Akashi- Kaikyo bridge (Pearl bridge), Japan - Research Paper Example

Akashi- Kaikyo bridge (Pearl bridge), Japan Seventeen long-span bridges, including ten suspension bridges, five cable-stayed bridges, one arch bridge, and one truss bridge, connect the main island of Honshu with the southern island of Shikoku in Japan. The Akashi Kaikyo bridge across the Akashi Strait, perhaps the longest suspension bridge in the world, is one of them. It forms a part of the 81-kilometre-long Kobe-Naruto highway and links Maiko, in Kobe, and Matsuho, on Awaji Island. The Akashi Kaikyo Bridge is known in Japan as the Pearl Bridge. Before the bridge became operational, the people had to mainly depend on ferry services to travel between the islands. The ferry travel was uncertain and rather dangerous as the area often experienced powerful winds, storms and typhoons, and horrible ferry mishaps did occur. Heeding to the public outrage that followed the sinking of two ferries in 1955, causing the death of 168 people - a majority of them children, the Japanese government undertook the construction of this bridge in 1988. Another reason for building the Akashi Kaikyo bridge, which was approved in the 1969 comprehensive Japan national development plan, was to promote local trade and assist the industrial development of the region. Together with another major suspension bridge namely, the Ohnaruto suspension bridge, completed in 1995 and connecting Shikoku Island with the southern end of Awaji Island, the economy of Awaji Island which is the sixth largest island in Japan, was expected to improve considerably (Cooper, 1998). Although the construction of the bridge was prioritised following the ferry disaster in 1955 and feasibility studies began soon thereafter, the actual construction could only begin in 1988 as the process was a difficult one. The Akashi Strait is four kilometres wide and where bridge was proposed to be built, the sea was 110 metres deep, with tidal currents of 4.5 metres per second. The problems of bridge design related firstly to the severe weather conditions existing in the Akashi Strait, such as strong winds (wind speeds of 80 metres per second), and even typhoons. Earthquakes were also a strong possibility 150 kilometers from the site and possibly, tsunamis. All these adverse conditions had to be withstood by the bridge. For this, the Japanese designers and engineers first built complex models and conducted the necessary scale model wind tunnel tests on them. This helped them refine the design so that the bridge could handle severe weather and typhoon conditions. Secondly, the Akashi Strait is an inland waterway and a busy shipping port, with nearly 1500 ships and barges crossing it everyday. That is, the bridge would have to span across the busy international shipping lane. Therefore, it should be sufficiently high to allow the large barges and ships pass under it without any difficulty and also, the construction work should cause least disturbance to the shipping activity in the busy Akashi Strait. Thirdly, the bridge had to be strong to support an expressway for vehicles. Only a suspension bridge could overcome all these problems especially because suspension bridges can be built high. Also, temporary central supports are not required to be built during the construction which means no disturbance to the busy waterways. Therefore, the Akashi Kaikyo bridge had to be a suspension bridge. The original plan was to build a road-cum-railway bridge which was later modified to a six-lane road bridge (Yim, 2007). The bridge was completed in 1998, with more than a hundred contractors involved in the job that cost an estimated 500 billion Japanese yen (that is, around U.S. $4 billion). Bridge architecture The Akashi Kaikyo bridge has three spans. The main span of the bridge was originally proposed to be 1990 metres with two side spans measuring 960 metres each. However, due to the January 17, 1995 Hyogo-ken Nanbu Earthquake which had its epicentre right between the two towers of the central or main span, the towers moved apart by almost a metre, increasing the main span length to 1991 metres. As the construction of the deck that is, the roadway had not yet commenced, this change was easily incorporated into the modified final design. The height of the towers is approximately 297 metres and a 65-metre clearance is maintained over the shipping lane for easy movement of ships. Anchorages at the two ends measure approximately 63 metres by 84 metres