On that cool blue morning 10 years ago when everything changed, Les Robertson was half a world away, hosting a dinner at a Hong Kong restaurant. The rattling of cell phones left on the table—“a detestable habit”—was the first indication that something had struck one of the Twin Towers. Robertson, the revered engineer responsible for their structural design, was at first unconcerned. “I just assumed that a helicopter had run into the Trade Center,” he said recently, speaking from his 47th-floor office, which looks out over Ground Zero. Such an event, unfortunate as it might have been, was well within the tolerances for which the towers were designed. A few minutes later, however, when those cell phones started buzzing once more with news of a second crash, he realized it was “quite another thing again” and excused himself to watch the unfolding events from a hotel room. In the weeks that followed Robertson declined all requests to speak publicly about the tragedy, even as the innovative structural design of the towers became the subject of public contention. “I thought at that time that my career as a designer of structures was over,” he says. For that matter, it seemed that his entire profession might have become obsolete, as fear spread that the attacks marked the very end of the age of the skyscraper. Anxious stories began to fill America’s newspapers. “Many workers fear that their lofty dwellings are more dangerous than glamorous,” the Wall Street Journal reported on September 19. USA Today, on the same date, was less guarded in tone: “It wasn’t just the World Trade Center that was obliterated last week. The future of the skyscraper as an American landmark may be teetering.” In the coming months the press would report on, and the U.S. Patent and Trademark Office would receive proposals for, any number of devices—parachutes, tethers, detachable vehicles—to facilitate escape from a burning building. The attacks precipitated a worldwide case of vertigo. Beyond anxieties about safety, there was a pervading sense that in the digital age the city, and the skyscraper along with it, had become a relic of the past. Henry Petroski, a professor of engineering at Duke University and author of numerous books on the design of the everyday world, made this argument emphatically, just five days after 9/11, in an essay, “Onward but Perhaps Not Upward,” in the Washington Post. The fast and easy telecommunications provided by the Internet, according to Petroski, translated into “a diminished need for compact contiguous space.” As it turns out, the various prognostications of the skyscraper’s demise turned out to be very much mistaken, the moment of questioning an altogether brief interregnum. “There were a lot of foolish predictions or claims that skyscrapers killed people,” says Carol Willis, founding director of New York City’s Skyscraper Museum. “Terrorists killed people. It wasn’t the buildings that were evil or dangerous.” Overseas, construction barely paused after 9/11. The furious urbanization taking place across the Pacific generated a huge demand for new skyscrapers there. “China, the Middle East, Asia? Nobody gave it a moment’s notice,” says T. J. Gottesdiener, managing partner of Skidmore, Owings & Merrill, an architectural firm synonymous with the design of corporate towers. “We had projects that were in the design stages, and they all continued.” Indeed, our very thinking about the skyscraper has changed dramatically since that day: we now understand that tall buildings can be something more than hubristic blots on our skylines. They just might be the most efficient and sustainable way of accommodating the flood of global urbanization. Going Up The past decade, in fact, has been the single greatest period of skyscraper construction in history. According to the Council on Tall Buildings and Urban Habitat (CTBUH), an organization that tracks skyscraper development, some 350 skyscrapers have been constructed since 2001, more than doubling their worldwide population. The number of “supertall” buildings (structures greater than 300 meters in height) has also doubled in that time. This boom is accelerating. Last year marked the completion of the Burj Khalifa in Dubai, which at 828 meters is not only the world’s tallest building but also exceeds its closest competitor (Taipei 101, completed in 2004) by 320 meters, about the height of New York City’s Chrysler Building. No building topped off this year or next will dethrone the Burj Khalifa, but 2011 will go down as the single greatest year for the construction of tall buildings in history, with more than 97 skyscrapers over 200 meters (including 22 supertalls) slated for completion. “The term you hear over and over again is ‘iconic,’” Willis says. “Clearly, clients are asking for iconic buildings, and they want them to be tall—taller than the most efficient way to get a return on their investment.” Most supertalls, in fact, cannot be justified on strictly economic terms. Once a building rises beyond roughly 70 stories (the exact figure varies depending on location), the added costs required to achieve structural stability and the added space necessary for elevators and other services generally preclude any direct financial profit. The overwhelming majority of these aspirational supertalls are rising outside the borders of the U.S. “Cities are using skylines to brand