The Japanese Akashi Kaikyo Bridge is the highest, longest and most expensive suspension bridge in the world. It stands against all odds in one of the most difficult places for its construction, because it is located in the path of typhoons, at the mercy of winds that reach the incredible speed of 290 km/h, a power capable of tearing off the roofs of houses and uprooting trees. In addition, it crosses one of the busiest and therefore most dangerous trade routes in the world, due to its naval transit, with the added bonus of being located in the middle of a major earthquake zone.
The Akashi Kaikyo Bridge is a massive six-lane highway connecting the dynamic metropolis of Kobe on the main island to Awaji Island to the south. For the inhabitants of the fishing villages there, it constitutes a vital link with the schools and hospitals of the main island city. The bridge represents a symbol of national pride for Japan and is the final link in a network of bridges that connect the four Japanese islands, providing fast and efficient transportation, opening access to trade, business and tourism throughout the area.
This bridge holds three world records: at 280 m high, it is the tallest suspension bridge in the world, each of its two towers measuring as long as an 80-story building. With a central arch of more than 1.6 km, it is the longest suspension bridge on the planet and almost doubles the length of the Golden Gate Bridge in San Francisco "San Francisco (California)"). And if this were not enough, it is also the most expensive bridge that has been built in history with a cost of more than three billion euros.
The Akashi Strait is a four-kilometer barrier of hostile sea that separates the island of Awaji from the rest of Japan. It is more than a hundred meters deep with a current close to 14 km/h on calm days. The area is frequently hit by typhoons and gusty winds that reach speeds of 290 km/h and destroy almost everything in their path. The strait is also one of Japan's busiest trade routes and the main artery connecting the four Japanese islands. Every day more than 1,000 ships pass through these densely trafficked waters, and in spring the dangers increase, a thick fog takes over the channel and causes hundreds of ships to sink every year.
The national and local governments studied for years the construction of a bridge over the Akashi Straits barrier, to connect one of the country's largest cities with the island of Awaji and its fishing villages. But a fatal disaster had to occur for the project to materialize, a collision between two Ferries transporting children to school, produced a serious tragedy with many deaths. The accident caused the government to rethink the need to carry out the bridge project.
To face such a challenge, the Japanese government created the Honshū Shikoku Bridge Authority, its mission was to build the impossible, it took 30 years of research into new technologies before starting to build the bridge. In May 1988, work began and the builders faced the most daring project of their careers. They had 10 years of unknown challenges, setbacks and natural disasters ahead of them. The construction of the largest suspension bridge in the world represented a monumental task, requiring billions of euros, 181,000 tons of steel and 1.4 million m of concrete. Its foundations are the size of a 20-story building, its towers are almost as tall as the Eiffel Tower in Paris, and its cables could go around the world seven times.
The Akashi Bridge was to be almost half a kilometer longer than any suspension bridge that had been built until then. In theory, the design of suspension bridges is very simple, two main cables held by two towers extend over the water, the road hangs from those cables that are anchored to both sides of it, it is a formula tested to the point and works excellently. But the length of suspension bridges has a limit, to prevent the cables and the road from collapsing, they have to be much stronger and as light as possible. The longer a bridge is, the more it weighs; A suspension bridge is designed primarily to support its own weight and the excess strength will be used to support the traffic load. The load supported by the Akashi Bridge comes from 91% of its own weight and only the remaining 9% corresponds to vehicle traffic.
In May 1988, the first problem that Japanese engineers faced was where to place the enormous foundations where the bridge would rest, since the treacherous Akashi Straits caused them more than one headache. The ideal location for its construction was in the middle of a busy shipping channel, and the foundations would be a major obstacle for the countless vessels that crossed it every day. The canal is almost 1.5 kilometers wide, and to avoid it safely they had to separate almost two kilometers, making Akashi the longest suspension bridge in the world, but an even bigger problem had to be solved. Normally the foundations of bridges are placed in the middle of the water, cylindrical sections are filled with concrete and they sink under their own weight, the process is repeated and the foundations are raised from the banks in different phases, but the Akashi Straits are 110 m deep and are much deeper than most foundations where bridges are built, what's more, the rapid currents prevent normal construction techniques from being used because the water carries everything away, so the The bridge's designers came up with a novel, risky and unproven solution on that scale. They proposed manufacturing two enormous steel molds in dry docks, one for each of the bridge foundations. Once manufactured, they were towed out to sea and sunk precisely at the exact point. Until then, no one had attempted to do anything like it on a similar scale.
