Despite the attack on the twin towers, plenty of skyscrapers are rising. They are taller and more daring than ever, but still mostly monuments to magnificence

SKYSCRAPERS are hard to build and even harder to make money from. Perhaps that is why they hold such an enduring fascination. “The problem of the tall office building”, wrote Louis Sullivan in 1896, “is one of the most stupendous, one of the most magnificent opportunities that the Lord of Nature in His beneficence has ever offered to the proud spirit of man.”

Sullivan, an architect, was enthusiastically observing Chicago at a time when steel-framed buildings clad in stone had reached greater heights than anything that had been built before. But he could have been talking about New York in the 1920s, Hong Kong in the 1980s, or one of the booming cities in Asia that, according to Emporis, a firm which tracks skyscrapers, now lead a boom in high-rise building undaunted by the attack on the World Trade Centre in 2001. Roughly 8% of the world's stock of tall buildings is currently under construction, according to Emporis. About 40% of the world's 200 tallest buildings have been completed since 2000.

Skyscrapers pose stupendous problems to designers and developers because they have two powerful forces working against them. The first is economic—exemplified by the number of big American companies that now prefer to operate from low-rise campuses built outside cities. There are of course exceptions. These tend to be large financial institutions, on which a skyscraper confers the permanence once embodied by marble halls and oak-panelled dining-rooms.

The second is physical: towers are always fighting against their own weight. As more parts of the building are devoted to holding it up, they encroach on the space for working or living in. Developers and leasing agents refer to the ratio between these two elements as a building's “net-to-gross”. In a really efficient skyscraper, nearly 70% of the building's volume is useable, with the rest taken up by lift-shafts, stairwells and pillars. In a well designed low-rise building, by contrast, more than 80% of the space can be sold or let.

These obstacles explain why, in each skyscraper boom, many more designs are proposed than built. London, for example, has five towers over 200 metres (656 feet) tall that either have planning permission or are in the planning process, by which stage they must have completed designs and a willing developer. Yet only a couple of them are likely to be built. Francis Salway, chief executive of Land Securities, a developer, predicts that “between one and three” will go up over the next decade. A lot has still to happen before a tower can metamorphose from a design with enthusiastic backers into a few hundred metres of vertical glass and steel.

Penthouse envy

From a developer's point of view, skyscrapers come in three varieties: speculative ventures, commissions for a company headquarters and government-sponsored exercises in gigantism. Tall buildings cost more, so all three types depend on finding someone who is prepared to pay extra to look down on the neighbours.

Of these three types, towers built speculatively are the most common. A developer finds an anchor tenant (which reduces the risk of a flop), borrows the construction costs and looks to fill the rest of the building when it is near completion. A lot of skyscrapers known by a company's name came about like this: Woolworth only ever took up two of the 57 floors of the Woolworth building in New York, for example—the rest was let out in smaller parcels. The new New York Times building, due to be completed this year, will house the newspaper of the same name, but half of its 52 storeys will be let.

These skyscrapers are the riskiest of all and need a rare combination of factors to come together if they are to make money. They include a short supply of land in a desirable location (as in Hong Kong), zoning laws that preserve this scarcity and easy access to finance. Other factors, like bedrock near the surface to drive foundations into, help to keep costs down. The areas around Wall Street and midtown Manhattan have such favourable geology, which partly explains how New York looks. But nature can be mastered: Chicago, home to the first skyscrapers, sits on mud; Dubai's giant towers are being built on sand.

Company headquarters, such as the HSBC and Citibank towers at Canary Wharf in east London, bend these rules but do not break them. Because a single company occupies them, they are less subject to changes in the rental market. Instead, they are part advertising hoarding, part management tool. And for some companies, this is worth paying extra—which removes some of the constraints that apply to speculative towers. Carol Willis, the curator of the Skyscraper Museum in New York, describes company towers as “the Cadillacs of skyscraper design”.

One company polishing the fins on its machine is Goldman Sachs, an investment bank that is not known for frittering away its shareholders' money. Goldman is building a tower in lower Manhattan, close to the controversial redevelopment of the World Trade Centre site, where the Freedom Tower and other buildings are planned. Goldman's aim is to bring together people from ten office buildings it has in New York. The bank already has a tower a short distance away in New Jersey, but gleeful Manhattan property developers say that Goldman's bankers refused to move to the cheaper office space just across the water. For a bank, then, paying a premium to be near lots of other bankers can be worthwhile.