and go down into the Kobe and granite layers at the site. The designing of the Akashi Kaikyo bridge pushed suspension bridge technology to the limit. Anchorage at the Honshu end had go down 61 metres below sea level for which the excavation had to be performed in open air. For this purpose, a circular slurry wall 85 metres in diameter and 2.2 metres thick, was constructed. Excavation was carried out within the slurry wall. Specially-developed, roller-compacted concrete, which was non-disintegrating underwater, was used to fill up the slurry wall to complete anchorage foundation construction (Cooper, 1998; Wilson, 2003). On the other hand, foundation for the Awaji anchorage was built with steel pipes and earth anchors to support the surrounding soil (Cooper, 1998). Specially-developed flowing-mass concrete that was also non-disintegrating underwater was used to fill the excavated foundation. At the anchorages at both the ends, a huge steel supporting frame was constructed to anchor the main suspension cable strands. The main tower piers were constructed in the Akashi Strait, approximately 60 metres under the sea. The tower piers can transmit 181,400 metric tons of vertical force to bedrock (Cooper, 1998). The construction of the foundation of the tower piers was made using the newly developed laying down caisson method. In this technique, pre-fabricated steel caissons were towed to the tower sites and set on the pre-excavated seabed using concrete. When the construction of the pier-foundation was over, the main steel towers were assembled on the concrete piers. Each tower consists of thirty prefabricated steel segments stacked on top of each other. In order to maintain proper tower alignment, laser measuring technologies were used during fabrication of each segment to control the dimensions. Each tower was provided with tuned mass dampers to reduce tower motion due to wind. The dampers will also reduce tower vibration due to earthquakes. A helicopter was used to place a pilot hauler rope attached to the anchorages at the top of the towers. A ramp was attached to the pilot hauler rope from where to work on the erection of the main cable. The main cables consisted of prefabricated strands of high-strength galvanised wires. The design of the Honshu-Shikoku bridges are generally based on the allowable stress design method. Due to the Hyogo-ken Nanbu earthquake of January 1995, the Awaji tower was displaced 1.3 meters to the west, and the Awaji anchorage was pushed 1.4 meters to the west, relative to the Kobe tower and anchorage. A one-month delay in the construction schedule was caused due to the earthquake as the bridge had to be carefully inspected for damage. Anchorages, piers, and towers were found to be mostly undamaged. Many unique technologies were developed for use on the Akashi Kaikyo bridge such as development of underwater non-disintegrating concrete; use of prefabricated parallel wire strand for cable fabrication and erection instead of traditional cable-spinning methods for on-structure cable fabrication, suspenders covered by polyethylene tubes, development of cable dehumidification system etc. (Wilson, 2003). A crucial maintenance protocol for steel bridges on the sea is prevention of corrosion of the steel members. The dehumidification method for the cables of suspension bridges is very important in this respect. The Akashi Kaikyo suspension bridge in Japan is a technological marvel. It remained unaffected by the strong earthquake, the Hyogo-ken Nanbu earthquake of January 1995 due to sound anti-seismic measures used in the bridge design and construction. Three major reasons could be given for the anti-seismic property of the bridge. They are: (1) site location of the bridge based on thorough geological survey; (2) the inbuilt flexibility of suspended structures; and (3) detailed seismic design techniques used to construct the suspended structures. References Cooper, J.D., 1998. Worlds Longest Suspension Bridge Opens in Japan. Public Roads, 62(1), Accessed 11 May 2010 from http://www.tfhrc.gov/pubrds/julaug98/worlds.htm Wilson, B., 2003. A master of many. Roads & Bridges, Volume : 41, Number: 8. Accessed 11 May 2010 from http://www.roadsbridges.com/A-master-of-many- article4363. Yim, W.T., 2007. The Bridge Engineering 2 Conference Akashi Bridge. Proceedings. University of Bath, U.K. Accessed 11 May 2010 from http://www.bath.ac.uk/ace/uploads/StudentProjects/Bridgeconference2007/ conference/mainpage/Yim_Akashi.pdf Read More