In March 1989, the gigantic steel molds for the bridge's foundations were finished, their hollow rings of two layers of steel measuring 70 m high and 80 m wide. At 5:30 p.m. on March 26, 12 tugboats left the dock dragging the first of the two large hollow structures out to sea. It was not an easy task because each mold weighed 15,000 tons, the equivalent of 40 Jumbo airplanes. Under the supervision of the coast guard, the barges towed the immense floating skyscrapers through the busy shipping route and over turbulent waters. It took 38 hours to move each of the two molds to their place, subsequently 32 water pumps were used to fill each of the gigantic molds with water, individually filling 250 million liters of water, taking more than 8 hours to complete this process to ensure that the foundations settled on the seabed correctly.
To complete the gigantic foundations, they had to fill them with concrete but there was a problem, the foundations were full of water and if ordinary concrete is poured it dissolves like aspirin. To solve that problem, engineers had to do something that had never been done before: create a super-concrete that hardened with water. The developed concrete was inserted to replace the seawater present in the foundations; When this operation began, it was filled with more than 265 m of concrete.
In the bridge's estimated 200-year life, it will face large earthquakes regularly, and the builders knew that concrete foundations could crack and collapse during an earthquake because they are not flexible enough. The engineers' plan was to manufacture a steel resistant to tremors up to a score of 8.5 on the Richter scale, causing the budget to skyrocket to 3 billion euros.
Each 283m bridge tower was made up of five 170-tonne sections each fitted on top of each other by more than 700,000 screws. Each section had to be perfectly flat, any irregularity would be magnified as the tower gained height, if the towers deviated by just a couple of centimeters when reaching their maximum height, the bridge could collapse. For all these reasons, its construction and assembly required absolute and detailed precision, taking 18 months to complete the entire construction process of the towers.
In November 1993, engineers began the most critical phase of the project: the construction of the gigantic main cable, more than a meter wide, from which almost the entire weight of the bridge would be suspended, a total of 160,000 tons, three times the weight of the Titanic. It took 300,000 km of cables, enough to circle the earth seven times, and each of the two main cables was made of 37,000 strands of wire. The weight of such large cables is one of the elements that limit the length of suspension bridges, the longer they are, the more they weigh and in the end the bridge sinks under its own weight.
To cover the 2 km central arc between the two towers, engineers had to develop a steel cable twice as strong as a conventional one, making it possible to use a single cable on each side instead of two. This super strong cable is only manufactured in Japan, its creators changed the composition of the steel by adding silicon alloys, achieving a cable that broke all world resistance records, in such a way that a 5 mm cable could be capable of supporting the weight of three family cars. In this way they used 37,000 cables to hold the bridge.
The manufacture of the main cables was also an unprecedented milestone, it had never been done on such a large scale, for this it was necessary to join 127 5 mm wires, which in turn were made up of 290 strands to create the main cables composed of a total of 37,000 cables. The final cable was more than 4 km long, but the construction of the cable was not the greatest challenge, this was presented below. Engineers had to lay the huge cable over the crowded shipping channel and cover a length more than 4 km wide. To do this, they first had to lay a guide rope over the Akashi Strait, only then could they take the cable to the other side, but the bridge builders could not close such a busy maritime artery, and they were forced to follow a much more dangerous route by air. To do this, they used a helicopter with an ultra-strong Kevlar rope and guiding it over the top of the towers would be like threading a needle with a helicopter. For this reason, they had to look for a pilot with extensive experience and qualifications.