Government-backed towers discard most of these rules. Whether built as a statement of economic intent (as in Dubai, where much of the building frenzy is financed by companies backed by the government), or as a concrete index finger held up to the outside world (the 330-metre high Ryugyong Hotel in Pyongyang, North Korea), any city that aspires to be taken seriously wants a skyscraper or two.

Building skyscrapers for governments can lead to some odd results. In Dubai, for example, skyscrapers stand one deep on either side of the Sheikh Zayed Road in the south of the city, with desert behind them stretching away into the distance. The international skyscraper style, which involves using acres of glass, does not always make sense. It works well in the parts of north America where it first appeared, but when transported to the Gulf, the giant greenhouses require a huge amount of energy to cool them and engineers have to find ways to keep the light out. Some architects have proposed designs with concrete walls and small apertures that recall the screens in early Islamic architecture, but they have been rejected. Skyscrapers don't look like that.

Taller, lighter, stranger

The engineers who have to make the towers stand up, can build taller and stranger shapes only when technology permits. Three sorts of changes have shaped the current wave of skyscraper design: materials, lifts and computing.

New materials have the most visible effect on skyscrapers' looks. In 1922 Ludwig Mies van der Rohe, a German architect who later moved to America, sketched a tower with a glass skin stretched around the floors of a translucent building that appeared to have no central core. Not only would the building have fallen down, but it would have been impossible to make glass panels that were light enough or that could have been bent into the shapes that the design demanded.

“It has taken 80 years for glass technology to catch up with what Mies was sketching,” says Neven Sidor, of Grimshaw Architects. Translucent towers, which aside from looking pretty also alleviate one of the worst things about skyscrapers—the long shadows they cast on the streets below—are now proposed by architects everywhere. Thin-film technology (coating the glass in glazes that repel heat, but let in light) and self-cleaning glass are becoming standard. And glass can be formed into shapes that now make Mies's conceptual design look rather conservative, as at 30 St Mary Axe in London (better known as the “gherkin”) or the Hearst building in New York.

Other changes to materials have helped towers weigh less, which allows them to go higher. Floors and walls have become thinner, thanks to innovations like slim-line insulation made of fibreglass and aluminium foil, an idea borrowed from containers used to transport blood. This brings its own problems, though. When a floor is really large, thin ones become like trampolines and engineers have to find ways to prevent the journey to the photocopier from becoming too bouncy.

Architects are also grappling with Mies's other idea: dispensing with the central core, or breaking it up. Skyscrapers up to 200 metres tall can stand up with a central core of steel and concrete that houses a building's lifts and the plumbing for support services. Any taller and the building needs outriggers, which provide support like the flying buttresses on a gothic cathedral. This structure can apparently be extended heavenwards indefinitely. The Burj Dubai is made up of a central core with outriggers. It is determined to claim the title of tallest building in the world—so determined, in fact, that its final height is a secret and subject to elongation to keep ahead of would-be usurpers.

But some towers (architects refer to them as tubes) use a web of steel struts to transfer the building's weight down to its foundations via a few large piers. These buildings tend to be easy to spot—as with the HSBC building in Hong Kong, their architects like to flaunt their structural elements. This allows the lift shafts to be broken up or even moved to the outside of the building. The skyscrapers that result can look much lighter.

As materials have improved, it has become possible to go higher than the 509-metre Taipei 101 tower, which is currently the world's tallest building. But engineers also have to work out how to get people to the top floors.

Tall buildings have always relied on changes to lifting technology to go higher—the first hydraulic lifts around 1870 made it possible to go higher than the steam-powered lifts they replaced. Now, however, the constraints come less from the ability of a lift to travel half a kilometre vertically than from how long people must wait in the lobby for a lift to take them to the 50th floor. That makes it necessary to find ways to speed up their journeys around the building.

Most tall towers now have at least two banks of lifts: one for the lower floors and one for the upper ones. In the tallest towers in Asia (home to eight of the world's ten highest giants) this still means waiting too long. So engineers run two or more lifts in each lift shaft, and build “sky lobbies” where passengers cross between lifts if they want to go the whole way down or up.

