Mile-high tower wars: How tall is too tall?
The Burj Dubai will soar 800m above the Arabian desert, but a new mile-high tower is set to dwarf it. Jimmy Lee Shreeve asks: how tall is too tall?
Wednesday 18 June 2008
Prince Charles famously doesn't care for skyscrapers. He sees them as a vain attempt to assert masculinity, like a rock star with a cucumber down his trousers – or, as he puts it: "Trying to make them ever taller than the other person's building is surely taking the commercial macho into the realms of adolescent lunacy".
Phallic icons or not, we're about to move into a new era of mega-tall buildings that will put structures like Chicago's Sears Tower, at 442 metres (1,450 feet) tall, and Taiwan's Taipei 101, at 508m, in the shade.
George Efstathiou, a managing partner at the architectural firm Skidmore, Owings & Merrill, declares that "the age of the super-skyscrapers is starting again". Considering the scale of the structures on the way, he could well be right.
The structure set to beat them all was announced at the end of March. The Mile High Tower, to be built in a "mini city" near the Red Sea port of Jeddah in Saudi Arabia, will be about 1,600 metres tall – seven times the height of the Canary Wharf tower in London Docklands, or four Empire State buildings on top of each other.
The tower is the brainchild of Prince al-Walid bin Talal, a member of the Saudi royal family and the owner of London's Savoy hotel. According to Forbes magazine, he's worth about £11bn and is the 19th-richest person in the world. He needs to be: the estimated cost of the Mile High Tower – which, it is reported, will be built by the British engineering firm Hyder Consulting – is £5bn.
The building will pivot on hi-tech wizardry. A giant computer-controlled damper (shock absorber) will stretch down several floors to counter the nausea-inducing sway caused by the wind. And two mini towers, attached by sky bridges, will flank the building's base, further improving its stability.
You'd imagine that the logistics and know-how needed to build this monolith would be enormous. They are, says Ron Klemencic, president of Magnusson Klemencic Associates, an engineering firm specialising in high-rise constructions. But such structures are now relatively easy to erect. "Structural engineering-wise, it's not even difficult," Klemencic says. Stronger concrete and steel and advances in designing building frames, he says, allow for the safe development of mega-skyscrapers.
The current record-holder as tallest man-made structure is still a construction site. The Burj Dubai in the United Arab Emirates edged past Taiwan's Taipei 101, reaching 509m in July last year. Sources say the tower – being built by Skidmore, Owings & Merrill – will reach just over 800m when it is completed by the end of next year. The spire will be visible from 95km (60 miles) away – and the views will naturally be stunning.
The quantity of materials involved in the building's construction is mind-boggling. For example, the 31,000 tons of steel rods used to reinforce the structure would, if laid end to end, stretch one-quarter of the way round the world. When the tower is finished, 230,000 cubic metres of concrete will have been used. It will weigh about 500,000 tons.
When structures are so tall, it isn't just materials you have to worry about; it's the tools needed to build them. The tower cranes used to build conventional high-rise buildings, which are fixed to the ground, can carry enormous weight – but for mega-skyscrapers, helicopters are needed to lift materials to the higher levels.
Numerous high-profile skyscrapers are now being built around the world, including New York's Freedom Tower on the World Trade Centre site. But none will exceed 700m in height.
The building set to replace Southwark Towers in London – the Shard London Bridge (also known as the "shard of glass") – will be a comparatively puny 310m tall and have 72 floors. When it is completed (estimates say in 2011) it will be Britain's tallest building. The 50-storey Canary Wharf tower at One Canada Square is the current record-holder at 250m.
Several other high-rises are proposed for the capital. But the Mayor of London, Boris Johnson, is critical of tall buildings and has vowed to strengthen regulations protecting views of historic buildings such as St Paul's Cathedral and the Palace of Westminster. By world standards, of course, what Johnson considers tall isn't at all.
