Article re: tall buildings

How to build today's supertalls
Elegance, not machismo, is behind Chicago's unprecedented reach for the sky
By Blair Kamin Tribune architecture critic
August 18, 2007

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A white hard-hat on his head, the earnest look of a professor on his face, Bill Baker has a ready metaphor to reveal the hidden structural logic behind Chicago's unprecedented reach into the sky. Mimicking the cores of concrete that shoot up the center of today's supertall skyscrapers, Baker stands like a soldier at attention, his feet touching. The silolike cores are too thin to single-handedly brace the towers against howling winds, he says. So Baker extends his left arm and puts it on the shoulder of a colleague standing with him in front of Donald Trump's ever-growing skyscraper on the Chicago River's north bank."That steadies me," says Baker, a 53-year-old partner and structural engineer at the Chicago architecture, engineering and planning firm of Skidmore, Owings & Merrill. "It's like ski poles."He has just illustrated the essence of a relatively new but little-noticed way of erecting skyscrapers, in which massive arms of steel or concrete extend outward from a building's core and grab high-strength structural columns at or near the perimeter, bracing the building against gravity and the overturning force of the wind. This is the method, called core and outrigger, that is propelling a skyscraper boom unlike any other in Chicago, birthplace of the skyscraper. Today, for the first time in its history, the city has three supertall skyscrapers -- those 1,000 feet or higher -- under construction simultaneously. And owing to shifts in both physics and aesthetics, they aim to become icons of a new post-industrial, post-lunch bucket city -- less about old-fashioned machismo than new-age elegance."It's the difference between somebody who is a gymnast and a dancer," said Zurich-based architect and engineer Santiago Calatrava, designer of the now-under-construction Chicago Spire, in a telephone interview from Spain. "We try to be elegant -- we are not being athletic. We are not showing muscles."
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At this stage, the most visible of the new giants is the Trump International Hotel & Tower, which now reaches more than 500 feet into the air on its way to total height of 1,362 feet -- 88 feet shorter than Sears Tower. Then there is the Waterview Tower, where the superstructure has just popped out of the ground at the southwest corner of Wacker Drive and Clark Street. This hotel and condominium skyscraper eventually will stretch to 1,047 feet, a foot taller than New York's celebrated Chrysler Building. Finally, there is the Spire, the twisting 2,000-footer at 400 N. Lake Shore Drive, which will be the nation's tallest building and the world's tallest all-residential structure. For now, it consists of a few holes in the ground, into which contractors will drive steel and concrete caissons reaching 120 feet down to bedrock.While other places have far more supertall towers under way -- the oil-rich Middle East playground of Dubai has a staggering 15, according to the Emporis international building database -- Chicago offers something that no other city can: a chance to view the present generation directly alongside the previous generation -- Sears Tower, the Aon Center and the John Hancock Center. That trio of giants, which was based on the structural concept of a "framed tube," redefined the skyline's silhouette between 1969 and 1974."What's going on here is fairly phenomenal," said Antony Wood, executive director of the Chicago-based Council on Tall Buildings and Urban Habitat, an international organization of architects, engineers, planners and builders. "It's a unique place to watch the evolution of the skyscraper art."That evolution is inseparable from the work of structural engineers such as Baker, who tend to work in the shadows of more celebrated architects. But maybe that is changing. The current cover of the trade journal Architectural Record carries the headline "Engineering the New Architecture." The lead story is titled "The Engineer's Moment." The point, as author Nina Rappaport asserts, is that engineers are assuming an expanded role in design rather than simply laboring as "consultants after the fact.""They're very important," agrees Chicago architect Adrian Smith, who collaborated with Baker on both Trump and the Burj Dubai, the underconstruction, mixed-use tower in Dubai that is expected to hit a record-shattering height of around 2,650 feet, more than half a mile into the sky. "The architects can conceive things to be built," adds Smith, who left Skidmore last year to start his own firm. "But very rarely do they have the expertise for how to keep it up and build it in the most efficient ways. You need a team of structural engineers to do that."Indeed, structural engineers have played an integral role in the development of tall buildings for nearly 125 years.Their contribution begins with architect-engineer William Le Baron