These arrangements, whereby cappuccino-carrying office workers or hotel porters are directed to a particular lift according to where they want to go, are collectively known as “hall call”. KONE, a Finnish lift company, is working on a lift system that sends text messages to people's mobile phones as they enter a building, informing them to take lift five, say, if they want to go to their desk or lift seven if they want the café on the 60th floor.

Contemplating buildings this complicated has been possible in recent years only because computers have became powerful enough to build three-dimensional models that developers, architects, structural engineers, mechanical engineers and builders can all work on. Before such computer systems arrived, design changes had to be made on several sets of drawings, which increased the chances of mistakes. Strange shapes constructed at lower levels were possible before computers sped up. But ambitious forms like the new 230-metre China Central Television building in Beijing (which looks a little like a bent croquet hoop) needed computer processors to design.

Computers have made other things possible, too. Engineers can use them to test how a building might stand up to a fire or an aeroplane crash. When the main tower at Canary Wharf was proposed in the 1980s, according to Peter Bressington of Arup, an engineering firm that is a prolific builder of skyscrapers, nobody was able to predict accurately how long it would take to evacuate if a fire broke out. Now Arup can run a simulation in which a fire starts on the 35th floor, one lift is out of action and a few thousand people have to get out, and see how long it takes.

As well as allowing skyscrapers to go taller, these changes have made them more efficient machines for living or working in and brought their running costs down. Since the construction of the Commerzbank building in Frankfurt (which is reckoned to be the first green tower) in 1997, architects and some planning authorities have pushed environmental designs. In New York, for example, planners now insist that developers build skyscrapers that conform to tough standards set by the Green Building Council. Boosters argue that green designs improve the productivity of their inhabitants. Workers with access to natural light and fresh air that has not circulated endlessly through a building's air-conditioning system seem to get ill less often and avoid symptoms of “sick-building syndrome” associated with the sealed towers built in the 1970s, when energy costs were high.

Building green is more expensive. But developers can demand a premium—partly because of the value to companies of being seen to be green and partly because such buildings use at least 35% less energy. The new Bank of America tower in New York is an example. According to its developers, the building will act as a giant air filter, sucking in dirty air from the city, cleaning it up and eventually sending it out cleaner. It will also have a heat-exchange system whereby heat produced by sunshine, people, computers and lights will be used to make ice, which in turn will be used to cool the offices the following day. “The pitch”, says one Manhattan-based developer, “is higher productivity, lower energy costs, nicer places to work.”

Yet all this innovation has not altered the biggest obstacle to towers built speculatively: the vagaries of the property cycle. Developers make money through good timing, releasing office space on to the market when it is picking up and prices are rising. But because skyscrapers take much longer to go from a design to occupancy, it is hard to get the timing right.

The first big attempt to study historical patterns in commercial property markets was made by two economists in the 1930s. Homer Hoyt and Roy Wenzlick looked at the market for office space in Chicago and found that big profits made by developers early in each cycle encouraged further building. This led to oversupply in the market and falling rents.

Monoliths and markets

Skyscrapers have an unfortunate habit of being finished just as the market starts to drop. The Chrysler building was being completed when the tumble of Wall Street became the crash of 1929; the Empire State in the nadir of the Depression (in 1934, a quarter was vacant, hence its nickname: the Empty State building). Work by Jason Barr of Rutgers University suggests that over the 20th century the tallest skyscrapers in New York have been completed at around the time when America's economy was about to turn downwards (see chart).

It is hard to know if this will hold true for the Asian cities that are now leading the way in skyscraper enthusiasm, since strong demand in the office market has not been around for long enough. Yet the timing of the 452-metre Petronas towers in Kuala Lumpur, which were completed a year after the East Asian financial crisis began in 1997, suggests it might.

It seems to apply to some other cities, as the history of skyscrapers in London shows. London's office market headed down in 1974, 1982, 1990 and 2002. The two most recent tumbles were marked by the completion of well-known skyscrapers: the main Canary Wharf tower in 1991 and 30 St Mary Axe in 2003. Skyscrapers in London take between five and ten years to go from idea to income, and each cycle lasts for some ten years. This makes it “virtually impossible to get the timing right on tall buildings,” according to Peter Damesick of CB Richard Ellis, a property agent.

Perhaps it is this defiance of gravity and the market that makes skyscrapers so alluring and explains why people continue to want to build them. Publicity for the Empire State Building claimed the feeling of looking out from its viewing gallery was better than air travel. Nobody explained that the platform was there only because the space could not be sold.