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|Publication Number: Date: July/August 1998|
Issue No: Vol. 62 No. 1
Date: July/August 1998
On April 5, 1998, 10 years after construction began, the ribbon was cut to open the world's longest suspension bridge, the Akashi Kaikyo Bridge in Japan. Following a parade of the 1,500 invited guests (including this author) across the bridge, the Crown Prince and Princess of Japan officiated the formal ceremony.
The Akashi Kaikyo Bridge, also known in Japan as the Pearl Bridge, has a record main span of 1,991 meters. By comparison, the bridge is 366 meters (almost Â¼ mile) longer than the previous record holder, the StoreBaelt (East Bridge) in Denmark, which was also opened in 1998. The Akashi Kaikyo Bridge is also 580 meters longer than the Humber Bridge in England, constructed in 1981; 692 meters longer than the longest suspension bridge in the United States, the Verrazano-Narrows Bridge in New York, built in 1964; and 710 meters longer than the Golden Gate Bridge in San Francisco, constructed in 1937.
It cost an estimated 500 billion Japanese yen (U.S. $3.6 billion) to build the bridge. Construction began in 1988 and involved more than 100 contractors.
The Akashi Kaikyo suspension bridge forms one link in the Kobe-Naruto highway route, which connects the main island of Honshu and the southern island of Shikoku. This route is the eastern-most route of three north-south traffic routes approved in the 1969 comprehensive Japan national development plan to stimulate local commerce and to facilitate the industrial development of the region. A central route, which connects Kurasiki on Honshu Island with Sakaide on Shikoku Island, was completed in 1988. A third route further to the west, the Onomichi-Imabari Route, will connect the Hiroshima greater metropolitan area with the Matsuyama area when completed in 1999.
The 81-kilometer-long Kobe-Naruto Route is marked by the construction of two major suspension bridges, including the world's longest - the Akashi Kaikyo Bridge. This bridge with a total length of four kilometers crosses the Akashi Straits, connecting the Kobe area of Honshu with Awaji Island, the sixth largest island in Japan. About 1,400 ships per day traverse the 1,500-meter-wide main shipping lane through the Akashi Straits into the inland sea. A second major bridge on this route, the Ohnaruto suspension bridge, was completed in 1995 and connects Shikoku Island with the southern end of Awaji Island. This link of modern highways, interchanges, and bridges will greatly expand the economy of Awaji Island.
Bridge Construction and Features
The bridge was constructed in 110-meter-deep water with tidal currents of 4.5 meters per second. Wind speeds of 80 meters per second and a potential 8.5 Richter magnitude earthquake 150 kilometers from the site had to be considered by the bridge owners, the Honshu-Shikoku Bridge Authority. Geology of the bridge site includes layered alluvium and diluvium deposits over what is called the hard Akashi or Kobe layer. Granite protrudes at the Awaji anchorage.
The main span was designed to be 1,990 meters with two side spans of 960 meters each. The bridge roadway surface is constructed on top of a 14-meter-deep by 35.5-meter-wide truss girder system suspended from main cables passing over two steel towers that rise 298 meters above main sea level. A 65-meter clearance is 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 were used in the superstructure, and 1.42 million cubic meters of concrete were used in the substructure.
Anchorages measure 63 meters by 84 meters in plan and extend into the Kobe and granite layers at the site. This required special foundation construction technology. The Honshu anchorage had to be embedded 61 meters below sea level, and the anchorage excavation had to be performed in open air. Therefore, 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.
Main tower piers were constructed in the Akashi Strait. The tower-pier foundations were designed to transmit 181,400 metric tons of vertical force to bedrock, approximately 60 meters below the water surface. The 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. Next, the main steel towers were erected on the concrete piers. Each main-tower height is 282.8 meters (297.3 meters with cable saddle in place) and was erected by stacking 30 approximately 10-meter-high prefabricated steel segments on top of each other. The segments are formed with three separate cells in plan view. Special procedures were used during fabrication of each segment to assure tight tolerances for proper tower alignment. The tolerances were maintained using laser measuring technologies for controlling all dimensions. The technology resulted in no major erection problems during field bolting and splicing together of the steel tower segments.
An independent, self-supporting, 145-metric ton, tower crane was used during tower erection. Tuned mass dampers were attached to each tower at varying stages of completion to reduce wind-driven tower motion and reduce tower vibration in the event of an earthquake. Prior to stringing the cable, a pilot hauler rope was attached to each anchorage and placed over the tower tops by helicopter. The pilot rope was used to suspend the catwalk from which work on the main cable erection would proceed. The main cables, which have a 1-to-10 sag ratio, were erected using the prefabricated strand method. Cable strands, comprised of 127 5.23-millimeter-diameter galvanized wires, were factory-fabricated in 4,085-meter lengths. High-strength wire with a tensile strength of 180 kilograms per square millimeter (kg/mm2) was used rather than the standard 160-kg/mm2 wire. Each strand was transported to the construction site where it was pulled from one anchorage over the saddle of each tower and fastened to the opposite anchorage frame. This procedure was repeated 289 times to fabricate each main cable. Each main cable was separated at the anchorage by a splay saddle prior to attachment to the steel frame inside the anchorage to equally distribute cable tension to the foundation. A specially designed cable-squeezing machine was used to compress the 290 parallel wire strands into the final 1.12-meter-diameter cable. Cable bands were placed to circumferentially compress the cable and to maintain the circular shape. Finally, suspender cable hangers were attached to the main cable to support the main stiffening truss.
