi Strait were designed to transmit 181,400 metric tons of vertical force to the bedrock, approximately 60 meters below the water surface. This foundation was constructed using a newly developed laying down caisson method. Steel caissons, 80 meters in diameter and 70 meters in height, were towed to the tower sites, submerged and set on the pre-excavated seabed. Pier-foundation construction was completed with the placement of concrete; following which, the main steel towers were erected on the concrete piers. Each main-tower, 282.8 meters in height, was erected by stacking 30 approximately 10 meter high prefabricated steel segments on top of each other.
The hanger cables or ropes were factory-fabricated from bundled, seven millimetre-diameter, 180 kg/sqm, parallel wire strands. Because higher strength wire was used, the two hanger ropes were required to support the panel points of the stiffening truss girders. Steel stiffening truss girder panels were fabricated off-site and transported via barge to the bridge tower piers, lifted to roadway elevation, and transported by traveller crane to the proper location for connection to the suspender ropes. This procedure allowed the uninterrupted use of the busy shipping lane of the Akashi Straits.
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