A winter gale enjoys an easy approach to Manhattan from the north-northeast. As the wind moves over the Hudson River, the waves put up a weak fight against the air at altitude. Coming off the water, though, the wind hits the trees and buildings of Hudson Heights, and the mounting obstacles create huge vortices of air that join the increasingly turbulent flow. At West 110th Street, the wind tumbles into Central Park and then, skimming over oak and beech trees, it picks up speed while some of the great gyres it conveys spin down and vanish. Yet when the wind leaves the park at West 59th Street, it still contains tumultuous traces of its history, of the trees, the buildings and water it has traversed. The wind, it can be said, has memory.
At last, the wind happens upon one of the supertall towers south of the park and reveals a far more wicked talent. It strikes the building’s face and rushes for the edges, whipping off the corners and spiraling tightly, creating a columnar vortex that sucks at the tower’s side and goes careering downwind. If air is moving quickly, these vortices form to a beat, pulling first one way, then the other. The gale is coming out of the north, but this force acts on the perpendicular, along the east-west axis, rocking the structure. Specialists call this the crosswind effect, and in certain circumstances, the rocking hits a building’s “natural frequency.” Imagine, says Derek Kelly, an engineer, that the hand of God were to reach down and gently pluck one of the skyline’s spires: The skyscraper would vibrate back and forth, like a guitar string. That is a building’s natural frequency. If it matches the crosswind tugs, the two are in resonance; the oscillations grow, like a child kicking on a swing. East then west, east then west. When a gale rolls in, a supertall will lean back, but it’s nothing compared with the potential power of the crosswind effect.
Today’s engineers have conquered gravity: With enough structural steel and high-performance concrete, a tower will soar. The more dogged foe is wind. While gravity pulls down, wind can come from any compass point. It can apply pressure or suction, or alternate between the two. The wind, unlike gravity, changes from city to city, from season to season. Most harrowing of all is the wind’s dynamism. It is changed by everything it touches, and the wind even shapes itself, with every current pulling on all its neighbors. Gravity is plodding and obvious, but give wind a chance, and it will gather itself together and riot.
When Citicorp Center, with its slanted top, was completed in 1977, it didn’t look as if it should be able to stand. At 915 feet, the structure was supported entirely by four nine-story columns, leaving an impressive hollow at its base. The structural engineer William LeMessurier was hailed, but the next year an engineering student pointed out that the building (now called 601 Lexington) might indeed fall — in a strong-enough wind. Welders rushed to make emergency reinforcements and, with Hurricane Ella threatening, the city contemplated evacuating the area. Ella turned out to sea, though, and Midtown was spared.
In the world of tall buildings, a novel kind of specialist has come to prominence: the wind engineer. As towers grow taller, they climb into more powerful winds, and lighter construction techniques can leave them more vulnerable. Developers have begun putting up very slender skyscrapers, like 432 Park Avenue in New York, and these are particularly sensitive to the aerial environment. When a wind engineer like Kelly looks at such a building, he understands that it is airborne, with one end pinned to underlying bedrock, the rest riding the winds of Manhattan.
Kelly is a principal at RWDI, one of North America’s top wind consultants. The company’s client list includes 432 Park Avenue and 111 West 57th Street, a 1,428-foot skyscraper set to be among the slimmest in the world. (Imagine a one-foot ruler, stood on end and stretched to roughly twice its height.) When testing shows too much sway in an initial design, a near certainty with slender supertalls, RWDI offers a “shaping workshop.” The architect, developer and engineer make the trek from their home metropolis to the company’s headquarters in Guelph, Ontario, with dibs for the day on a wind-test tunnel and a cadre of model makers so that ideas can be tried in the tunnel and improved upon. The goal is to find ways that the building might, as these specialists say, “confuse the wind.” Designers of airplane wings want a smooth rush of air, to generate lift; designers of buildings want to divide the wind and leave it in disarray.
Dubai’s Burj Khalifa, at 2,717 feet the planet’s tallest man-made structure, is asymmetrical, winding down from the top in a series of steps, like an expanding spiral staircase. The crosswind effect depends on a building’s width, and so at each level on the Burj, the wind beats at a different frequency: confused and frustrated, like a toddler kicking wildly on a swing that won’t get going. Another favorite weapon of wind confusion, seen on many skyscrapers, is cut corners, which disrupt suction forces along the side. Pinnacles and antennas are subjected to the kind of scrutiny given America’s Cup yachts. In the case of 432 Park Avenue, the design team used five gap layers, each two floors in height, where the facade opens to allow air to pass through, sapping vortices. These horizontal bands give the tower a visual rhythm, but they are there because of the wind. In the natural world, wind sculpts sand dunes and cuts the snow, carving rings where it has whipped around a tree. It leaves its marks on buildings too.
In February 2014, the white-haired Uruguayan architect Rafael Viñoly delivered a lecture, sponsored by the Skyscraper Museum, on 432 Park Avenue. A tall building can be made eminently safe, capable of withstanding hurricanes and earthquakes, but no amount of beefing up its steel and concrete skeleton can force it to hold still. Which raises the question: For a penthouse in the $100 million range, how much sway is too much? Viñoly described a project-team trip, arranged by RWDI, to a facility in St. John’s, Newfoundland, that houses a marine simulator, a covered platform on six hydraulic jacks mocked up as a ship’s bridge. Now they would simulate a penthouse: Out with the ship’s controls, in with chairs, a sofa and a coffee table. Through the windows, rolling North Sea waves were replaced with a 360-degree vantage of the city from a suitably astonishing height. As Viñoly described the feel of the building behaving badly, before final engineering, he rocked the lectern. “If you’re standing here, your cup of tea moves,” he said. “And if you are tacky enough to have a chandelier, your chandelier also moves.”
If shaping and structural tweaks reach their limit, engineers can reduce motion further by installing “tuned mass dampers” near the apex. One version consists of an enormous mass on a suspension system with pistons that resist the mass’s movement. The damper acts as a pendulum, but set just off the building’s natural frequency, meaning that whenever the tower lurches, the mass drags, out of sync, steadying it. The 1,667-foot Taipei 101 is damped with a 728-ton ball that does double duty as a tourist attraction. From the observation deck, the ball appears to swing in heavy winds, though actually the tourists are also in motion.
Hidden at 432 Park Avenue, some 1,300 tons of combined mass stroke away on two dampers. The building’s engineer, Silvian Marcus, the U.S.A. director of building structures at WSP/Parsons Brinckerhoff, visited one of the top floors with a group and asked if anyone felt anything. No, they said. He rested a laser pointer on the floor, aimed it up and stood back. The dot wandered as the tower flexed. “They said, ‘It’s unbelievable; we feel nothing,’ ” Marcus told me. With high-end damping, most people will not sense motion in normal weather. For supertall residential skyscrapers, tuned mass dampers are the rare luxury amenities that go unseen.
Very tall buildings are a recent invention, and the public has not yet developed an intuitive sense for them. “We still have this innate understanding that a building we enter will remain stationary,” says Melissa Burton, the global head of civil structures for BMT Fluid Mechanics. “It scares us when it moves.” You can choose to make a home in the clouds, comfortably isolated from the elements, but you can never escape the wind. The walls, and everything they contain, will always be in motion. Most of the time, this will fall beneath your notice. Yet someday a storm will come, the wind will riot and you will feel its touch. ♦
Gareth Cook is a contributing writer for the magazine and a Pulitzer Prize-winning journalist. Illustrations by Brian Rea. Animation by Pablo Delcan.