Is the Sky the Limit?

“There is a limit – but it’s today’s limit, not a future limit,” says Jeff  Smilow, director of building structures at WSP in the USA. Innovation and evolution in a whole range of technologies, from damper systems to high-strength materials, will enable designers to create higher and higher building structures. “There are always new technologies. If you talked to engineers 50 years ago or 70 years ago, you’d get a different answer than today.” What about a theoretical limit past which  technology can’t take us? “You could find some very abstract limits on the strength of materials, but we are far away from reaching those limits,” adds Dr Ahmad Rahimian, director of building structures at WSP in the USA. “People’s aspiration, or the lack of it, is the real limit.”

In fact, it’s not a question of height so much as width. As buildings become taller, they must also get stronger and stiffer to remain stable in increasingly high winds. “You could easily build a building as tall as Mount Everest by making it as broad as Everest at the bottom,” says John Parker, senior Technical director in London. Everest is 8,848 m high, more than ten times taller than the Burj Khalifa. Even with a slenderness ratio of 24:1, equal to 111 West 57th Street, it would still be 368 m wide at its narrowest point – almost as wide as the Empire State is high. Quite apart from the sheer amount of land required, there would be no access to daylight through much of the floorplate, Parker points out. “There’s a limit on how far from the window people would want to be.” Pumping water to the top, and sewage back down, would also test engineers' ingenuity, as would pumping up the concrete for the structure itself.

And imagine how long it would take to get to the top. Vertical transportation is the other critical factor limiting a building’s height, and swelling its width. “The taller the building, the more lifts you need,” explains  Parker. “The lifts can only go so far in one go because the cables will stretch, so you  have to go up a certain distance, get out at a sky lobby and change to a different lift. The taller the building gets, the more lift changes you’ll have to make and the longer it’s going to take to get to the top floor. At some point, the guy who buys the penthouse is going to get fed up with spending 15 minutes to get to his front door.” Apart from the length of the journey, this also means a vast area is given over to VT. According to the traditional rule of thumb, a tower’s core accounts for around 20% of its area, so in a cylindrical Everest-sized tower, it would have to be at least 164 m wide – roughly equivalent to the base of the Burj Khalifa. But that proportion also rises with  height, says Parker. “A tower 8,500 m tall would have a lower efficiency because the lifts and stability system would occupy much more than 20%.” Which would mean an even larger chunk out of the usable, and crucially saleable, floor space. This is the real crux of the matter: how many people would want to live, work or stay in such a tall tower, and how much they would be willing to pay for the privilege. “At the end of the day, height is limited by whether it’s economically viable,” says Rahimian. “The drivers for high-rise buildings are the fundamentals of the economy – how strong the market is, how much demand there is for the product.”

The Burj Khalifa was anticipated to be home or workplace for around 32,000 people. So a building ten times as tall might have a population of 320,000 – the  same size as Iceland. We’re getting used to the idea of towers as vertical cities, but how long before we start thinking of them as vertical countries?

(This article was first published in our publication High Rise)