By Eur Ing Juhani Virola
Introduction
Akashi Bridge (Japan)
(Photograph courtesy Honshu-Shikoka Bridge Authority)
In this article, two long-span suspension bridges are described: the Akashi-Kaikyo Bridge in Japan and the Runyang South Bridge in China. The Akashi Bridge is dated 1998. With its main span of 1991m, it is the longest-span bridge ever built and the greatest bridge of the past Millennium.
The Runyang Bridge (1490m) commenced in 2000 and is due for completion in 2005. It will then be the third longest-span bridge ever built, and for some time the greatest bridge of the new Millennium. While these two stand out, there are many other important bridges in that part of the world. For example, the Tsukuhara Bridge is located in Honshu island, a few kilometres away from the Akashi Bridge. The Tsukuhara Bridge consists of two adjacent bridges, and at completion in 1998 it was the longest-span (180m) 'extradosed prestressed concrete bridge' in the world. Soon thereafter, however, some longer-span such bridges were built. To mention just two such bridges in Japan: the Kiso Bridge (3 x 275m main spans) and the Ibi Bridge (4 x 271.5m).
Fixed links between Japanese main islands
The area of Japan is 378,000km2 and the population is about 126 million (in 1998). Japan consists of four main islands. These are Kyushu in the south-west, Shikoku and Honshu in the centre, and Hokkaido in the north-east.
Between Kyushu and Honshu, the largest island, there are tunnel and bridge links. For example, the Kanmon Strait Bridge is a notable suspension bridge, 712m main span, and was the longest-span bridge in Asia at the time of its completion in 1973.
Between Kyushu and the smallest island of Shikoku there are not yet fixed connections, but plans for a fixed link between Kyushu and Shikoku and also for some other long-span bridges in Japan are under consideration.
Between Honshu and Hokkaido under the Tsugaru Strait, the Seikan Tunnel was completed in the 1980s. With a length of nearly 54km (53km 850m) it is the world's longest railway tunnel: the Eurotunnel under the English Channel is narrowly shorter than 50 km.
Between Honshu and Shikoku, there are existing three fixed links: from east to west, Kobe-Naruto, Kojima-Sakaide, and Onomichi-Imabari. These routes contain several great bridges, some of them world record bridges. The easternmost link, west of Osaka, is the Kobe-Naruto route, length 89.0km, completed in Spring 1998. Its most remarkable structure is the Akashi-Kaikyo Bridge, the longest-span (1991m) bridge in the world.
Of these three links, only the middle one or the Kojima-Sakaide route (length 37.3km) carries both road and rail traffic, while the two other routes convey only road traffic. The Kojima-Sakaide route is located some 100 km west of Kobe and it was completed in April 1988. It contains notable suspension and cable-stayed bridges. This bridge combination is known also as the Seto-Ohashi Bridge. Its most remarkable structure is the Minami Bisan-seto suspension bridge, main span 1,100m, which was until 1997 the world's longest span bridge for road/rail traffic. The overall length of the Seto-Ohashi Bridge is 12km 306m, the longest road/rail bridge in the world.
The westernmost Honshu-Shikoku fixed link is the Onomichi-Imabari route, length 59.4km, completed in May 1999. It is situated some 200km west of Kobe, and it also contains a number of great bridges. Internationally the most remarkable of those is the Tatara Bridge, the world's longestspan (890m) cable-stayed bridge. Another notable bridge is the Kurushima-Kaikyo Bridge. It is a unique combination of three successive suspension bridges, six towers, four anchorages, main spans 1,030 + 1,020 + 600m, and total length 4,015m including the side spans. This 4015 m is a record: it is the world's longest suspension bridge structure, longer than the suspension bridge structure of the Akashi Bridge, which is 960 + 1,991 + 960 = 3,911m.
The total length of these three Honshu- Shikoku routes is about 186km. Their construction was commenced in the 1970's, and the total cost was about JPâ 3,400 billion.