Hanger cables or ropes were factory-fabricated from bundled, 7-millimeter-diameter, 180-kg/mm2, parallel wire strands. Because the higher strength wire was used, two (rather than the usual four) 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 by barge to the bridge tower piers, lifted to roadway elevation, and transported by traveler 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.
Several unique technologies were developed to support the design and construction of the Akashi Kaikyo suspension bridge. The aerodynamic stability of long suspension bridges poses major challenges to designers. To verify the design of the world's longest suspension bridge, the Honshu-Shikoku Bridge Authority contracted with the Public Works Research Institute to construct the world's largest wind-tunnel facility and to test full-section models in laminar and turbulent wind flow. Other innovations resulting from wind-tunnel testing included installation of vertical plates at the bottom center of the highway deck to increase flutter speed. Methods of improved prediction of flutter speed and gust response will be used in future bridge designs. A second unique technology developed for use on the Akashi Kaikyo Bridge was the use of parallel wire strand for cable fabrication and erection. Rather than using traditional cable-spinning methods for on-structure cable fabrication, individual parallel wire strands were fabricated off-site, transported to the bridge site, and strung parallel to each other to form the main cable. The advantage of using the new method is that the strands are continuous from anchorage to anchorage and eliminate the in-place spinning of cables, thus reducing the probability of accidents occurring. To use the parallel wire strand method, a unique cable-squeezing machine was designed to form the parallel strands into the final circular shape. The use of higher strength wires (180 kg/mm2 ) reduced the number of strands required, thus saving erection time and cost. Use of the higher strength wire also reduced (from four to two) the number of suspender ropes needed to connect each stiffening truss panel point to each cable hanger attachment on the main cable. This accounts for reduced erection time and cost savings.
Performance in Earthquake
Of particular interest was the performance of the bridge in the Jan. 17, 1995, Hyogo-ken Nanbu Earthquake, which provided a full-scale test of tower response. The complete bridge structure was designed to resist a 150-kilometer-distant, 8.5-Richter-magnitude earthquake. Fortunately, erection of the bridge stiffening truss had not begun. The Nojima fault zone passes between the towers of the bridge, and the earthquake caused a permanent lateral and vertical offset of the Awaji tower and anchorage. Ground fault rupture was visible on the northern tip of Awaji Island, approximately two kilometers from the Awaji anchorage. The Awaji tower was displaced 1.3 meters to the west, while the Awaji anchorage was displaced 1.4 meters to the west, relative to the Kobe tower and anchorage. This resulted in a 0.8-meter increase in span length between the main towers and a 0.3-meter increase in the southern side span length. The Awaji tower pier was displaced 0.2 meters vertically downward, while the Awaji anchorage rose by 0.2 meters. The sag in the main cable was reduced by 1.3 meters.
The earthquake caused a one-month delay in the construction schedule during which the bridge was carefully inspected for damage. This lost time was made up during the remaining three-year construction period, and the bridge was opened to traffic on schedule. The increased distance between towers was accommodated by the redesign of the two center stiffening panels, which are 0.4 meters longer than originally designed. Other minor damage was inflicted to the cable-squeezing machine, which was quickly repaired. Anchorages, piers, and towers were otherwise undamaged.
The bridge was under construction for 10 years. In spite of the dangers associated with this type of construction project, elaborate safety procedures paid off. A couple of accidents resulted in six injuries and no deaths, a world-class safety record.
Now that the world's longest suspension bridge is in service, sights are being set on even longer spans. Preliminary plans are underway to investigate the feasibility of even longer span bridges in Japan. Japanese officials indicate that they are looking to extend main span lengths to 2,400 meters, clearly a daunting challenge for suspension bridge design in the new millennium.
The author has followed the design and construction of the Akashi Kaikyo Bridge since 1985, and he was one of only a dozen foreign officials invited to attend the opening of the bridge. He presented a congratulatory letter from Federal Highway Administrator Kenneth Wykle to the executive director of the Honshu-Shikoku Bridge Authority.
James D. Cooper is chief of the Structures Division in the Office of Engineering Research and Development at the Federal Highway Administration's Turner-Fairbank Highway Research Center in McLean, Va. He received his bachelor's and master's degrees in civil engineering from Syracuse University. He is a licensed professional engineer in the District of Columbia.