The Economist — June 15, 2013
WHEN Elisha Otis stood on a platform at the 1854 World Fair in New York and ordered an axeman to cut the rope used to hoist him aloft, he changed cityscapes for ever. To the amazement of the crowd his new safety lift dropped only a few inches before being held by an automatic braking system. This gave people the confidence to use what Americans insist on calling elevators. That confidence allowed buildings to rise higher and higher.
They could soon go higher still, as a result of another breakthrough in lift technology. This week Kone, a Finnish liftmaker, announced that after a decade of development at its laboratory in Lohja, which sits above a 333-metre-deep mineshaft which the firm uses as a test bed, it has devised a system that should be able to raise an elevator a kilometre (3,300 feet) or more. This is twice as far as the things can go at present. Since the effectiveness of lifts is one of the main constraints on the height of buildings, Kone’s technology—which replaces the steel cables from which lift cars are currently suspended with ones made of carbon fibres—could result in buildings truly worthy of the name “skyscraper”.
The problem with steel cables (or “ropes” as they are known in the trade) is that they are heavy. Any given bit of rope has to pull up not only the car and the flexible travelling cables that take electricity and communications to it, but also all the rope beneath it. The job is made easier by counterweights. But even so in a lift 500 metres tall (the maximum effective height at the moment) steel ropes account for up to three-quarters of the moving mass of the machine. Shifting this mass takes energy, so taller lifts are more expensive to run. And adding to the mass, by making the ropes longer, would soon come uncomfortably close to the point where the steel would snap under the load. Kone says it is able to reduce the weight of lift ropes by around 90% with its carbon-fibre replacement, dubbed UltraRope.
Carbon fibres are both stronger and lighter than steel. In particular, they have great tensile strength, meaning they are hard to break when their ends are pulled. That strength comes from the chemical bonds between carbon atoms: the same sort that give strength to diamonds. Kone embeds tubes made of carbon fibres in epoxy, and covers the result in a tough coating to resist wear and tear.
According to Johannes de Jong, Kone’s head of technology for large projects, the steel ropes in a 400-metre-high lift weigh about 18,650kg. An UltraRope for such a lift would weigh 1,170kg. Altogether, the lift using the UltraRope would weigh 45% less than the one with the steel rope.
Besides reducing power consumption, lighter ropes make braking a car easier should something go wrong. Carbon-fibre ropes should also, according to Mr de Jong, cut maintenance bills, because they will last twice as long as steel ones. Moreover, carbon fibre resonates at a different frequency to other building materials, which means it sways less as skyscrapers move in high winds—which is what tall buildings are designed to do. At the moment a high wind can cause a building’s lifts to be shut down. Carbon-fibre ropes would mean that happened less often.
All of which is worthy and important. But what really excites architects and developers is the fact that carbon-fibre ropes will let buildings rise higher—a lot higher.
Lighter, stronger ropes mean the main limiting factor in constructing higher skyscrapers would become the cost, says Antony Wood, an architect at the Illinois Institute of Technology, in Chicago. Dr Wood is also executive director of the Council on Tall Buildings and Urban Habitat, which, among other things, lists the official heights of skyscrapers. At present the tallest is the Burj Khalifa in Dubai, which was completed in 2010 and, at 828 metres, shot past the previous record-holder, the 508-metre Taipei 101 tower. The Mecca Royal Clock Tower in Saudi Arabia, completed in 2012, is now, at 601 metres, the second-tallest. The Freedom Tower in lower Manhattan, built near the site of the World Trade Centre’s twin towers (417 metres and 415 metres) that were destroyed by al-Qaeda in 2001, had its spire added in May to reach 541 metres. But work has now started on the Kingdom Tower in Jeddah, Saudi Arabia. Its exact proposed height is still a secret, but it will be at least a kilometre.
With a big enough budget it would, says Dr Wood, now be possible to build a mile-high (1,600-metre) skyscraper. Even with carbon-fibre ropes few of such a building’s lifts would go all the way from the entrance lobby to the observation deck. Most would debouch into intermediate sky lobbies, where passengers could change lifts (not least because a mile-high lift which seemed to stop on every other floor would not be popular; it would be unutterably tedious and might force the poor souls on board to make eye contact).
Such an arrangement is already familiar. Many skyscrapers are more like three-stage rockets, with different buildings stacked one on top of another—offices, a hotel and apartments. Sky lobbies mark the frontiers between these uses. But carbon-fibre ropes will allow each of these stages to be taller, too.
The sky’s the limit
Nor need carbon-fibre lift-cables be confined to buildings. They could eventually make an idea from science fiction a reality too. Space lifts, dreamed up in the late 1950s, are a way of getting into orbit without using a rocket. Building one would mean lowering a cable from a satellite in a geosynchronous orbit above the Earth’s equator while deploying a counterbalancing cable out into space. The cable from Earth to the satellite would not be a classic lift rope because it would not, itself, move. But it would perform a similar function of support as robotic cars crawled up and down it, ferrying people and equipment to and from the satellite—whence they could depart into the cosmos.
There are, of course, many obstacles to building such a lift. But the answer to one—finding a material that is light and strong enough for the cable—might just have emerged from that mineshaft in Finland.