But how high can skyscrapers be? Is the sky the limit? David Scott, chairman of the Council on Tall Buildings and Urban Habitats (CTBUH) and principal at the New York-based engineering firm Arup, doesn't believe there is an absolute limit. "Mount Everest is essentially a pile of stone," he says. "You don't need a lot of technology to create it. Just a lot of money."
Engineers and architects have always speculated about how tall skyscrapers could be. Frank Lloyd Wright designed a mile-high tower, the Illinois, to be built in Chicago. It was proposed in 1956. Most experts agree that the technology was there to build it at the time, but not the investment.
But money isn't the only limiting factor. Elevator technology lags behind building technology, and one obvious issue is lift cables; if they had to raise a lift one mile, they would be far too heavy. In Burj Dubai, no elevator goes all the way from the ground to the top.
There's also we frail humans to consider. If an express elevator – at speeds up to 25mph – went from the ground floor to the top, we could pass out due to changes in the air pressure.
Then there's the problem of building movement. Most skyscrapers can sway a few metres in the wind without tumbling down, but the people inside might feel uncomfortable, if not downright nauseous, especially on higher floors. So the tallest structures need sturdy central cores to anchor them. The Empire State Building in New York, and other skyscrapers of that time, had steel beams wrapped around their elevator shafts.
The Burj Dubai has a "buttressed core", or concrete hub, with three wings spreading out to form a kind of tripod. When the wind blows against two of the wings, the third supports them. Taipei 101 has a 730-ton pendulum in the top of the building; the giant ball swings against the movement to keep the upper floors steady.
More difficult are natural disasters, particularly earthquakes that could topple a skyscraper. In areas of high seismic activity, such as California, strict building-codes are in place to protect against earth tremors. The most common defence is to use mass dampers – essentially seismic shock absorbers – made of giant springs or hydraulic systems that move in the opposite direction to the earthquake's oscillations.
And today, there's the threat of terrorism. Tall buildings are a target because they are iconic, and because they contain so many people. Lessons have been learnt from the attacks on the World Trade Centre, which led to the collapse of the towers. More internal supports make collapse far less likely. Stairwells are made wider to aid emergency evacuation, and ventilation systems force smoke out rather than letting it spread upwards, as if in a chimney.
But the fact is that there are hazards everywhere in life. And most experts agree that the benefits of skyscrapers outweigh the downsides. Bill Baker, chief structural engineer at Skidmore, Owings & Merrill, believes that building high is not only safe, but necessary to stop cities devouring green-belt land. "Urban density is good," he says. "Everyone uses public transportation, people walk to lunch. Look at the Sears Tower in Chicago. It has 4.4 million square feet – that's 100 acres on one city block."
David Scott of the CTBUH agrees. "If you look at the holistic impact of tall buildings on urban living, they offer many benefits," he says. "Not only do they reduce things like car ownership, but they can use waste heat in winter to warm them, and they have the potential to generate more open spaces at ground level, as well as reducing suburban sprawl. In the case of London, it can either expand outwards or upwards."
But George Efstathiou, the architect, perhaps has the real reason for our love of skyscrapers: "Tall buildings are a matter of ego. Tall buildings are a sign of success."
*The 60-storey Woolworth Building in New York, built in 1913, required 17 million bricks, 7,500 tons of terra cotta, 53,000 pounds of bronze and iron hardware and 87 miles of electric wiring.
*The Empire State Building in New York required 60 miles of water pipe and 3,500 miles of telephone and telegraph wire. Rain and snow can sometimes be seen rising instead of falling because of the wind patterns around the building.
*The World Trade Centre in New York (built in 1970-71), used enough concrete to lay a pavement 5ft wide from New York City to Washington, DC, a distance of 204 miles. The electrical wiring for the twin towers would have reached from New York to Mexico (about 1,500 miles). More than a million cubic yards of earth and rock were excavated to make way for the World Trade Centre. The material went into the Hudson river, creating 23.5 acres of new land.
*The Sears Tower in Chicago contains enough concrete to build an eight-lane highway five miles long. If laid out flat on the ground, the black "skin" of the tower would cover an area of 28 acres.
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