themselves,” says Antony Wood, executive director of the CTBUH. “The skyline is seen as an important symbol to portray that a country has arrived on the scene and is a First World country.” Of the 20 tallest buildings completed in 2010, only one, Chicago’s Legacy tower, is on American soil (and at number 19, it barely cracks the list). One World Trade Center, formerly known as the Freedom Tower, is only the fourth-tallest building under construction worldwide. It will eventually top out at a symbolic 1,776 feet (541 meters). China is leading the skyscraper boom. According to a 2009 report by the McKinsey Global Institute, China’s cities will swell by 350 million people by 2025. By comparison, the transformational migration of African-Americans from the Jim Crow–era South to northern American cities between 1915 and 1970 entailed a population shift of just six million. Yet even if they are not constructed in the U.S., supertall buildings remain very much a national idiom, their design dominated by American architectural and engineering firms. “If you’re going to spend $3 billion on a building,” Willis says, “you want to have someone who has done it before.” Given this essential conservatism, it should not be surprising that advances in the design and engineering of the basic structure of these buildings over the past decade have been more incremental than radical. “In terms of grand thought, there’s very little that’s really new in the design of structures,” Robertson says. One of the world’s most visually dramatic new skyscrapers is a shimmering 76-story, 265-meter residential tower in Lower Manhattan designed by architect Frank Gehry (who compared its rumpled steel facade to the sculpture of Bernini) that, from a structural standpoint, is quite standard. Earthquakes still pose a serious challenge to structural engineers. Buildings located in earthquake zones must be stiff enough to carry all of the weight of a supertall yet supple enough to sway when the ground moves. Designs that have “high mass and high stiffness make things worse because a building’s mass is what creates your earthquake forces,” says Leonard Joseph, a structural engineer who specializes in supertall buildings for engineering firm Thornton Tomasetti. The lighter and more flexible structures required in earthquake zones must also resist the push of the wind. Among the most inventive engineering solutions to this dilemma have been tuned mass dampers such as the one specified by Joseph and his team at Thornton Tomasetti for Taipei 101, which was until last year the world’s tallest building. This 660-metric-ton steel ball is suspended from the 92nd floor to minimize uncomfortable motion. It sways in opposition to the building, pushing and pulling against giant shock absorbers that convert the edifice’s motion to harmless heat. Although the idea seems counterintuitive, today’s tall buildings actually offer some advantages over low-rise structures during earthquakes. “When the ground moves suddenly, the skyscraper will ‘roll with the punches,’ absorbing the sudden movements by deforming,” Joseph says. “Short buildings cannot absorb sudden ground movements that way because too much movement would need to be absorbed over too short a height.” Worst-Case Design Of course, 9/11 taught us that tall buildings may have more to contend with than earthquakes and typhoons. There are limits, however, to the extent to which the design of tall buildings can provide protection against terror attacks, especially given the size and speed of contemporary aircraft. The World Trade Center was designed to withstand the impact of a Boeing 707, which weighs in around 20 percent of an Airbus A380, today’s largest passenger jet. Roughly a month after the attacks, Robertson found himself back in Hong Kong, forced to explain this reality to a nervous development corporation. The intervening weeks had allowed him a bit of perspective, and he was now prepared to make a case for tall buildings. “I espoused to the board that the responsibility was to keep airplanes away from the buildings and not to design the buildings for that circumstance,” he says. Architects and engineers, however, are not without recourse. Improving communications systems for firefighters has been a major priority, solved in part by placing radio repeaters throughout stair towers. “Unquestionably, as a result of 9/11, there has been greater consideration of life safety,” Gottesdiener says. On the other hand, few of the more outlandish emergency egress systems that found their way to the patent office have made much headway with designers. Structural engineers consider these measures extraneous at best. “The building has to give people the option of getting out in a conventional way,” says Guy Nordenson, a professor of architecture at Princeton University. “You really want everyone to be able to get out safely.” One safety-enhancing design feature that is only beginning to be implemented is the use of sky bridges between buildings, a staple of futuristic renderings of the early 20th century. Several of the unrealized proposals for Ground Zero took advantage of these passages. The most prominent standing example is the Chinese Central Television (CCTV) headquarters, which is formed from two towers that rise and connect at a crooked angle in the sky. The effect is not just visually striking, though it is certainly