The Akashi-Kaikyo Bridge - greatest bridge of the past millennium
On the Kobe-Naruto route, a smaller Awaji Island is located between Honshu and Shikoku, and the 4 km wide Akashi Strait is located between Awaji and Honshu. The Akashi-Kaikyo Bridge is named thereby: the Japanese word 'Kaikyo' means 'Strait' in English. The bridge across the Akashi Strait is located some tens of kilometres west Kobe, which itself is west of Osaka.
On the same Kobe-Naruto route, south of the Akashi Strait and between Awaji and Shikoku islands, the Ohnaruto Bridge was completed in 1985. It is a suspension bridge, main span 876m.
The Akashi Bridge was first proposed in the 1950s. With successive proposals, the main span was all the time increasing. In the 1960s, it was considered to be 1,300-1,500m, a world record in those times. In the 1970s, the main span was increased to 1,780 m and the bridge was proposed for both road and rail traffic. In 1985, the rail alternative was abandoned and a decision was made to restrict the bridge for highway use only. After careful consideration of the heavy sea traffic along the Akash Strait (some 1,400 ships a day), which dictates a long main span for the bridge, and the maximum current speed in the strait of 4.5m/s, and difficult foundation conditions, the optimal main span range appeared to be 1,950-2,050 m. The final main span was chosen to be 1,990m, annoyingly less than the magic 2km mark. Even this exceeds by 41% the previous record: Humber Bridge in Britain, which has main span 1,410m, and dates from 1981.
Construction of the Akashi Bridge was commenced in May 1988, and the construction period lasted ten years. As to its type, the bridge is a three-span twohinged suspension bridge with steel truss stiffening girders for deck structure, designed spans 960 + 1,990 + 960 = 3,910m, actual spans somewhat longer. The bridge is located between Maiko (in Shikoku) in the north and Matsuho (in Awaji) in the south. At each end there is a short viaduct made of concrete.
It should be mentioned that although this suspension bridge has a total length of nearly 4 km, it is roughly only 10% the length of the longest bridge in the world. The record belongs to the Second Lake Pontchartrain Causeway, in Louisiana, USA, dated 1969. It is a multi-span bridge, total length 38km 422m, but its spans are on average only about 25m. So the Akashi Bridge is the world's longest-span bridge. But it is definitely not the longest bridge in the world, though this impression might be given in some sources.
The main span of the Akashi Bridge is the first ever to exceed the English mile (1,609m), and currently the only one exceeding even the nautical mile (1,852m). There is good reason to say that in many respects this is the most remarkable bridge of the past millennium. During the new millennium, some longer-span bridges may be built and the list of proposals below contains some gigantic bridges of the future:
- across the Messina Strait between Sicily and mainland Italy: a suspension bridge, main span 3.3km.
- across the Tsugaru Strait in Japan: a hybrid suspension/cable-stayed bridge with two successive main spans of 4km.
- across the Gibraltar Strait between Spain and Morocco: a suspension bridge with two successive main spans of 5km, or a cable-stayed bridge with an enormous main span of 8.4km.
Foundations and towers
The huge cable anchorages (anchor blocks), made of concrete, are located on both shores. To prevent the corrosion of the cable wires, the relative humidity of the air inside anchorages is kept at max. 60%, practically about 40%. The two towers, located in the sea, are founded on large caissons 80m in diameter, foundation depth about 70m below water level, which is near a record. Among longspan bridges (>1000 m), the south tower of the Ponte 25 de Abril in Lisbon has the world's deepest foundation depth below water level (80m).
The towers of the Akashi Bridge are made of steel, and their height above the piers is 283m, 297m measured from water level. These are the tallest bridge towers in the world. Due to the curvature of the earth surface, the distance between the towers is 93mm longer at top than at base. Among long-span suspension bridges (>1,000m), the majority of bridges have steel towers. Some bridges, mainly in Europe and China, have concrete towers, like Great Belt, Runyang (1,490m; to be completed in 2005), Humber, Jiangyin, Tsing Ma, H¶ga Kusten.
