Suspended steel links the world
1991 metres of road hovers between Kobe and Awaji, in Japan. Akashi Kaikyo, the world’s longest suspension bridge, symbolises the victory of engineers over nature. But shipwrecks and collapse have also played their part.
On 5 December 1785, a ferry boat is making the short hop from mainland Wales to the island of Anglesey, when the powerful current makes it run aground. The ferry passengers all abandon the boat, climbing out onto a sand bank. But the tide is rising, and will soon cover the sand bank on which they shelter.
As darkness falls, other boats can hear the 55 shipwrecked people cry for help, but the tide rises too quickly and the current runs too violently to allow a rescue mission. Only one person survives.
This disastrous shipwreck in 1785 was just one of hundreds in the treacherous Menai Strait. Yet ever more people needed to cross the waters; Anglesey was becoming a key path to and from Ireland, which joined the British Union in 1800.
Something had to be done. So in 1815 the decision was made to build a bridge, but
the distance was too long for a traditional stone bridge to span. Innovative thinking was required, and the result was the creation of the modern suspension bridge.
Today, Japan’s Akashi Kaikyo suspension bridge is the longest suspension bridge in the world, its central span of 1991 metres more than half as long again as the Golden Gate bridge in San Francisco.
Between Menai and Akashi Kaikyo there were many bitter lessons to be learned by engineers in their struggles with the powers of nature, including a famous US bridge collapse in 1940. Yet the concepts behind the suspension bridge were far from new.
Hang it all
The suspension bridge dates back far before Menai. Tibetans built them in the Himalaya as early as the 15th century, and Incas in Peru also built them in the Andes. These bridges were made of wood and rope, attached to rocks or trees and suspended across water. Many modern examples are still in use within such mountainous regions.
But rope sets a limit on the load these bridges could bear, and on the length of the gaps across which they could be built. If they were to carry carriages and span big rivers, they had to be made of sterner stuff.
Stone is strong, and an arched stone bridge can carry lots of weight. But the bridge itself is extremely heavy, and difficult to construct so that ships can pass beneath it. A suspension bridge design is more suitable for spanning long distances.
The bridge at Menai solved the problem, and after its completion in 1826, it became the template for all the major suspension bridges we know today. Civil engineer Thomas Telford was the man with the plan, hired in 1815 to bridge the strait between Wales and the island of Anglesey. It had to be strong, and it had to allow sailing ships to pass underneath, so its deck had to hover high above the water.
To obtain the correct height, Telford built two offshore towers and suspended powerful cables between them. The cables were subsequently equipped with vertical cables of different lengths, ensuring that the deck remained horizontal whatever the distance to the load-carrying cables.
The design today looks entirely familiar, because it is used in famous suspension bridges from the Golden Gate in California to the Great Belt Fixed Link in Denmark. But before the Menai bridge, horizontal suspension bridge decks were difficult to construct, because the deck was typically supported directly by the load-carrying cables. For Menai, Telford found a more robust design, with shorter suspender cables transferring the deck’s weight to the main supporting cables, and the main cables’ tension then converted to compression force in the piers.
However, Telford’s design did not allow sufficiently for the worst enemy of suspension bridges: the wind. More than 100 years after the completion of the Menai bridge, high wind caused history’s perhaps most famous bridge collapse: Tacoma Narrows.
Wind tunnels prevent accidents
The construction of a modern suspension bridge begins under water, where the bridge pier foundations are to be made. The river bed or ocean floor is often soft and sandy, so that engineers can’t simply lay down concrete in situ. Instead they have developed hollow boxes called caissons, which are lowered to the bottom and then emptied of water to provide a dry working area.
When one of the world’s most famous suspension bridges, Brooklyn Bridge in New York, was built in the 1870s, it required the use of wooden caissons that were 50 metres long, 30 metres wide, and 4 metres high.
The caissons were weighed down on the river bed, the water was pumped out, and workers entered to dig down to a harder surface, before the moulds were filled with concrete. But once the foundations are in place, the biggest challenge is the wind.
