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World's Longest Suspension Bridge Opens in Japanby James D. Cooper
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. 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. Background
Bridge Construction and Features 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 Safety First Looking Ahead 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.
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