Confusing The Wind: The Burj Khalifa, Mother Nature, and the Modern Skyscraper

“This causes a regular, or periodic, force,” continues Garber, “that pushes the building side to side across from the wind flow. The frequency at which that happens will vary with wind speed and if those vortices can align with the frequency that the building wants to oscillate at then you can get some very larges forces developed.”

“What they’ve done on the Burj Khalifa deliberately,” explains Garber, “is keep introducing changes to the shape of the building with height so that the flow pattern can’t organize itself. It’s almost like you have several different buildings with height and each one of them has different vortices shedding at different winds speeds. All of those things can’t happen at the same time so what you’re left with is very little vortex shedding.”

If not properly addressed, wind stress from vortex shedding could theoretically cause major structural damage or even collapse. No need for queasiness though, as today’s skyscrapers are strong enough to withstand the most extreme winds speeds, making true structural failure a near impossibility. Skyscrapers are engineered according to a 50- or 100-year return rule, meaning that, on average, engineers expect winds to reach structurally dangerous speeds only once in a half century or more. Just to be safe, designers then increase the strength of the structure by an additional 60% or so to account for uncertainty in their measurements. When all this is taken into account, says Garber, “you’re talking about something along the lines of a 500- or 1000-year event.” The bottom line, he says, is that these buildings aren’t in any risk of falling over.9

Still, wind stress can still cause all kinds of problems in tall buildings. It can break windowpanes, damage the outer façade, stress building joints, cause leaking, crack walls and create myriad other issues. In addition, it can result in unnerving, even nauseating, swaying.10

“If the building’s moving too much, sometimes you can hear it creaking,” says Garber. However, “the most common concerns are of excessive motions. You might get people complaining that they feel the building moving or they might even feel sick.” Such was the case in the former Gulf & Western building in New York City. As a result of wind stress, the 44-story building developed cracks in stairwells and interior walls. In addition, office workers on the upper floors frequently complained of motion sickness on windy days. To fix these problems, owners invested over $10 million to add a massive steel brace to steady the structure.11

Indeed, measures to counteract the wind are undertaken as much for comfort as for safety. The happiness of occupants is an especially important issue to structural engineers, says Willis. “People are more sensitized than structures are to wind. Tuned mass dampers, for example, are used to address acceleration and peoples’ queasiness and response to the sway of buildings.”

Wind stresses grow dramatically the higher you build. Not only do wind speeds increase with height, but the force of the wind also increases with the square of its velocity. That means rapidly growing wind stress as the height of the building increases, which can cause even the most rigid skyscrapers to sway slowly back and forth.

“In any building,” says Garber, “the amount of motion you’d expect is on the order of 1/200 to 1/500 times its height.” For the Burj Khalifa, this translates into about two to four meters. “It’s not much, but certainly enough to make residents queasy if they can sense this motion. That’s why one of the chief concerns of architects and engineers is acceleration, which can result in perceptible forces on the human body.”

In carnival rides, cars and planes, physicists often think about forces in terms of “g’s,” multiples of the force of gravity. “When we are looking at buildings,” explains Garber, “we’re talking about milli-g’s of force.” As long as the occupants can’t feel the building moving, a certain degree of sway is acceptable and even expected. Humans can sense acceleration as small as about 5 to 25 milli-g’s, far less than what the structure can actually withstand.12 In most cases, such as the John Hancock building and Taipei 101, tuned mass dampers are installed not to ensure structural stability but to prevent queasiness.