Main cables and hangers
The Akashi Bridge has two main cables, each 1,122mm in diameter, a world record. Each cable consists of 290 parallel-wire ropes, each containing 127 high-strength wires 5.23mm in diameter. Thus each cable contains 36,830 parallel wires, the total length of those being about 300,000km. The wires are of special high-strength steel, ultimate tensile strength 1,800MPa (180 kgf/mm2). Thus only two main cables are needed, while the previous grade 1,600MPa would have needed four main cables.
The 10 mm-diameter poly-aramid fiber pilot rope was carried across the strait by helicopter, a method used for the first time for a major suspension bridge. The wires of the cables are protected carefully against corrosion. Also the atmosphere environment inside the cables is improved by injecting dried air into the cables.
With the majority of long-span suspension bridges (>1,000m), the main cables are built using the conventional AS-method (Air Spinning), used since the late 1800s. In that technique, a few parallel continuous steel wires are hauled with several reels at a time between the anchor blocks. In the modern Prefabricated Parallel Wire Strands (PPWS) technique the cables consist of shop fabricated parallel-wire strands. This technique was applied in the Akashi Bridge and in some other long span suspension bridges in Japan (Minami, Kurushima, Kita), and also in China (Jiangyin, Xiling, Boca Tigris, and presumably also Runyang, but not Tsing Ma). Those countries have factories needed for this technique.
The hangers of the Akashi Bridge are vertical. This is the case with all long-span suspension bridges after the completion of the Humber Bridge in 1981. Only three major suspension bridges have inclined hangers: Humber, Bosporus, and Severn.
Deck and under clearance
The deck of the Akashi Bridge consists of strong steel truss girders, width 35.5m, depth 14.0m. A streamlined steel box girder was studied, but in this case its upper main span limit appeared to be 1,600m, which the Akashi Bridge exceeds by nearly 400m. It should be mentioned that the following long-span bridges (>1,000m) have steel box girder decks: Great Belt, Runyang, Humber, Jiangyin, Tsing Ma, H¶ga Kusten. Fatih Sultan Mehmet, Bosporus, Kurushima.
The effective width of the Akashi Bridge is 30,0m and the under clearance is 65 m, while the highest underclearance among major suspension bridges (>1,000m) is the 70m of the Ponte 25 de Abril in Lisbon. The deck of the Akashi Bridge accommodates 3 + 3 lanes for motorway traffic, design speed for vehicles 100 km/h, no lanes for light traffic. The maintenance traffic uses the wide steel grid platform at the lower level of stiffening girder trusses. On the author's visit, the party walked along that service route from the south anchorage to the south tower.
Depending on loads, the maximum displacements of the deck may vary within the following ranges: vertically 8m upwards and 5m downwards; horizontally 27m each direction. There are expansion joints at anchorages and hinges at towers.
Completion
A greenish-gray colour was selected for the bridge to harmonise with the sea and sky in the Strait. It was officially inaugurated on 5 April 1998, just a few weeks ahead of the second longest-span bridge, the Great Belt East Bridge (1,624m), which was inaugurated on 14 June 1998 in Denmark.
Severe earthquake at Kobe
The Akashi Bridge is designed for winds up to 80 m/s and also for severe earthquakes of 8.5 on the Richter scale. On 17 January 1995, with the bridge part-constructed, an earthquake of magnitude 7.2 occurred in the Kobe area and about 6,000 people were killed. The epicentre of the earthquake was located close to the south end of the Akashi Bridge, several kilometres away from Kobe City itself. The bridge experienced a severe earthquake resistance test, as the magnitude at the bridge site was nearly 8 on the Richter scale. When the earthquake occurred, the towers and the cables of the bridge were newly completed, and the construction of the deck was yet to be commenced. Buildings and bridges collapsed even at a distance of 50km from the Akashi Bridge.
It appeared that the Akashi Bridge survived with only minor damage. Because of the movement of the earth, the south tower moved 80cm southwards, and the tops of the towers tilted 10cm southwards. The main span increased 80cm and became nearly 1991 m. Also, the south side span increased 30cm. Accordingly, the total length of the suspension bridge increased 110cm. As a result the sag of the main cables decreased by 130 m. It is estimated that the damages would not have been this slight had the cables not been already installed. Despite the earthquake, the Akashi Bridge was completed according to the original schedule in Spring 1998.