that. In the event of fire or other calamity, occupants have multiple routes to safety. “From any point in the building, you can go down and out or over and out,” says Nordenson, who engineered an even more ambitious Beijing project, the Linked Hybrid complex, along similar lines. Conceived by New York City architect Steven Holl, the mixed-use development spreads 644 apartments, a hotel, a movie theater and even a Montessori school across eight towers, all connected by skywalks. Today’s eye-catching buildings are designed with software that is changing not just the way skyscrapers look but the way they operate. Computer-aided modeling technologies allow architects to adjust their designs on the fly. Just as transformational has been the introduction of building information modeling software, which allows architects to integrate the skyscraper’s complex mechanical systems—the need to heat, cool, and move people and materials into and out of the building—into the earliest stages of the design process. The same technology will also allow building managers to test how occupants would react to unique situations such as fire alarms, thereby improving performance. Scale Factor The biggest change in skyscrapers over the past decade, however, may not have anything to do with the design of the buildings themselves or with their size but with how we think about them. There was a time—and not long ago—when the skyscraper was understood to be the built equivalent of the SUV, an energy-and-resource-sucking drain on the environment. “The notion that skyscrapers are green is so contrary to what was the standard position a decade ago,” says Terence Riley, curator of a landmark 2004 exhibition on tall buildings at the Museum of Modern Art in New York City. “In a lot of people’s minds, living green means living in the country.” In fact, just the opposite is often true. Residents of cities with dense urban cores, such as New York or Chicago, use far less energy per capita than those who live in suburban or rural areas. “There are clear environmental benefits in high density and in reducing urban sprawl by bringing spaces for living and working together within a single, compact footprint,” says British architect Norman Foster, whose office is engaged in the design of numerous supertall buildings. His Commerzbank headquarters in Frankfurt, completed in 1997, is widely considered the world’s first “green” skyscraper, notable for its natural-ventilation system, spiral of sky gardens and daylight-accessible workspaces. “The higher the building, the more viable it becomes to take advantage of economies of scale,” Foster says. “And by bringing together different functions, we can balance energy needs across these uses, generating even greater environmental benefits.” Perhaps most important of all, the skyscraper, centrally located, encourages the development and use of public transportation systems. The most environmentally sophisticated office building in the U.S. is the Bank of America Tower, an asymmetrically capped, 366-meter-tall white pylon just a block from Times Square. It is the first commercial high-rise to receive the Leadership in Energy and Environmental Design (LEED) “platinum” certification, the highest designation awarded by the U.S. Green Building Council. The Bank of America Tower creates two thirds of its own energy (it has a natural gas generator), filters its incoming air for volatile compounds and recaptures its storm-water runoff. Its floor-to-ceiling, thermally insulated windows and internal glass partitions allow daylight to penetrate deeply into the building and ensure that a high percentage of occupants has a view out. Travel down to the lowest basement level, and you will find what amounts to the largest ice cube tray on earth. Every night chillers freeze the water in 44 enormous tubs, each three meters tall and two and a half meters in diameter. When these ice batteries melt during the day, they provide a significant portion of the building’s air-conditioning, shifting its energy consumption to off-hours. For all its high-tech systems, however, if you ask Robert F. Fox, Jr., a partner in Cook + Fox Architects, the building’s designers, what feature is most crucial to the tower’s sustainability, he will answer with the real estate agent’s maxim: location, location, location. “We can no longer have one person in a car spending fossil fuel to get to work,” he says. “A three-story building in the suburbs in the future won’t allow for the necessary density, the access to public transportation, or the ability for lots of people to get together and collaborate.” That’s right. In direct contrast to Petroski’s dire forecast immediately following 9/11, it seems that even as we spend so much of our time on social media (or perhaps because we spend so much of our time on social media), a desire for the human contact afforded by cities—and the skyscrapers within them—are more powerful than ever. Indeed, Google, the standard-bearer of the digital economy, recently spent $1.8 billion on a new building in Manhattan (albeit one only 15 stories tall). As it is, more and more people are seeking that kind of human contact—or at least the jobs that cities generate. According to the CTBUH, worldwide roughly one million people move to urban centers every week. “Cities have to go vertical,” Wood says. They are doing just that.
