15 feb 13 | Popular Science
Rise of the Supertalls
By Clay Risen
Engineering advances have architecs striving for the mile-high skyscraper.
On the morning of September 11, 2001, Bill Baker, a structural engineer with the architecture firm Skidmore, Owings and Merrill (SOM), was at his office in downtown Chicago. SOM is the undisputed leader in skyscraper design, and, at least on the engineering side, Baker is its undisputed king. In the past 30 years, he has overseen or worked on six of the world's 15 tallest buildings. But 9/11 was a bad day to be king: As the World Trade Center collapsed and rumors circulated about a rogue plane headed for the Sears Tower, Baker and his colleagues watched as the symbols of their profession became objects of terror.
A few days later, Baker and some of his co-workers drove to New York. The contractors at ground zero needed volunteer engineers to help take apart the towers. "They broke up the site into four zones," he said. "Each zone had four structural-engineering teams, and we were the Chicago team." As Baker picked through the rubble, it was hard not to question the future of high-rise architecture. One article in The Associated Press noted that architects were asking bluntly, "Should we ever build iconic skyscrapers again?"
Barely 18 months after 9/11, Baker returned to New York—this time to talk about designing the world's tallest building. The firm won the contract; six years later, the Burj Khalifa in Dubai topped out at 2,717 feet, more than half a mile tall.
Rather than an era of architectural modesty, the decade since 9/11 has seen a flowering of skyscraper construction. In the 70 years before 9/11, the record for the tallest building grew 230 feet. Since then, it has shot up 1,234 feet. And it's poised to rise much higher over the next decade. Today's tallest skyscrapers are new in every respect: new structures, new materials, designed and tested with new methods. The result isn't just taller buildings but an entirely new category of building: the supertall skyscraper.
Technically, the supertall category, as defined by the Council on Tall Buildings and Urban Habitat, covers anything taller than 300 meters, or 984 feet. That includes the 1,250-foot Empire State Building, a supertall half a century before the term's invention. The two World Trade Center towers, which began to rise in 1966, reached 1,368 and 1,362 feet. But only within the past 15 years have architects and engineers begun to see supertalls as a separate class, with its own challenges and opportunities. "When you get above the World Trade Center size, you've got to change your fundamental thought process," Baker says.
Baker is a tall, professorial type given to illustrating his comments with back-of-a-napkin sketches. Last October, we met for coffee across the street from 30 Rockefeller Plaza in New York. The iconic 850-foot tower opened in 1933, capping a frenzied era of ultra-tall-skyscraper construction. Then the growing stopped. For the next 30 years, steel-frame towers like 30 Rock and the Empire State Building seemed to be as high as architects could go.
That began to change in the mid-1960s, when an engineer named Fazlur Khan, one of Baker's predecessors at SOM, introduced a new structural system called the tube. Khan replaced the traditional internal steel frame with a series of columns running up the outside of the building. The columns are connected to one another and to the building's core, which houses the elevators, stairs, and utilities. That way, the strongest part of the building is on the outside, where it can best resist wind—which, above 40 stories or so, can be a greater concern than gravity.
The advent of the tube set off a surge in tall buildings in the '60s and '70s, including the John Hancock Center, the Sears Tower, and the World Trade Center. But by the time Baker arrived at SOM in the early 1980s, architects and engineers had run into new problems. The tube has a major limitation: It can go as high as an architect wants but only if the base grows proportionally. "If you make it twice as tall, you have to make it twice as wide and twice as deep, and the volume goes up by a factor of eight," Baker says. That won't work for a supertall building—150 floors means several million square feet of office space, much of it deep inside the building, enough to make investors nervously loosen their ties and look for the closest exit.
In the mid-1990s, two things happened that helped push architects beyond the floor-space conundrum, both of which were critical in unleashing the supertall revolution. The first was economic. The tallest skyscrapers used to contain mostly office space. Now supertalls are home to hotels, condominiums, shopping centers, and restaurants. Residential and retail spaces require narrower floor plates than offices, which allows buildings to go higher with the same amount of material while also providing a diversity of real-estate options that make very tall buildings easier to fill. In 2000, only five of the 20 tallest buildings in the world were mixed-use; by 2020, only five won't be.
The move to mixed-use towers facilitated the second big shift in skyscraper design: discarding the tube itself. In 1998, Baker and Adrian Smith—an SOM architect who designed many of the firm's tallest projects, including the Burj Khalifa, before leaving to start his own company—released their plan for Chicago's 7 South Dearborn. The tower was supermodel-slim: It would have risen 2,000 feet on just a quarter of a city block. Instead of a tube, they used a "stayed mast," which featured a central core closely surrounded by eight enormous columns, out from which cantilever the top 60 of 108 stories of mixed-use space.
The dot-com recession scotched the construction of 7 South Dearborn, but its innovative approach inspired architects and engineers to design dozens of "post-tube" skyscrapers. Baker and Smith teamed up again on the Burj Khalifa, and again they came up with an entirely new structural system, the "buttressed core." It involves a central, hexagonal, concrete core, on three sides of which they placed triangular buttresses. Imagine a rocket ship with three long, thin stabilizing fins.
Of course, it's not enough simply to design a tall building; architects and engineers also have to figure out how to move people through it. They've turned to solutions including sky lobbies, double-decker elevators, and so-called destination-dispatch elevators. Still, even the smartest elevators can rise at only about a kilometer a minute and descend at only about two thirds of that—otherwise most passengers' ears can't withstand the pressure.
