World's tallest towers

Seventy-five years after it opened its doors, the Empire State Building looms large--and not just on the Manhattan skyline. In many minds, it remains the ultimate skyscraper, though it has been more than three decades since the Art Deco tower ceded its title of tallest in the world.

Today, the Empire State is the ninth-tallest building (not counting communications or observation towers) and soon will be bumped down even further by new projects. Financing extremely tall towers can be complicated--even transporting people up and down can be difficult. But construction technology doesn't limit the heights of our skyscrapers, according to experts. And since humans remain ambitious and nations ever-desirous of flaunting their wealth and know-how, buildings are getting loftier by the year.

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"I think for a while we will keep building up and up and up," says architect Cesar Pelli, whose projects include the Petronas Towers in Malaysia, currently the second- and third-tallest towers in the world. "The desire is there."

When plans for the Moscow City Tower, which is expected to be completed in 2010, were unveiled in March, it was touted as "Europe's tallest building." At an estimated 2,000 feet high, it would actually be the tallest in the world if completed now. But other structures still on the drawing board, including the Freedom Tower in New York City and Burj Dubai in the UAE, are in an ongoing race for the sky.

For much of the 20th century, the US, the world's economic powerhouse, dominated the skyscraper scene. Steel frames and elevators had made tall buildings achievable and urbanisation made them necessary. Again and again, American buildings topped one another--the Chrysler Building was the tallest in 1930 but was overtaken by the Empire State a year later. That was surpassed when the World Trade Center arrived in 1973, but two years later, Chicago's Sears Tower became the tallest at 1,454 feet.

The landscape looks very different in the 21st century, with other countries and continents dominating the top of the tallest list.

"We're seeing a tremendous amount of activity overseas, whether it's in Asia or the Middle East," says Ron Klemencic, chairman of the Council on Tall Buildings and Urban Habitat and president of Seattle-based structural engineering firm Magnusson Klemencic Associates. "For countries that are emerging on the economic scene, very tall buildings are symbols of their economic strength."

The tallest building in the world, Taipei 101, was completed in 2004. It is a national symbol for Taiwan, as are the Petronas Towers for Malaysia. A government plan to bring Malaysia into the developed world by 2020, Klemencic points out, included a scheme to garner attention with very tall buildings.

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Building technology is easily able to keep up with development demands, says Leslie Robertson, head of Leslie E Robertson and Associates and the structural engineer for the World Trade Center. So where are the limits?

Pelli, for one, believes that they are going to come from human psychology and physiology--our willingness to ride elevators for extended periods and our ability to withstand pressure changes.

"You may get the bends if you go up or down too fast," Pelli says. "Around 150 stories, that may start happening. We don't know yet. You could pressurise whole buildings, but I don't think people want to live in a sealed building."

Safety is also a factor, of course, especially in the aftermath of the Sept 11, 2001, terrorist attacks. But, surprisingly, architects and developers continue to push taller and taller towers. At the time of the attack, one of Pelli's towers was under construction in Hong Kong; it took just 20 minutes for his clients to decide to proceed with the project, he says. And the Freedom Tower, which will be built on the World Trade Center site, is designed to be taller than the original complex.

The economics of very tall buildings can be complicated. The higher you go, for example, the more elevators you need, says James Sanders, a New York-based architect and author of Celluloid Skyline: New York and the Movies. "They keep eating area out of the lower floors," he says. "You get to a point where going taller is not gaining you square footage."

In some places, such as China's crowded cities, tall may still make good economic sense. That's not necessarily the case in places like the Middle East, where populations are small and there is more space.

Burj Dubai, however, is more than a skyscraper. The residential and hotel tower, expected to top more than 2,000 feet, is part of a complex. "It's the shopping center, golf course and the smaller towers that, in total, make it work economically," Klemencic says. Where as the tall buildings of the 20th century were primarily office towers, many of today's are mixed-use; part hotel, part condo, part retail space and part offices--all in amounts that the local market can absorb.

There is plenty of debate about what makes one building taller than another--should spires be counted or only occupied floors, for example? We used the rankings from Emporis, a real estate information company based in Darmstadt, Germany.

The list includes buildings that have reached their full height, even if they are unfinished. They are ranked by structural height, measured from the base to the "highest architectural or integral structural element of the building." (Spires count, but not antennae.) But the list only includes buildings, not communications towers or other structures, which is why Toronto's CN Tower doesn't make the cut, galling as that may be to the Canadians.

Perhaps height is less important than it once was--ask someone on the street what the tallest building in the world is, and they probably won't know. They may well guess it is one of those memorable buildings, like the Chrysler or Empire State.

"How much are they able to capture your imagination and your heart?" Pelli says. "Being tall doesn't necessarily do it."

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