Freestyle Formwork

Strips of Steel Swimmingly Shape Aqua’s Flowing Facade

by Tudor Van Hampton

A sea change is coming to Chicago’s skyline, and the $475-million, curvy skyscraper rising along North Columbus Drive is already making waves with the workers.

“This is my favorite spot,” says David Alexander, senior vice president of Chicago-based general contractor James McHugh Construction Co. Alexander’s eyes ignore the trash dumpster and perimeter fence near him and instead focus on the mesmerizing, southwest-corner slab edges of “Aqua,” which is nearly 50% complete after breaking ground in February 2007.

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The 2.3-million-sq-ft project is scheduled for phased occupancy starting in 2009 and completion in 2010.

Rising more than 870 ft and 82 stories out of an eight-level podium with 140,000-sq-ft average floor plate in the podium, Aqua has all the elements of an urban landmark.

“Having never designed a tower before, we started thinking about it in a fresh way,” says Jeanne Gang, design architect with the Chicago-based firm bearing her name. “With towers, you look up, so you have in mind how would this building be perceived from the street.”

A Curvaceous Condo

Supported from a central core, columns, shear walls, outrigger walls and belt walls, Aqua’s curvy slabs, averaging 16,000 sq ft in concrete, cantilever far past a glass-perimeter window-wall from zero to 12 ft to take advantage of city and lake views.

The protruding slab edges undulate, giving the effect that the facade is rippling in the breeze. No two slabs are the same. Even in the negative space between the curves, it appears as though someone randomly erased the edges of the building. The project team has dubbed those edges the “burn” zones.

The slabs span an average of 28 ft between columns and are 9 in. thick, only about 1 in. thicker than normal, says Ron Klemencic, the structural engineer with Seattle-based Magnusson Klemencic. They taper to 4.75 in. thick at the extreme edges to facilitate drainage. Eventually, they are painted a soft-white that lights up in the sun to contrast against blue-green window glass.

About 7 mi of balcony railings will contain the curves, which are extremely long for building balconies. “If we start to go farther than 5 to 6 ft with the cantilevers, we start to get a little concerned,” Klemencic says. So the relatively thin slabs and wide bays called for careful construction methods and bigger columns, averaging 30x50 in. and that bear on superhigh-capacity rock caissons.

The site’s 31 rock drilled shafts, between 5 and 10 ft in diameter, are socketed 6 ft into dolomite down to 112 ft below grade. The piles consist of a 9,000-psi concrete tested to hold 250 tsf using an Osterberg load cell. A set of 274 smaller, belled caissons that bear on shallower, hardpan clay dot the soil underneath the podium.

Only the up-and-coming Trump International Hotel & Tower and Chicago Spire exceed such foundation capacities, city records indicate. Trump has bearing pressure of 270 tsf, and the Spire has bearing pressure of 300 tsf.

A New ‘City within the City’

If Chicago’s Marina City towers defined modern construction in the last century, Aqua is poised to break the next mold. The 61-story, corncob-shaped twin Marina City towers, completed in 1964, were called “The City Within the City,” a place where residents could live, work and play.

Aqua, too, ties multiple uses: hotel, rental, condo, office and retail spaces serving as a bookend to the $4 billion Lakeshore East campus with Lake Michigan on the other side.

That level of diversity in one building, “Nobody’s ever tried it in Chicago,” says Jim Loewenberg, architect of record and co-chief executive of Lakeshore East owner Magellan Development Group.

Occupancy has not been a problem for Marina City, and it appears the same may hold true for the nearly sold-out Aqua. It will feature 968 housing units 474 rental apartments, 264 condominiums, 225 hotel rooms and nine park homes.

Like the concrete corncobs, Aqua requires special construction, and the same contractor is reprising the role. For shaping the funky slab edges, McHugh tapped concrete superintendent Paul Treacy, who experimented before settling on a light-gauge steel, about 1/8-in. thick. Surveyors plot radius points every 1 to 4 ft from design drawings and feed them into a robotic total station, which strikes them out in the field. Engineers also use a computer-modeling program more commonly employed on serpentine highways to lay out the design.

With enough formwork to shape half the tower’s footprint at a time, the flexible steel articulates to follow survey points but returns to a straight, reusable edge when removed. Anchor clips, welded to the plates every 16 in., nail to the formwork below.

“Essentially, you connect the dots,” Alexander says. Workers form the “burnt” edges using straight plywood or a 1x10 board. Edges are quickly troweled smooth because it would be cumbersome to hang a swing-stage scaffold from the building, says Treacy.

Gang specified a tolerance of 2 to 3 in., but the survey technology is allowing workers to form to 1/8 in, according to McHugh. Weight is a bigger issue. The builder cannot shore the slabs after pouring because the long cantilevered balconies, with their 5,000-psi concrete, are not allowed to take “wet” loading.

The builder opted to employ 25x50-ft Aluma steel-truss forms, which hang on brackets bolted to the 5,000- to 12,000-psi columns. There is a three-day pour cycle on the project.

Four, 7,000-lb-capacity, temporary rack-and-pinion hoists shuttle workers and materials into the building. But they, too, cannot bear on the slab edges. McHugh is building nearly 300 hoist bridges, which cantilever over the 12-ft-long balconies but bear on the slabs behind the perimeter columns. Workers bolt the hoist towers’ support struts into threaded coils cast into the concrete slabs.

Attending to Hotel Guests

Hotel facilities in the podium and lower-tower floors will open prior to substantial completion, and guests will not want to look at construction. So, McHugh has concealed a single Liebherr tower crane near the building’s southeast corner, between the columns and core.

The chase is sealed as the crane’s base climbs. It services the flying forms, while a dedicated Putzmeister placing boom in the core supports concreting. The core rises from a self-climbing scaffold.

For reinforcement, McHugh ordered steel bars in three different sizes rather than fabricating rebar to custom lengths in the field. “Although we are paying more for the extra steel, the labor savings more than offsets it,” says Alexander, who adds that McHugh is looking to reuse the steel forms on other projects.

With nearly half of the slabs compete, Alexander says completion is all about maintaining speed and repetition to keep up a three-day-pour cycle.

Efficiently shaping complex slabs is what made Marina City a technical signpost, Gang says. Likewise, she says Aqua “is going to be the first building that will mark that transition between old technology and new.”

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