Quantities and Costs
The quantity of concrete that went into the bridge construction was 1.44 million m3. Large quantities of steel were also involved for substructure (68,000 tonnes) and superstructure (193,300tonnes), making in total 261,300tonnes, or 2.2tonne/m2. The total cost of the bridge was JPâ 500 billion. Dividing this by the length of the suspension bridge (3,911m) and by its effective width (30.0m), the area cost is about JPâ 4.2million/m2.
The bridge is operated by the Honshu- Shikoku Bridge Authority (HSBA), having its Main Office now in Kobe (earlier in Tokyo).
The costs are amortised by toll payments, and the vehicles are divided into five categories. Between Tarumi (in the north) and Awaji (in the south) interchanges, to mention two of five categories, the cost for an ordinary passenger car is JPâ 2,600 and for a large special vehicle JPâ 7,650 one way. These payments apply the 5-year period 1 April 1998 until 31 March 2003. The toll payments are gathered in each direction, and so, for instance, a round trip between these two interchanges costs JPâ 5,200 for an ordinary passenger car (about $80 Australian). During 1998, the daily traffic volume was on average about 27,000 vehicles.
The Runyang Bridge - longest-span bridge of the new millennium
In China, a large bridge complex is under construction across the Yangzi River in Jiangsu Province, downstream of Nanjing. Due to the island of Siyezhou in the river, the crossing consists of two major bridges which will link Zhenjiang on the south bank of the river and Yangzhou on the north. The south bridge is a suspension bridge with a main span of 1,490m, while the north one is a cable-stayed bridge with a main span of 406m.
The bridge complex is known as the Runyang Bridge, and its total length is about 23 km. The new connection will form an important link in the Beijing-Shanghai Expressway.
Construction of the Runyang Bridge was commenced in October 2000, and is due for completion in October 2005. The top of the towers of the suspension bridge are 215 m above water level, and those of the cable-stayed bridge 150 m. Only the main span of the suspension bridge is suspended, its two approach spans are not. The main span of the suspension bridge consists of a streamlined orthotropic steel box girder, depth 3,0 m. The width of the deck of the main span is 34,3 m between hangers, accommodating 6 traffic lanes, and the overall width is 39,2 m, with a narrow walkway at each outside edge for maintenance access. The underclearance is about 50 m. With its main span of 1,490 m the suspension bridge will rank as third in the world, after only Japan's Akashi (1991 m) and Denmark's Great Belt East (1,624 m) bridges.
Some other long-span suspension bridges are planned in China, for instance Tsing Lung (span 1,418 m) and Lingdingyang (1,450 m), Jiaozhouwan (1,652 m or 1,800 m), and Qiongzhou (2,000 m to 2,500 m).
Epilogue
Long-span bridges have interested us always, (my wife and I), particularly great suspension bridges and also cable-stayed bridges such as Tatara. During October 1998 we visited the mighty Akashi-Kaikyo Bridge in Japan, and some other Honshu- Shikoku great bridges, too.
Hitherto, we have visited the following long-span cable-supported bridges (in order of main span): Akashi, Great Belt, Humber, Verrazano, Golden Gate, H¶ga Kusten, Fatih Sultan Mehmet, Bosporus, George Washington, Kurushima-3, Ponte 25 de Abril, Forth, Severn, Tatara (890m), Transbay (2 x 704m). Excluding the two Istanbul suspension bridges, we were permitted to enter the top of the towers of the bridges mentioned above. The three Kojima-Sakaide suspension bridges (Minami, Kita, Shimotsui) we crossed only by train at nightfall, so those are not counted.
During our visit to the Akashi-Kaikyo Bridge, we were kindly guided by the Kobe Office staff of the Honshu-Shikoku Bridge Authority-HSBA. We were permitted to enter the top of the south (Awaji side) tower by internal elevator at a height of 297m. The weather was misty and this had an effect on some of our photographs. At the end of the visit to the Akashi Bridge, we were kindly guided to the Tsukuhara Bridge, in Honshu, a few kilometres away. |
