Tall Buildings Sway More Than You Think
Yesterday, the world's tallest building (for now) opened in Dubai. The Burj Khalifa has moved ahead of other tall buildings that have recently debuted and in the next decade or so, it will no doubt be eclipsed by even taller buildings, mega-skyscrapers.
I got to wondering about these kinds of buildings, why people build them, how they build them and the engineering challenges they face. So I called up Ron Klemencic, president of Magnusson Klemencic Associates, Inc., a structural and civil engineering firm headquartered in Seattle, Wash., and asked him. Here are some of the things we talked about.
Tall Buildings Sway...More Than You Think
I shouldn't be surprised, I guess, that a tall, relatively slender object would sway in the wind. If it were a tree, yes. But a building? According to Klemencic, engineers use a pretty basic formula to get a good idea of how much a building will move back and forth. Take the height of the building and divide by 500. So a 1,000-foot building would sway two feet to the left or right. At 2,716 feet, the Burj Khalifa would sway back and forth about 5.5 feet. Some people feel the motion when the building changes direction and accelerates and it's one of many factors that architects and engineers take into consideration when designing the building.
"Structural engineers spend a lot of time trying to figure out how to distribute the material in the building," said Klemencic. "They work closely with the architect to shape the building to minimize the sway effect."
Skyscrapers Are Like Rocks in a Stream
Wind flows around buildings similar to the way water flows around rocks in a stream. As the water flows over the rock, small vortices or eddies swirl near the rock. Wind does the same thing. If those wind vortices break off the building in an organized, rhythmic fashion, the building will rock.
"If you can have the vortices break off at random or unorganized points in time, you can reduce the resonance," said Klemencic. The Burj Khalifa achieves this with a design that's terraced at different levels, which disrupts the wind vortices.
Are Skyscrapers Terrorist-Proof?
"I don’t know of any buildings that can withstand the impact of an aircraft," said Klemencic. "In the building industry, the view is that [9/11] wasn’t a building problem, but an airport security problem. While we are taking measures to enhance safety and security at the ground, there’s not a lot of emphasis placed on missiles, if you will."
Why Do People Build Mega-Skyscrapers in the First Place?
Tall buildings such as the Petronas Towers or the Taipei 101 generally are not built to make money, said Klemencic. If they're lucky, the developers will break even and may even lose money.
"The purpose of building these things in the last 10 or 15 years is a symbolic gesture," said Klemencic. "Planting your flag in the ground, as a developing nation, [saying] 'we as a nation have arrived and here’s our big building to prove it.'"The real money, he said, is made off of the surrounding developments, the retail, hotel, commercial and residential buildings that go up in the building's shadow.
AP Photo/Kamran Jebreili