The Longest Suspension Bridge

 

Akashi Kaikyo is the longest suspension bridge in the world, and it is probably Japan’s greatest engineering feat now. It took two million workers and ten years to construct the bridge with 181,000 tonnes of steel and 1.4 million cubic metres of concrete. It has six lanes and links the island of Awaji and the mainland city of Kobe, four miles.

Before the Akashi-Kaikyo Bridge was built, ferries carried passengers across the Akashi Strait in Japan.
This dangerous waterway often experiences severe storms, and in 1955, two ferries sank in the strait during storm, killing 168 children. The ensuring shock and public outrage convinced the Japanese government to draw up plans for a suspension bridge to cross the strait. The original plan was for a mixed railway-road bridge but when the bridge was begun in April 1986, it was restricted to road only. Actual construction did not begin until May 1988, and the bridge was opened for traffic on April 5, 1998.

 

Location

Opened on April 5, 1998, it became  the longest bridge of its kind in the world, surpassing the Humber Bridge in the UK that has a central span of 1,410 metres. The bridge was designed by engineer Satoshi Kashima and built by Matsuo Bridge Co., who is also responsible for building some of the most famous bridges to date. The Kobe earthquake on January 17, 1995 had its epicentre just between the two towers of the Akashi Strait Bridge. The initial length was 1.990 meters planned for the main span, but the quake moved the towers 1-metre separating, when they had not started construction of the deck yet. The change in length was absorbed into the final design which slightly altered in some metres. . The bridge towers have mass dampers in order to reduce vibrations in the structure during earthquakes and typhoons.

 

 

The bridge connects Kobe, on the island of Honshu, with Iwaya on Awaji Island, Japan, crossing the busy Akashi Strait, which records an intense traffic maritime - over 1,400 ships per day, with an important port of supply and release of goods. At the time of design, engineers had to think of a bridge which would not block sea traffic. They also considered the weather as Japan experiences some of the worst weather on the planet. Winds cross the Strait, the rains are intense, and hurricanes, tsunamis or earthquakes strike the island almost annually.

The Akashi Strait, connecting the bay Osaka and Harimanada, is about 4 miles wide. The segment crossed by the bridge has a maximum depth of 110 metres and a maximum current speed of 4.5 metres per second. The Strait has been a productive fishing area since ancient times, and is an important waterway, used by more than 1,400 boats a day. To ensure the safety of maritime traffic has been established by law, an international waterway 1,500 metres wide for safety of maritime traffic.

 

 

Geologically the floor of the strait is composed of diluvial sediments and granite. Akashi side strata consist of 40% of gravel of between 10-20 cm in diameter and Kobe consists of a layer made of hard unconsolidated muddy and sandy silt or sandstone.

Structure

The bridge was built under severe conditions, such as extremely strong currents Strait and tidal depth, using the latest technologies developed for the construction of bridges. The Japanese engineers familiar with the terrible weather conditions placed the bridge deck on a support frame formed by a complex network of triangular support below the road. The open network of triangles gave rigidity to the bridge and allowed the wind to pass through the structure.

Foundation

Two main springs as an extension for laying the foundations of the towers were built with round and flat, the largest with a diameter of 80 m and the other 78 m. Each anchor required an average of 350,000 tons of concrete.

Also, large deep foundation anchors were built on reclaimed land with many new technologies. All foundations were well designed to cope with the strong earthquakes with a seismic design method of new research, together with a new kind of concrete result of a mixture of different water-resistant cements and erosion. The foundations have withstood the earthquake of January 17, 1995 with almost no incidents, just with the displacement of 1m of the towers; and the movement can be considered minimal.

 

 

Dampers

In the two main towers 20 mass dampers, TMDs, were placed, pivoting in the direction opposite to the wind when it blows on one side of the bridge, dampers sway in the opposite direction, effectively balancing the bridge and placed negating the influence of wind. In the bridge design system strengthening beams with two hinges allow the structure to withstand winds of 290 km / hour, earthquakes with a magnitude of up to 8.5 on the Richter scale and strong currents was also applied. The bridge also contains pendulums, which are designed to operate at the resonant frequency of the bridge to dampen forces.

