The Akashi Kaikyo Bridge (AKB) is a three-span, two-hinged stiffening girder system suspension bridge that spans the Akashi Strait, connecting the Japanese mainland at Maiko, Taruni-ward in Kobe with Matsuho on Awaji Island. It is part of the ongoing Honshu- Shikoku Bridge Authority (HSBA) project to link the islands of Honshu, Awaji and Shikoku.
After 10 years of construction and a total cost of $3.6 billion, Akashi Kaikyo Bridge opened on April 5, 1998, measuring 3,911 m long with a center span of 1,991 m. AKB is the longest suspension bridge in the world by 581 m, surpassing the Humber Bridge in England, which has a center span of 1,410 m.
Initially, the bridge measured 3,910 m but was lengthened 1 m mid- construction when the bridge was shaken by the Great Hanshin Earthquake measuring 7.2 on the Richter scale in January 1995. The bridge suffered no major damage other than the vertical and lateral offset to the Awaji tower and anchorage, resulting in the AKB being lengthened by one meter.
In June, the Akashi Kaikyo Bridge was presented the George S. Richardson Medal for a single, outstanding achievement at the International Bridge Conference by the Engineer's Society of Western Pennsylvania. (See Richardson and Roebling Awards Presented at IBC, July 1998, p 19). In recognition of this honor, ROADS & BRIDGES provides an in-depth examination, deconstructing the elements necessary to piece together the engineering marvel known as the AKB.
Akashi Kaikyo Bridge Design Standards
As the length of a bridge's center span increases, the structure's stability becomes more susceptible to natural forces. When designing the Akashi Kaikyo Bridge, the Public Works Research Institute (PWRI) constructed a 1:100 scale model and put it through repeated testing in order to develop a superstructure stable enough to withstand the 80-m-per-second winds of the Strait.
"The PWRI played a vital role in the aerodynamic investigation of the bridge," said James D. Cooper, chief of the Structures Research Division, Turner-Fairbank Highway Research Center, Federal Highway Administration. "The HSBA contracted with the PWRI to build a large wind tunnel which was then used to investigate the aerodynamic behavior of the 1:100 model of the AKB."
Additionally, two types of earthquakes were calculated as factors in designing the bridge: an earthquake of magnitude 8.5 on the Richter scale with an epicenter distance of 150 km; and an earthquake occurring with a recurrent cycle of 150 years within a 300 km of radius of the bridge site.Results from these tests were then incorporated into the design standards of the Akashi Kaikyo Bridge.
Main tower foundations
The foundations of the two main towers of the Akashi Kaikyo Bridge are structurally vital in that they transfer the approximately 120,000-tons of downward load from the huge main towers to the supporting ground.
"The most challenging parts of the bridge's construction were physical," said Cooper. "First, designing a bridge at the threshold of a 2,000-m main span length that could carry a significant live load using advanced traditional materials or high strength steels while working under severe constraints imposed by the environment including tidal currents of up to 4.5-m-per-second, winds up to 8-m- per-second and tectonic activity at the bridge site."
Strong currents (4.5-m-per-second), deep water (110 m), wind and waves are what workers battled when it came to laying and completing the foundations for AKB's main towers. Designed to transmit the 120,000-ton weight of the bridge from the support towers to the support ground, which is 60 m under water, excavation by a grab bucket dredger proved to be tedious. Additionally, the excavation process had to be within a vertical variation of /-10 cm for the caisson installation in order to prevent the foundation caissons from tilting.
Manufactured beforehand, the caissons were towed to the site, submerged, filled with underwater and standard concrete and then sunk.
"Due to the extremely demanding physical conditions of the Akashi Strait, special considerations had to be given to ensure navigational safety and environmental preservation," said Yoshikazu Fujiwara, president of HSBA.
It took two days to set the caissons in place because careful attention was paid to the positioning of caissons and the sea level at each particular moment. Lasers and ultrasonic measuring devices were used to guarantee precise and accurate installation.
A variety of high-tech equipment was introduced and new technologies and materials were developed especially for the construction work. For the foundations, a new type of underwater concrete, "underwater nondisintegration concrete," was developed. Advantageous in terms of fluidity and consistency, it can be poured for long distances without a weak layer forming on its surface. "Construction progressed steadily through comprehensive use of advanced bridging technology and where necessary the development of new technology," said Fujiwara.
Constructing the Anchorages for the Akashi Kaikyo Bridge
Since the anchorages, which hold AKB in place, had to be constructed on the shores of the Strait. This was possible only after the work bases had been built on reclaimed land. According to the HSBA, the underground slurry wall method was employed for the 1A anchorage on the Kobe side and artificial bedrock was constructed, creating one of the largest bridge foundations in the world. For the 4A anchorage on the Awaji Island side, a spread foundation construction method using retaining walls was employed. The supporting stratum of the site was inclined both in the bridge axis direction and in the transverse direction. Because of this, retaining walls were installed in six layers of blocks fitting the contours of the site.