The ’new’ Tacoma Narrows bridge felt the consequences of this on 7 November 1940. Winds of 19 metres per second made the bridge deck wobble as if it were made of rubber, until it finally split in two, crashing into the river 59 metres below.
But why? The winds were within the bridge’s design parameters, and the Tacoma Narrows collapse mystified experts for 50 years. Finally in 1990, an engineer called Robert Scanlan discovered that eddies were the decisive factor. When the wind encounters an object, it can either pass around it or become deflected, causing turbulent eddies. Scanlan proved that Tacoma Narrows collapsed through a phenomenon he named aeroelastic flutter. Eddies set the bridge in motion, the motion caused more powerful eddies, self-perpetuating until collapse.
Back in the 1940s after the collapse, the Americans didn’t know about aeroelastic flutter. But they still wanted to build a new bridge, and they used a new method to avoid repeating the collapse: wind tunnels.
Engineers built a 30-metre-long model of the new bridge, which was to be 1800 metres long, and placed it in a tunnel where fans could mimic the powers of nature. The wind tunnel tests were passed, and cars are still passing over the redesigned Tacoma Narrows bridge that was completed in 1950.
Wind tunnel tests are still used today, but they are only one of the methods that stabilise huge modern structures against everything from earthquakes to typhoons.
Bridge pushes the boundaries
A distance of 3.9 kilometres separates the Japanese mainland from the island of Awaji. Yet in 1998 the Akashi Kaikyo fixed link was completed. The bridge has the longest central span of any suspension bridge: 1991 m.
The bridge is constructed according to the same principles that were established by the Menai bridge, but several innovative new features have been added.
The bridge’s cables are made of a steel alloy that includes silicon, giving individual cables 10% more tensile strength than that of ordinary steel. Some 290 small cables are ’braided’ into the bridge’s two load-carrying cables, each with a diameter of 112cm. The Akashi bridge contains a total of almost 300,000km of wire, and the cables were lifted into place using a helicopter. The cables are supported by two bridge towers which rise 282 metres above the Akashi Strait, at which altitude wind speeds can reach 300km/h.
The engineers behind Akashi Kaikyo had learned from history, and tested a model of the future bridge in one of the world’s biggest wind tunnels. The wind caused turbulent eddies around the model bridge, so engineers installed a plate alongside the deck of the real bridge, designed to interrupt the motions of the wind around it. The Akashi Kaikyo bridge has proven to resist even typhoons without any destructive motion.
Norwegian super bridge
The Japanese bridge has held its record of longest span for more than two decades; 2km seemed to be the limit with current methods. But engineers have now developed a new method that should be able to stretch the bridges to more than double this length.
How long exactly? Engineers from the UK’s University of Warwick have calculated that a suspension bridge can theoretically have a maximum span of 5000 metres before the weight of the load-carrying cables become too much for the materials we have at our disposal today.
In the early 2000s, Chinese engineer T.Y. Lin proposed a suspension bridge across the Strait of Gibraltar, joining Europe to Africa. This would require a 5000-metre span. That remains only a proposal, so that Norwegian engineers hope to beat Lin to a new record with their plan for a suspension bridge across the Sognefjord, the largest and the deepest fjord in Norway. Their plan includes a span of 3700 metres – almost twice as long as the existing record. The engineers’ major challenge is that the Sognefjord is 1300 metres deep at its deepest point, making it difficult to construct bridge towers.
So the engineers propose the use of the material graphene – carbon atom structures which are a mere atom thick. Graphene can improve the tensile strength of concrete significantly, allowing a longer central span than would otherwise be possible.
If that idea for a new-material classic suspension bridge doesn’t get off the ground, the Norwegian engineers can offer another option: a suspension bridge built under the water. Instead of an open deck, the bridge would suspend a concrete tunnel through the water, affixed to floating pontoons.
Since the Menai bridge was completed, the possible length of the central span of suspension bridges has been increased more than tenfold, and clearly engineers are not yet done with new ways of stretching the bridges further. Perhaps the question now is whether the classic suspension bridge will continue to dominate. Will a clever engineer invent a brand-new type of bridge – or will bridges instead head under water?