Jenney, who was the first to use an internal frame of metal to support the floors and outer walls of a skyscraper in his now-demolished, nine-story Home Insurance Office Building in Chicago, completed in 1885. With its rigidly connected girders and columns, this method of skeletal, or "portal frame," construction liberated the skyscraper from the constraints of thick, load-bearing masonry walls and reached its apogee in the 1,250-foot Empire State Building of 1931.But portal-frame construction was supplanted in the 1960s and 1970s by the framed tube, in which the building's outer shell bore the forces of the wind and much of its own gravity loads. The Hancock Center, a collaboration of Skidmore, Owings & Merrill architect Bruce Graham and engineer Fazlur Khan, led this charge in 1969 with its industrial-strength X-braces and undeniable economy. The steel-framed, 100-story "Big John" was built for the same cost as a 45-story office building with a conventional structural cage. The Aon Center (originally the Standard Oil Building), Sears Tower and New York's World Trade Center all followed its pathbreaking example.Yet another revolution would sweep aside the framed tube in the 1980s and 1990s, this one spawned by architects' desire to break free from the aesthetic bonds of structural expressionism -- and by the irony that the closely spaced perimeter columns in some of the framed-tubed skyscrapers were cutting off the very panoramic views that their great height made possible.Instead, following the example of the unbuilt Miglin-Beitler Skyneedle -- a superskinny 125-story office building proposed for Chicago in 1988 and designed by New Haven, Conn., architect Cesar Pelli with New York structural engineers Thornton Tomasetti -- architects shifted from steel to new, high-strength concrete and ditched the framed tube for a combination of a concrete core attached to perimeter super columns.Here, science met art in a shift that allowed supertall towers to be both tall and thin.And the trend goes on.The Spire, for example, will have a height-to-width ratio of 10-1, compared with Sears' much-chunkier 6½-1. It will, its backers claim, be the most slender supertall skyscraper in the world. Trump, for its part, will have a roof that reaches slightly higher than its counterpart at the Hancock Center (1,133 feet versus 1,127 feet). But the building is 25 feet narrower at the base, skinniness already apparent in the building's flatironlike appearance when it is seen from the north, south and west.While such narrowness leads to fewer units per floor, it also means that "no part of the interior is very far from a window," says R. Shankar Nair, the former chairman of the Council on Tall Buildings and Urban Habitat and the structural engineer for the Waterview Tower. The lost square footage is made up by simply stacking more units in the sky.The core and outrigger system "is the standard for supertall skyscrapers today," Nair says.But unless you are standing on Wacker Drive and peering into the cleft in Trump's glass wall that is left open for the hoists that whiz up and down the tower's south side, all the structural heavy lifting is largely concealed from view. Baker, a genial man with gold-colored, wire-rimmed glasses, is happy to show it off, however.His tour of the tower, the tallest American skyscraper to be built since Sears Tower in 1974, begins near the entrance to the hoists where a sign on the wall says "Suntan Lotion (Non-Greasy) Please Us Often!" -- the "e" in "Use" is missing -- and simple, soaplike dispensers stand ready to pour out the goo.Above a construction elevator entrance, another sign -- this one displaying a picture of the tower's famous namesake pointing at the viewer like Uncle Sam in the old "I Want You For [the] U.S. Army" recruitment poster -- urges: "'Safety First' or You're Fired!"Nearby is the tower's core, which rests on caissons reaching down 110 feet to bedrock. The core looks very different from the core of a typical tube structure. In such a building, Baker explains, the center of each floor is porous, with elevators and exit stairs inserted into shafts formed by irregularly spaced columns and surrounded by drywall partitions."It's not a structural core that helps resist wind loads," he says.At Trump, however, there is no mistaking the core's heft. At the tower's base, it measures 150 feet long by 45 feet wide, made of high-strength, steel-reinforced concrete. It consists of five parallel walls, each shaped like an I-beam, with ends four feet thick. Escape stairs and elevators are placed inside. Such walls presumably would have been far more difficult to pierce than the cores of the World Trade Center's twin towers, which collapsed after fuel-laden, hijacked jetliners knifed through them Sept. 11, 2001.Baker declines to discuss the threat of terrorism, but Calatrava, who has designed a similarly robust core for the Spire, speaks directly to the issue. "Being so massive, it creates a kind of enormous protective barrier for the escape stairs and elevators," he says of the Spire's core.Soon, the hoists