Megastructures - The Akashi - Kaikyo Suspension Bridge

The Akashi Kaikyo Suspension Bridge is the longest suspension bridge in the world and it is probably Japan’s greatest engineering feat.

It took two million workers ten years to construct the bridge, 181 000 tonnes of steel and 1.4million cubic metres of concrete. The steel cable used would circle the world seven times.

It has six lanes and links the island of Awaji and the mainland city of Kobe, a distance of four miles. The concept of building a bridge across the Akashi Straits became urgent after a disaster in 1955. A ferry carrying over one hundred children sank after colliding with another ferry, in the busy shipping lane. One hundred and sixty eight children and adults died in the disaster. Political pressure for a bridge increased and in 1988 construction began.

The Akashi Straits is four miles wide at the bridge site with sea depths of one hundred metres and currents averaging fourteen kmph. The Akashi Straits is one of the busiest sea lanes in the world with over a thousand ships per day travelling through it. Furthermore, the bridge is in a typhoon region in which winds can reach speeds of 290 kmph.

The construction of a suspension bridge involves the use of two main cables stretching between two towers. The roadway beneath these is suspended by more cables. To stop the towers, roadway and cables collapsing, they are held at either end by large anchor blocks (the Akashi anchor blocks weigh 350 000 tonnes). In the case of the Akashi-Kaikyo Bridge, suspension bridge technology was pushed to the limit.

The Japanese designers and engineers tested their designs by building complex models. These were tested in wind tunnels which helped them refine the design so that the bridge could cope with severe weather and typhoon conditions. The photograph opposite shows 40 metre long model, set up for a variety of scientific tests.

After vigorous testing had taken place, construction of the real bridge could begin.

The two towers stand on two large circular foundations. The moulds for the two foundations were built in dry dock weighing 15 000 tonnes and 60 metres in height. In March 1989 a major stage of construction was reached with the moulds for the foundations to the towers being towed out to their positions in the sea by numerous tugs. When in position the moulds were flooded with two hundred and fifty million litres of water, taking eight hours to complete. By the time the moulds were full, they were resting on the sea bed.

Each of the two foundations were filled with 265 000 cubic metres of concrete. However, ordinary concrete does not mix with water and so the Japanese had to develop special concrete which was capable of mixing with sea water.

In 1989 work on the two towers began. Each is nearly as high as the Eiffel Tower and is designed to have a two hundred year lifespan. The towers are 283 metres in height and if the foundations are included, this adds a further 60 metres.
Each tower is made up of 90 sections and they were built with absolute precision as the design allowed only a 25mm offset at the top. In order to achieve this level of accuracy each of the blocks were ‘surface ground’ to a precise finish. 700 000 bolts were used to fix each of the towers together.
Each tower is designed to flex / move in storm force conditions. They and even have a special mechanism that counteracts and dampens movement.

When the towers were completed a temporary cable was stretched between both and a wire mesh gangway built so that workers could start construction of the main cables. This temporary gangway can be seen in the photograph to the right. Workers and machinery pulled the main cables from one tower to the other.

Once the main cables and the vertical cables were in position the deck / roadway was fixed hanging below them. This work took place in 1994. Large purpose built cranes were used to lift the sections, 4000 tonnes each, were bolted into position, one after another. 290 sections make up the entire bridge.
The photographs to the right show the cranes in operation and the deck as it was fixed in position, section by section.
Each section has a triangulated form. This means that weight is kept to a minimum and yet each section has maximum strength.

The final section of the deck was bolted in position in September 1998 and the bridge was opened to the public on the 5th of April of the same year.