Towers

The two main supporting towers rise 282.8 metres above sea level, 297.30 m to the end of the anchor cable. Steel cables with a diameter of 112 cm contain 36,830 lines of wire. The bridge is supported by two cables basically the central section, considered the toughest in the world constructed. The primary tower is made of steel, and the shaft has a cruciform cross section, which is insensitive to wind-induced oscillation. However, tuned mass damper is installed within the shafts to suppress oscillation is anticipated that during erection of the tower, and even at the stage of completing the bridge. A shaft of the tower is divided into 30 levels and almost all levels are composed of 3 blocks. Each block was made in factories and transported to the site and then hoisted by crane climbing tower that had a lifting capacity of 160 tons. Articulating high tension bolts were used to connect field.

 

It took two million workers and ten years 
to construct the bridge with 181,000 tonnes of steel and 1.4 million cubic metres of concrete

 

 

Circulation

The 4 km of the pathway for vehicles that crosses the Akashi Strait is divided into 6 lanes, is suspended and supported by the cables cover is held by its own weight, resulting in thousands of steel beams positioned to form a triangular grid. It took 15 months to put the 280 beam sections.

As added reinforcement, below deck, a vertical stabilizer with a form like the fin of a plane that runs through the centre of the bridge and balances the pressure, both below and above deck. A steel mesh was placed in the centre and side rails to allow passage of the wind.

The main material used in the bridge superstructure is steel and reinforced concrete. For the foundation, underwater towers developed a new type of mixing, called a “concrete submarine that is not broken” (non-disintegration concrete).

Lighting: The Akashi Kaikyo Bridge-has a total of 1,737 light fixtures: 1,084 for the main cables, 116 for the main towers, beams and 405 to 132 for the anchors. In the main cables are placed three high capacity lighting tubes, red, green and blue. The RGB model and computer technology contribute in making a wide variety of combinations. A minimum of 28 patterns are used for special occasions such as regional public holidays or commemorations.

Cables: The length of the cables used in the bridge amounts to 300,000 kilometres, enough to circle the earth 7.5 times. Steel cables with a diameter of 112 cm contain 36,830 lines of wire.

Beams: In beams, 90,000 tons of reinforcement steel were used. Due to the large size of the bridge, the wind load which it must face is higher than that of any other existing bridge. Using steel with high tensile strength for the beams was made which are very strong yet lightweight, and thus cheaper.

The reinforcing elements were prefabricated shaped panel which were transported to the construction site where they were erected into the interior of the anchorages and towers with floating cranes.

 

 

Form

The Akashi-Kaikyo Bridge expresses its structure in a pure and clear form. Every element of the bridge was showed clearly such as the supporting tower, the deck, the stiffening truss, etc. From its appearance, you can easily identify the function of each part of the bridge. Although it had the world longest suspending span, the truss structure would still give people a feeling of stability. It has an impression that the bridge can withstand every kind of load caused by typhoon, earthquake or heavy traffic and suspension bridge was the best suitable bridge type for this wide strait by having only two main supporting towers without disturbs the marine traffic.

The Akashi-Kaikyo Bridge had a very good order. The amount of lines and edges were just enough. And all straight line appeared on the bridge are very clear, showing a very good order in the bridge. The Akashi-Kaikyo Bridge shows a very good combination between light and shadow that it expresses essence of beauty of the Seto-Inland Sea, harmony and relentless change of the light and the shadow. The Akashi-Kaikyo Bridge looks beautiful across a wide span of water as a suspension bridge, its simple combination of tower and cable gives people a feeling of claim, as claim as the gentle sea water. The designer chose the colour of green-grey for the bridge. It was because the colour matched the design themes and evoked forest-rich Japan. In addition, the bridge illumination was arranged, in which the colour of cable illumination can be changed monthly or seasonally following pre-determined plan.

 

 

The Akashi-Kaikyo Bridge was designed on a two hinged stiffening girder system, which allows the bridge to withstand 286 kilometres per hour (178 mph) winds, earthquakes measuring up to 8.5 on the Richter Scale, and harsh sea currents. The bridge also contains pendulums that operate at the resonant frequency of the bridge to dampen forces on it. The two main supporting towers are 298 metres above sea level.

Since the nearest seismic fault was just 90 miles away from the bridge site. So, the engineers had to take some necessary precautions, designing the bridge to withstand an earthquake that would measure 8.5 on the Richter scale.

References

  1. https://www.roadsbridges.com/akashi-kaikyo-bridge
  2. https://en.wikiarquitectura.com/building/akashi-kaikyo-bridge/

Sudeshna Mukherjee  
Assistant Editor
Civil Engineering and Construction Review