head to the 29th floor, a no-frills version of one of those glass-walled atrium hotel elevators. It is no accident that the three supertalls are clustered within less than a mile of one another on a new "gold coast" along the Chicago River. Peering outward from the freshly installed windows on Trump's 29th and 30th floors, where the distance between columns is a generous 25 feet, the visitor sees rumbling elevated trains that resemble centipedes crawling through the Loop. The clock tower of the neighboring Wrigley Building looks like an oversize toy. Because the tower rises from a bend in the river, it offers straight-shot views down the waterway and out to Lake Michigan.Despite the prospect of the wealthy enjoying such glamorous vistas, the skyscraper at this point resembles a vertical factory, a hive of 600 construction workers -- ironworkers, drywallers, plumbers, surveyors and others -- laboring away inside.The windows come shrink-wrapped in crates that travel up the hoists -- a far more efficient way of transporting them than having the project's two tower cranes carry them into place. The structural columns narrow as the tower rises because they don't have to bear as much weight. Less bulk means more room for condo and hotel dwellers -- and less money spent on concrete and rebar. On the upper floors, there are portable toilets on wheels for workers. At the Burj Dubai, Baker says, there even are upper-level dining areas for the construction workers, which saves them the trouble of traveling down 140 or so stories to eat lunch. (When completed in 2008 or 2009, the tower is expected to have 162 stories.)Now Baker descends concrete fire stairs to the 28th floor, where the outriggers can be found. They turn out to be 17½-foot-high, 5½-foot wide walls of concrete that reach out like arms from the core to the super columns on the tower's perimeter. Set perpendicular to the outriggers, and just as high, are so-called belt walls of concrete that, like a belt holding up a pair of pants, tie together the super columns."You want all the columns to work together, not like individual pick-up-sticks," Baker says.Air-handling equipment, electrical transformers, water pumps and other mechanical equipment will be threaded through openings in the outriggers. The ceilings above the outriggers are 30 feet high. With exposed concrete everywhere, the spaces have the feel of a nuclear reactor -- or, maybe, a spot for an edgy get-together."Great for a party!" Baker says.In the 1980s, a conventional wisdom developed that it no longer made sense to build supertall office buildings. More height would necessitate more material, it was said, and would leave more building surface to be blown around by the wind. In addition, more elevators would chew up more valuable floor space, rendering such towers uneconomical.Yet while it does take more time (and, thus, more money) to build the complex outrigger floors, the benefits of this way of building are now clear. They represent, in some respects, a return to the principles of the first skyscrapers that arose in Chicago, albeit in supersize scale.In the core and outrigger system, a tower can be more open to the outside than in a framed tube, taking advantage of surrounding views and bringing in more natural light. In addition, most of this new breed are residential and mixed-use buildings, which means they need far fewer elevators than do office buildings.The enormous weight of the concrete core and the super columns, meanwhile, helps reduce the wind-caused oscillation that can lead to whitecaps in toilets and swinging chandeliers -- an issue not unique to Chicago. While old-guard modernists might lament the split between a tower's bones and its skin, the freedom allowed by the core and outrigger system helps reduce the phenomenon known as "vortex shedding," in which swirls of wind bounce off one side of the tower and regroup to attack the other.Structural engineers and architects now do wind tunnel testing early in a skyscraper's design rather than waiting until the end to confirm whether their guesswork was accurate. That's one of the reasons Trump's corners changed from an angular look in early versions of the design to their current rounded, almost nautical shape, which suggests the streamlined ships, trains and cars of the 1930s.What this new way of building tall will mean for the public realm remains unclear, though a visit to the three-level riverfront walkway at the base of Trump reveals a strikingly open, handsomely proportioned public space, with views every bit as compelling as those upstairs. Yet new structural techniques by no means guarantee aesthetic success. Architecture remains a mix of technology and art, and the outcome of that story will only become apparent when the new giants finally assume their place on the skyline --Trump in 2009, Waterview in late 2009 or early 2010, the Spire in late 2010.For now, quiet engineering heroics are at center stage, making possible Chicago's next round of architectural fireworks.bkamin@tribune.com
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