The Akashi Kaikyō Bridge, or the metre an earthquake added
The two towers were supposed to stand 1,990 metres apart. On January 17, 1995, a magnitude-7.3 earthquake struck the Akashi Strait with its epicenter sitting almost exactly between them, and when the shaking stopped, the gap had grown to 1,991 metres. The towers had not fallen. They had not cracked. They had simply moved, and Satoshi Kashima’s engineering team updated the blueprints.
The Akashi Strait had been pressing the case for a bridge for decades. In September 1955, the Shiun Maru — a ferry crossing the strait in a typhoon — capsized and killed 168 passengers, many of them schoolchildren. The strait is four kilometres wide, logs roughly a thousand ships a day, and sits directly above one of the most seismically active fault lines in Japan. Planning for a fixed crossing began in the 1960s. Ground was broken in April 1988. Kashima, leading the structural design for the Honshu–Shikoku Bridge Authority, had one charge: connect Kobe, on Honshu, to Iwaya on Awaji Island with a suspension bridge no one had yet built.
What his team built was, for the next twenty-four years, the longest single-span suspension bridge on earth. The main span stretches 1,991 metres — nearly two kilometres of cable and steel across open, current-churned water — held up by twin towers that rise 282.8 metres above the strait. Each of the two anchorage blocks consumed 350,000 tonnes of concrete. The cables are made from 36,830 wire strands totalling roughly 300,000 kilometres of wire: enough to loop the equator seven times. Total cost: ¥500 billion, a decade of labour, more than a hundred contractors.
Then the earthquake. The Great Hanshin quake killed 6,434 people in Kobe and left a third of the city in rubble. Its epicenter fell in the strait between the two half-finished towers. The south tower had shifted 120 centimetres from its original position. Because the deck sections had not yet been hung, the engineers could absorb the revision into their calculations — the final main span, 1,991 metres, was simply accepted as the new design. The earthquake that devastated the city that the bridge was meant to serve had also, inadvertently, proved that the towers could take a direct hit from a magnitude-7.3 event and remain plumb.
The Akashi Kaikyō opened on April 5, 1998, and immediately became the standard that long-span bridge engineers argued from rather than toward. Tuned mass dampers inside the towers — pendulum counterweights that cancel resonance — entered standard practice for structures of this scale. The aerodynamic truss design of the stiffening girder addressed, directly, the lesson of Galloping Gertie in 1940: wind does not behave the way intuition suggests. Every long-span suspension project designed after 1998 inherited something from Kashima’s calculations.
The bridge held its record until March 18, 2022, when Turkey’s 1915 Çanakkale Bridge edged past it. What it kept was harder to beat: the demonstration that you can build the largest things in the places most determined to tear them down.
Sources
- Akashi Kaikyo Bridge — Wikipedia — opening date, span and tower specs, cable figures, the Shiun Maru disaster, the 1995 earthquake’s 1-metre span increase, cost.
- Akashi Bridge — We Build Value — 120 cm south-tower shift, earthquake death toll, construction materials, record superseded by Çanakkale in March 2022.