For the 1A anchorages, retaining walls arranged in circular form were installed first and the soil inside these retaining walls was excavated in the open-air while the ground water inside was pumped out. A continuous underground wall with 92 sections of the same length was constructed using an excavator for continuous wall construction. Using this retaining wall, the 85-m-diam area was excavated. The excavation work was started at 2.5 m above sea level and reached 61 m below sea level, taking about 11 months in total to complete, during which approximately 330,000 cu m of soil was excavated. After the excavation, roller-compacted concrete was applied to make a foundation consolidated with the retaining wall. The supporting ground for the 4A anchorage was granite. Among the four foundations supporting the bridge, only this ground was strong enough to support the bridge by itself. This supporting ground, however, was slanted. Because of this, a spread foundation was chosen while stability was secured through structural design. In consideration of the fact that the supporting ground existed, 15-25 m below the surface level and that this was reclaimed land, a pillar- supported continuous retaining wall method was used. In this method, pillar piles are driven into the ground first to construct retaining walls and then the inside is excavated.
Next came the installation of the cable anchor frame, a steel structure used to tie down a cable, that was eventually buried by cement in the anchorage.
The main bodies of the anchorages, which support the tension of the cables, were made from highly workable concrete. This concrete, which is highly fluid and needed no compacting, greatly increased efficiency in casting and reduced construction time.
Erecting the main towers
According to the HSBA, while constructing the 282.2-m-high main towers, it was important to maintain vertical precision. Therefore, in manufacturing its structural members, precision processing was applied. A high level of precision was realized by checking the gap between the two touching surfaces with a gap gauge that was only 0.04 mm thick.
The tower top saddles transmit the 100,000-ton weight of the bridge from the cables to the foundations. The towers are divided horizontally into 30 tiers of approximately 10-m-high prefabricated steel on top of each other. Each of the segments are divided with three separate blocks so as not to exceed 160 tons in weight. During construction, a climbing crane was used, and the crane operator and foreman concentrated their efforts to place blocks one on top of the other with great precision.
At a height of 282.8 m, rivaling the height of the Tokyo Tower, the towers are subject to influence from the wind. To counteract this, they were designed cruciform in cross section and have been equipped with stabilizers called tuned mass dampers (TMD). The TMD's are located in each tower to counteract deflective and torsional vibration caused by wind and reduce tower vibration during an earthquake. Weighing about 10 tons each, 20 of the TMD's are distributed on the 17th, 18th and 21st tiers of the tower.
Installing the cables
The first stage of cable installation was the pilot rope spanning, which was done by helicopter. Using light- weight, high-strength poly-aramid fiber rope measuring 10 mm in diameter, the pilot rope, attached to each anchorage, was installed over each span in succession. It was used to suspend the catwalk from which work on the main cables would proceed.
Each cable, 4 km in length, is composed of 290 strands, each strand containing 127 wires made of high-tensile galvanized steel and measuring 5.23 mm in diameter. The strands are hexagonal in shape and factory produced beforehand, in what is known as the prefabricated strand method.
One of the biggest technological advances achieved in constructing the bridge was the improvement in wire tensile strength. Although prior to construction of the AKB the wire tensile strength was only 160 kgf/sq mm, a new wire with an increased tensile strength of 180 kgf/sq mm was developed for the AKB, making it possible to use only one cable per side instead of two.
"The use of higher strength steel kept the diameter of the main cable to a reasonable 1.12 m," said Cooper. "The most significant achievement was the advancement of technology and use of prefabricated parallel wire strand for the construction of the main cable. This technology probably also improved worker safety conditions on the job, relative to worker exposure when using cable spinning on the job.
"Another unique feature of the main cable system is the corrosion protection system selected, which incorporates a dehumidified system to reduce moisture content of the air contained in the enclosed cable," said Cooper.
Stiffening steel girders
In the final stage before completion, 90,000 tons of steel was used in constructing the stiffening girders, which are 35 m wide and 14 m high. The girders were assembled into large blocks of roughly 3,000 tons, which were then installed as single units using floating crane ships. Due to the tremendous size of the bridge, the wind load they sustain is greater than that of any other bridge in existence, according to the HSBA. Using high tensile strength steel made the girders very strong, relatively light and therefore more economical. The stiffening girder construction, by the plane block method, began at the main towers and anchorages, where the floating crane was used to install six panel blocks on the towers and eight on the anchorages.
To reduce stiffening girder torsional vibration caused by wind, stabilizing plates were installed under the median strip of the deck. The stabilizers act to guide the wind, reducing torsional vibration by achieving a balance between pressures on the lower and upper surfaces.
"The Japanese skillfully developed and honed their design, fabrication and construction techniques to the point where the challenge of building the world's longest suspension bridge could successfully be met," said Cooper.
Experience Aids in the Completion of the Akashi Kaikyo Bridge
Honshu-Shikoku Bridge Authority, established in 1970, gained significant experience in the design and construction of the Seto-Chuo Expressway route, according to Cooper. The first of three crossings between the islands of Honshu and Shikoku, major experience was gained in the design and construction of the Seto Ohashi Bridge on that route, which provided the experience and confidence to propose, design and construct the Akashi Kaikyo Bridge. "The safety record for the AKB project sums up the handling of the construction process," said Cooper. "No fatalities, with a half dozen injuries on the world's longest suspension bridge construction project that spanned more than 10 years."
Fujiwara added, "With the opening of the Bridge, I express my deepest gratitude to the local residents and all of those who have rendered their tremendous assistance and support."
Image credit: By Tysto - Self-published work by Tysto, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=477955