To go even higher will require a radical rethinking of the elevator itself. "If you're going really tall, then you've got to get rid of the cables," says Leslie Robertson, the chief structural engineer for the original World Trade Center. The practical limit of conventional hoist elevators, he said, is about 1,500 feet. "You need, for example, a car that's driven electromagnetically. That's certainly the wave of the future."
Last year, a company called MagneMotion unveiled a cableless elevator powered by a linear synchronous motor, akin to the maglev motors on some trains. MagneMotion's elevator, developed for the U.S. Navy, is designed to move ammunition around a ship, but the company says it could easily adapt it for passengers.
Today's supertalls are different both in design and composition. Steel was once the material of choice for high-rise buildings, but engineers have begun to jettison steel in favor of concrete. Leonard Joseph, a structural engineer with the firm Thornton Tomasetti, says, "This concrete is not your grandpa's cement and stone and water." Rather, it involves complex recipes of chemicals and advanced materials, including microfibers that can replace bulky steel rebar.
Structural steel has a compressive strength of about 250 megapascals; in the 1950s, the strongest concrete could withstand about 21 megapascals, limiting all-concrete structures to about 20 floors. Today's strongest concrete tops 130 megapascals, and the addition of microfibers could nearly double that number. Another advantage is that concrete structures have a greater mass than steel structures—thus a concrete tower can be thinner than a steel one and still have the same resistance to wind forces. Concrete, unlike steel, doesn't need fireproofing.
As some engineers move toward concrete, others are already thinking beyond it, to carbon-fiber composites, the same lightweight, superstrong material that provides the structure in racing bikes and jet aircraft. But scientists will need to work out some significant challenges. Not only is carbon fiber very expensive, but its advantage—its lightness—would also be disturbing for anyone inside the building. People are used to the solidity of concrete and steel under their feet; in a carbon-fiber building, they would feel like they were walking on a drumhead, a disconcerting sensation at 1,500 feet.
As buildings rise taller, they face a series of increasingly complex forces. At ground level, a breeze might barely register. A hundred floors up, it could be gusting at 40 mph. Of particular concern to engineers is something called vortex shedding: As wind passes the sharp edges of buildings, it creates eddies, which pull on the structures in unpredictable ways.
The ability of engineers to model external forces has also enabled the growth of buildings. Until the 1970s, engineers had to overdesign towers with redundant strength because there was no way to test a building until it was built. Around that time, engineers began wind-tunnel-testing models. But it wasn't until fast, cheap computing power and 3-D printing arrived that design firms could test many scenarios rapidly.
These days wind-engineering firms can churn out multiple 3-D models of a building in hours, then test them in quick succession in a specialized wind tunnel. "They can go through 18 variations in a day," says Baker. "It's a long day, but still." Hundreds of sensors cover each model, taking hundreds of pressure readings a second that engineers later feed into a computer simulation that shows where the building is weakest. Toward the end of the process, they even re-create a scale version of its surroundings: hills, other buildings, even pedestrians, all of which create complex wind patterns.
Wind-tunnel analysis has helped engineers develop solutions to vortex shedding, such as rounded edges and notches at a building's corners, and dampers—similar to shock absorbers—that reduce a tower's tendency to move in the breeze. Without them, many supertalls would sway wildly; even if they didn't fall apart, they'd be impossible to work in. "You're on top of a wet noodle, and you get a really sickening ride," Joseph says.
In 1906, not long into the dawn of the skyscraper age, the landscape architect H.A. Caparn called the new building type "a revolt against the laws of economics." The only justification for going so tall, he said, was ego and money. More than a hundred years later, critics still level that charge. It's no coincidence, they say, that supertalls are concentrated in places like the Persian Gulf and China. They're like architectonic hothouse flowers, growing in the artificial climate of money and bad sense.
Yet rather than a revolt against economics, supertalls could be its purest expression. Dubai and Shanghai aren't ancient Egypt or 17th-century France, where a monarch could will a pyramid or palace into existence. The market, not the man, determines whether a supertall gets built.
Take, for example, the Burj Khalifa. On its own, the building represents valuable real estate. But its developer, Emaar Properties, also made it the centerpiece of a new business and residential district, charging a premium for properties with clear views of the skyscraper. Even if the Burj Khalifa fails to turn a profit, Emaar is betting that its presence will raise the surrounding property value enough to more than offset the difference.
Real-estate bets aside, something more fundamental drives the proliferation of supertalls: demographics. By 2050, the world population will have grown to nine billion, from about seven billion today. Some 70 percent of that population will live in urban areas.
For much of the 20th century, urban planning in the developed and developing world was antiurban; the dense verticality of the industrial city was supposed to be a thing of the past. Supertalls represent not just the rejection of that vision but also an embrace of a new synthesis: vertical urbanism.
Buildings like the Burj Khalifa and the Shanghai Tower are often called vertical cities, but they have none of the cluttered vibrancy of 19th-century London or New York's Lower East Side. In Hong Kong, the 1,588-foot International Commerce Center has its own airport rail link; that combined with a high-end mall, office space, and a hotel inside the tower means visitors can fly into the city, spend weeks in the I.C.C.—and never take a breath of the local air.
Whether we like it or not, that's the promise of supertall skyscrapers. In 2017, Kingdom Tower in Jeddah, Saudi Arabia, designed by Adrian Smith, will open at an estimated 3,280 feet, replacing the Burj Khalifa as the world's tallest building. Sitting inside the café at Rockefeller Center with Baker, I asked him whether the Kingdom Tower, at well over a half-mile high, might represent the outer limits of what man could design. Could he do, say, a mile? He thought about it for a moment. "Sure," he said. All he needed was the right client.