“I just assumed that a helicopter had run into the Trade Center,” he said recently, speaking from his 47th-floor office, which looks out over Ground Zero. Such an event, unfortunate as it might have been, was well within the tolerances for which the towers were designed. A few minutes later, however, when those cell phones started buzzing once more with news of a second crash, he realized it was “quite another thing again” and excused himself to watch the unfolding events from a hotel room.
In the weeks that followed Robertson declined all requests to speak publicly about the tragedy, even as the innovative structural design of the towers became the subject of public contention. “I thought at that time that my career as a designer of structures was over,” he says. For that matter, it seemed that his entire profession might have become obsolete, as fear spread that the attacks marked the very end of the age of the skyscraper.
Anxious stories began to fill America’s newspapers. “Many workers fear that their lofty dwellings are more dangerous than glamorous,” the Wall Street Journal reported on September 19. USA Today, on the same date, was less guarded in tone: “It wasn’t just the World Trade Center that was obliterated last week. The future of the skyscraper as an American landmark may be teetering.”
In the coming months the press would report on, and the U.S. Patent and Trademark Office would receive proposals for, any number of devices—parachutes, tethers, detachable vehicles—to facilitate escape from a burning building. The attacks precipitated a worldwide case of vertigo.
Beyond anxieties about safety, there was a pervading sense that in the digital age the city, and the skyscraper along with it, had become a relic of the past. Henry Petroski, a professor of engineering at Duke University and author of numerous books on the design of the everyday world, made this argument emphatically, just five days after 9/11, in an essay, “Onward but Perhaps Not Upward,” in the Washington Post. The fast and easy telecommunications provided by the Internet, according to Petroski, translated into “a diminished need for compact contiguous space.”
As it turns out, the various prognostications of the skyscraper’s demise turned out to be very much mistaken, the moment of questioning an altogether brief interregnum. “There were a lot of foolish predictions or claims that skyscrapers killed people,” says Carol Willis, founding director of New York City’s Skyscraper Museum. “Terrorists killed people. It wasn’t the buildings that were evil or dangerous.”
Overseas, construction barely paused after 9/11. The furious urbanization taking place across the Pacific generated a huge demand for new skyscrapers there. “China, the Middle East, Asia? Nobody gave it a moment’s notice,” says T. J. Gottesdiener, managing partner of Skidmore, Owings & Merrill, an architectural firm synonymous with the design of corporate towers. “We had projects that were in the design stages, and they all continued.”
Indeed, our very thinking about the skyscraper has changed dramatically since that day: we now understand that tall buildings can be something more than hubristic blots on our skylines. They just might be the most efficient and sustainable way of accommodating the flood of global urbanization.
Going Up The past decade, in fact, has been the single greatest period of skyscraper construction in history. According to the Council on Tall Buildings and Urban Habitat (CTBUH), an organization that tracks skyscraper development, some 350 skyscrapers have been constructed since 2001, more than doubling their worldwide population. The number of “supertall” buildings (structures greater than 300 meters in height) has also doubled in that time.
This boom is accelerating. Last year marked the completion of the Burj Khalifa in Dubai, which at 828 meters is not only the world’s tallest building but also exceeds its closest competitor (Taipei 101, completed in 2004) by 320 meters, about the height of New York City’s Chrysler Building. No building topped off this year or next will dethrone the Burj Khalifa, but 2011 will go down as the single greatest year for the construction of tall buildings in history, with more than 97 skyscrapers over 200 meters (including 22 supertalls) slated for completion.
“The term you hear over and over again is ‘iconic,’” Willis says. “Clearly, clients are asking for iconic buildings, and they want them to be tall—taller than the most efficient way to get a return on their investment.”
Most supertalls, in fact, cannot be justified on strictly economic terms. Once a building rises beyond roughly 70 stories (the exact figure varies depending on location), the added costs required to achieve structural stability and the added space necessary for elevators and other services generally preclude any direct financial profit.
The overwhelming majority of these aspirational supertalls are rising outside the borders of the U.S. “Cities are using skylines to brand themselves,” says Antony Wood, executive director of the CTBUH. “The skyline is seen as an important symbol to portray that a country has arrived on the scene and is a First World country.”
