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How the world's longest suspension bridge was built



Connecting the two cities of Kobe and Iwaya, the Akashi Kaikyō Bridge has allowed 23,000 cars to travel between the Japanese island of Awaji and the mainland of Iwaya every day since its completion in 1998. With a span of almost 4km, it holds the record for the world’s longest suspension bridge.

(Image via Thomas Schmitz / Unsplash)

The history

The bridge was actually planned as just part of a larger transportation link known as the Honshu-Shikoku bridge project, connecting the two cities by both highway and rail. Originally it was due to be a mixed railway-road bridge, but construction was ultimately restricted to road only.

Plans for a bridge to be built came after public calls for a safe means of transportation across the Akashi strait. This major shipping channel was originally crossed by several ferries every day, but the prevalence of regular, severe storms made it one of the world’s most dangerous waterways. In 1955 two ferries were overturned by rough weather, killing 168 passengers. The public demanded a solution and a massive bridge was decided upon. 

Plans to build the Akashi Kaikyō Bridge took a long time to come to fruition though, with construction finally started in 1988. Because of the length of the bridge and the need to withstand heavy storms, the bridge was a massive engineering challenge, taking ten years to be completed by over 100 contractors. 

(Image via Ecmadao / Unsplash)

The engineering

The bridge features three spans supported by two towers. The central span - the longest span in the world - measures 1,991m. Originally it was supposed to be an even 1,990, but during construction the towers themselves were moved 1m apart by the Great Hanshin earthquake in January 1995. 

Because of the prevalence of storms and earthquakes, the bridge had to be flexible to stay standing. In fact it actually expands and contracts several times per day, with a complex system of counterweights and pendulums keeping it stable. 20 tuned mass dampers (TMDs) swing against the wind sway in each tower, balancing out the bridge against the forces. 

350,000 tonnes of concrete were used in the anchorage of the bridge, and a system of dual-hinged steel-truss girders allow it to withstand winds of up to 286km/h (178mph), earthquakes up to magnitude 8.5 and tidal forces that regularly reach a water velocity of 7 knots (3.6m/s).

In order to anchor the cabling between spans, and to keep the bridge high enough for boats to pass underneath, the towers had to be built at 297m tall - making the bridge one of the world’s tallest as well as the world’s longest. 

The bridge is estimated to have cost around ¥500 billion (around US$3.6 billion). 2 million workers contributed to its construction, using 180,000 tonnes of steel and 1.4 million cubic metres of concrete. 

The finished structure

Construction was completed in 1998 and opened to traffic that April in a ceremony officiated by Crown Prince Naruhito. At the time it was the world’s longest bridge, and it remains the longest suspension bridge and contains the longest central span of any suspension bridge in the world. 

As well as fulfilling its function by providing transport for 23,000 cars per day, it is a popular tourist site with a circuit of observatory walkways accessible under the bridge’s platform, providing views of the bridges interior as well as the Akashi Strait. An exhibition centre on the Kobe side offers information about the history and construction of the bridge. 

At night it is lit in different colours by 1,737 bulls along its cables, towers, girders and anchorages. Twenty eight patterns are used at different times, for different occasions and have been said to give the bridge the appearance of a pearl necklace. For this reason the Akashi Kaikyō Bridge is often nicknamed "The Pearl Bridge". 

50 Years of Engineering

Fircroft is celebrating its 50th anniversary this year with a look back at a great engineering feat made each year from 1970-2020. Read last year’s look back at 1996 and the launch of the Toyota Prius - the first mass-produced hybrid car.

Find out how Fircroft’s 50 years of experience can help you find your next job on a major engineering project. Register your CV with us for free today. 

Recent Comments
An amazing structure and an engineering masterpiece to be proud off. Fircroft shares its 50th birthday with myself. Happy birthday Fircroft.
Paul Yates, 06 August 2020
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How the world's longest suspension bridge was built - Time to read 4 min
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