In December 1994, after six and a half years of adventures, the largest suspension bridge in the world was half-finished in the middle of the Akashi Strait. The engineers' next step was to build the four-kilometer-long, six-lane highway that would cross the strait; It was undoubtedly the most complex and critical part of the project, and the most exposed to the unpredictable forces of nature. The road deck is literally suspended by cables and held in place by its own weight. If the design were to fail, gales could blow the platform away like a toy, causing a true cataclysm. To overcome the forces of the wind, the engineers came up with an incredible idea, building the roof with thousands of steel beams, placed in the shape of a triangular grid, which is one of the most resistant designs in engineering. To increase its strength, they added a vertical stabilizer that runs through the center of the bridge. It has a shape similar to the fin of an airplane and hangs under the deck. When the wind blows, the stabilizer balances the pressure above and below the road and reduces vibrations. They also installed a steel mesh in the center of the road and along the sides, allowing the wind to pass through it, thus stopping the pressure from building underneath.
In January 1995, the final phase of the bridge construction began, that is, the construction of the road. The structure continued to be very vulnerable until the highway was finished, the bridge designers call this phase “temporary condition”, because it is the most dangerous moment for a bridge, especially in a country prone to earthquakes like Japan, which is why the engineers worked very hard to finish the bridge as soon as possible. However, on January 17, 1995 at , an earthquake shook the city of Kobe. It was the largest earthquake recorded in Japan since 1923 and scored a catastrophic 7.2 on the Richter scale, destroying practically the entire city. In a matter of minutes, 100,000 buildings collapsed and 40,000 people were injured, the death toll rose to more than four thousand people, and highways, railways, bridges, etc. were fractured. The epicenter of the earthquake was 20 km from the city of Kobe and just 4 km from the Akashi Bridge, with the road unfinished, the structure was markedly vulnerable. Fortunately, the engineers breathed a sigh of relief when they found that the bridge was still in one piece. Initial inspections did not reveal any damage. However, later days, conducting a more detailed examination, they found that a fault had opened up in the seabed right in the middle of the two towers of the bridge. This produced an alarming fact: the anchorage and the tower on the coast of Awaji Island had moved more than a meter to one side and what was more worrying, the earthquake had stretched the length of the bridge by more than a meter, becoming a hard rock. blow for the designers, since this setback could have caused a significant delay in construction. But the engineers were very lucky, ironically and despite their fears, the bridge was still standing because it was not yet finished, since if it had had the road installed it would have suffered more serious damage. The towers had survived thanks to their flexible steel and also their special earthquake-proof design, inside each of the gigantic steel towers are 20 huge structures that absorb impacts and help the towers stand firm in the face of strong winds and earthquakes. These are giant pendulums that can swing in any direction. If an earthquake pushes the bridge to one side, the pendulums move to the opposite side. It is the only bridge in the world that has survived such a large vertical impact during its construction.
A month after the earthquake, the engineers resumed the work again, but to do so, they first had to resolve an urgent issue, modifying the design, lengthening the length of the beams and the distribution of the suspension cables. Although it may seem incredible, the work was finally only delayed one month more than the initial prediction. In June 1995, the completion of the bridge began with the assembly of the road, taking more than 15 months to place the 280 sections of girders piece by piece on the narrow bridge. On September 18, 1996, the last section was snapped into place.
On April 5, 1998, the bridge was officially inaugurated, becoming a milestone of civil engineering, reducing travel time from 40 minutes by ferry to 5 minutes by car. Currently more than 23,000 cars circulate on it daily, but although the bridge is designed to last 200 years, its maintenance takes place 24 hours a day, 7 days a week. All aspects of its operation are monitored from the bridge control center. The suspension system from which the entire bridge hangs has its own air conditioning system to prevent the cables from corroding and there are wind measurement sensors that record alterations in the bridge deck. Since its inauguration, the bridge has only been closed three times due to bad weather.