Of the 20 tallest buildings completed in 2010, only one, Chicago’s Legacy tower, is on American soil (and at number 19, it barely cracks the list). One World Trade Center, formerly known as the Freedom Tower, is only the fourth-tallest building under construction worldwide. It will eventually top out at a symbolic 1,776 feet (541 meters).
China is leading the skyscraper boom. According to a 2009 report by the McKinsey Global Institute, China’s cities will swell by 350 million people by 2025. By comparison, the transformational migration of African-Americans from the Jim Crow–era South to northern American cities between 1915 and 1970 entailed a population shift of just six million.
Yet even if they are not constructed in the U.S., supertall buildings remain very much a national idiom, their design dominated by American architectural and engineering firms. “If you’re going to spend $3 billion on a building,” Willis says, “you want to have someone who has done it before.”
Given this essential conservatism, it should not be surprising that advances in the design and engineering of the basic structure of these buildings over the past decade have been more incremental than radical. “In terms of grand thought, there’s very little that’s really new in the design of structures,” Robertson says. One of the world’s most visually dramatic new skyscrapers is a shimmering 76-story, 265-meter residential tower in Lower Manhattan designed by architect Frank Gehry (who compared its rumpled steel facade to the sculpture of Bernini) that, from a structural standpoint, is quite standard.
Earthquakes still pose a serious challenge to structural engineers. Buildings located in earthquake zones must be stiff enough to carry all of the weight of a supertall yet supple enough to sway when the ground moves. Designs that have “high mass and high stiffness make things worse because a building’s mass is what creates your earthquake forces,” says Leonard Joseph, a structural engineer who specializes in supertall buildings for engineering firm Thornton Tomasetti.
The lighter and more flexible structures required in earthquake zones must also resist the push of the wind. Among the most inventive engineering solutions to this dilemma have been tuned mass dampers such as the one specified by Joseph and his team at Thornton Tomasetti for Taipei 101, which was until last year the world’s tallest building. This 660-metric-ton steel ball is suspended from the 92nd floor to minimize uncomfortable motion. It sways in opposition to the building, pushing and pulling against giant shock absorbers that convert the edifice’s motion to harmless heat.
Although the idea seems counterintuitive, today’s tall buildings actually offer some advantages over low-rise structures during earthquakes. “When the ground moves suddenly, the skyscraper will ‘roll with the punches,’ absorbing the sudden movements by deforming,” Joseph says. “Short buildings cannot absorb sudden ground movements that way because too much movement would need to be absorbed over too short a height.”
Worst-Case Design Of course, 9/11 taught us that tall buildings may have more to contend with than earthquakes and typhoons. There are limits, however, to the extent to which the design of tall buildings can provide protection against terror attacks, especially given the size and speed of contemporary aircraft. The World Trade Center was designed to withstand the impact of a Boeing 707, which weighs in around 20 percent of an Airbus A380, today’s largest passenger jet.
Roughly a month after the attacks, Robertson found himself back in Hong Kong, forced to explain this reality to a nervous development corporation. The intervening weeks had allowed him a bit of perspective, and he was now prepared to make a case for tall buildings. “I espoused to the board that the responsibility was to keep airplanes away from the buildings and not to design the buildings for that circumstance,” he says.
Architects and engineers, however, are not without recourse. Improving communications systems for firefighters has been a major priority, solved in part by placing radio repeaters throughout stair towers. “Unquestionably, as a result of 9/11, there has been greater consideration of life safety,” Gottesdiener says.
On the other hand, few of the more outlandish emergency egress systems that found their way to the patent office have made much headway with designers. Structural engineers consider these measures extraneous at best. “The building has to give people the option of getting out in a conventional way,” says Guy Nordenson, a professor of architecture at Princeton University. “You really want everyone to be able to get out safely.”
One safety-enhancing design feature that is only beginning to be implemented is the use of sky bridges between buildings, a staple of futuristic renderings of the early 20th century. Several of the unrealized proposals for Ground Zero took advantage of these passages. The most prominent standing example is the Chinese Central Television (CCTV) headquarters, which is formed from two towers that rise and connect at a crooked angle in the sky.
The effect is not just visually striking, though it is certainly that. In the event of fire or other calamity, occupants have multiple routes to safety. “From any point in the building, you can go down and out or over and out,” says Nordenson, who engineered an even more ambitious Beijing project, the Linked Hybrid complex, along similar lines. Conceived by New York City architect Steven Holl, the mixed-use development spreads 644 apartments, a hotel, a movie theater and even a Montessori school across eight towers, all connected by skywalks.
Today’s eye-catching buildings are designed with software that is changing not just the way skyscrapers look but the way they operate. Computer-aided modeling technologies allow architects to adjust their designs on the fly. Just as transformational has been the introduction of building information modeling software, which allows architects to integrate the skyscraper’s complex mechanical systems—the need to heat, cool, and move people and materials into and out of the building—into the earliest stages of the design process. The same technology will also allow building managers to test how occupants would react to unique situations such as fire alarms, thereby improving performance.
Scale Factor The biggest change in skyscrapers over the past decade, however, may not have anything to do with the design of the buildings themselves or with their size but with how we think about them. There was a time—and not long ago—when the skyscraper was understood to be the built equivalent of the SUV, an energy-and-resource-sucking drain on the environment.
“The notion that skyscrapers are green is so contrary to what was the standard position a decade ago,” says Terence Riley, curator of a landmark 2004 exhibition on tall buildings at the Museum of Modern Art in New York City. “In a lot of people’s minds, living green means living in the country.” In fact, just the opposite is often true. Residents of cities with dense urban cores, such as New York or Chicago, use far less energy per capita than those who live in suburban or rural areas.
“There are clear environmental benefits in high density and in reducing urban sprawl by bringing spaces for living and working together within a single, compact footprint,” says British architect Norman Foster, whose office is engaged in the design of numerous supertall buildings. His Commerzbank headquarters in Frankfurt, completed in 1997, is widely considered the world’s first “green” skyscraper, notable for its natural-ventilation system, spiral of sky gardens and daylight-accessible workspaces.
“The higher the building, the more viable it becomes to take advantage of economies of scale,” Foster says. “And by bringing together different functions, we can balance energy needs across these uses, generating even greater environmental benefits.” Perhaps most important of all, the skyscraper, centrally located, encourages the development and use of public transportation systems.
The most environmentally sophisticated office building in the U.S. is the Bank of America Tower, an asymmetrically capped, 366-meter-tall white pylon just a block from Times Square. It is the first commercial high-rise to receive the Leadership in Energy and Environmental Design (LEED) “platinum” certification, the highest designation awarded by the U.S. Green Building Council.
The Bank of America Tower creates two thirds of its own energy (it has a natural gas generator), filters its incoming air for volatile compounds and recaptures its storm-water runoff. Its floor-to-ceiling, thermally insulated windows and internal glass partitions allow daylight to penetrate deeply into the building and ensure that a high percentage of occupants has a view out. Travel down to the lowest basement level, and you will find what amounts to the largest ice cube tray on earth. Every night chillers freeze the water in 44 enormous tubs, each three meters tall and two and a half meters in diameter. When these ice batteries melt during the day, they provide a significant portion of the building’s air-conditioning, shifting its energy consumption to off-hours.
For all its high-tech systems, however, if you ask Robert F. Fox, Jr., a partner in Cook + Fox Architects, the building’s designers, what feature is most crucial to the tower’s sustainability, he will answer with the real estate agent’s maxim: location, location, location. “We can no longer have one person in a car spending fossil fuel to get to work,” he says. “A three-story building in the suburbs in the future won’t allow for the necessary density, the access to public transportation, or the ability for lots of people to get together and collaborate.”
That’s right. In direct contrast to Petroski’s dire forecast immediately following 9/11, it seems that even as we spend so much of our time on social media (or perhaps because we spend so much of our time on social media), a desire for the human contact afforded by cities—and the skyscrapers within them—are more powerful than ever. Indeed, Google, the standard-bearer of the digital economy, recently spent $1.8 billion on a new building in Manhattan (albeit one only 15 stories tall).
As it is, more and more people are seeking that kind of human contact—or at least the jobs that cities generate. According to the CTBUH, worldwide roughly one million people move to urban centers every week. “Cities have to go vertical,” Wood says. They are doing just that.
