Concrete Puts the Waves In Aqua
Jeanne Gang, principal of Studio/Gang/Architects in Chicago, began planning the design of Aqua, her first high-rise project, by examining the way tall buildings relate to their surroundings. Gang's 822-foot, 2.2-million-sf mixed-use tower has been designed to fit into the context of its downtown Chicago neighborhood near the highly popular Millennium Park. But its location in downtown affected the views from the tower as well as views of the tower from outside the structure. The view of Lake Michigan would be great from the top, but what about the views from those middle floors that were blocked by other high-rises? Gang's answer was to make the view from each condominium or hotel balcony unique.
Gang and her design team constructed a model of the neighborhood, then used string to plot the endpoints of the views from each unit of the structure, which will feature condominiums, apartments, an 18-floor hotel, retail, and office space. By adding balconies that sweep in and out along the perimeter of the tower, Gang has created views that would not necessarily be available in a box-like building, while also forming undulating waves on the building's concrete exterior. Some units will look down on the city's Millennium Park, others will highlight the view of Michigan Avenue. The terraces don't just extend out to create exterior waves, either. Each balcony provides shade and an obstructed view from the floors above and below it. No two balconies are exactly alike. That makes Aqua equal parts sculpture and functional mixed-use tower.
The design presents construction challenges for local general contractor James McHugh Construction, whose concrete portfolio includes the city's corncob-like Marina City Towers of more than 40 years ago up to the new Trump International Tower along the Chicago River.
By adding balconies that sweep in and out along the perimeter of the tower, Gang has created views that would not necessarily be available in a box-like building, while also forming undulating waves on the building's concrete exterior.
“The balconies extend a maximum of 12 feet from the perimeter,” said David Alexander, SVP at McHugh. “So the perimeter of the building is essentially different on every single floor. Because of the cantilevered balconies, no hoist can be closer than 12 feet to the structure. The framing of each floor will be difficult, as the balconies will be cantilevered off the column line and cannot hold support for upper floors.”
To support the building while each floor plate is constructed, McHugh will build a solid temporary structure set more than 12 feet away from the building in all directions that will rise to the 82-story height of Aqua. To create each balcony, Alexander said McHugh workers will use 130 bridges that account for the length of each different balcony in the project.
“No two balconies are the same, so you have to build them uniquely every time,” Alexander said.
McHugh will also have to position its four construction hoists outside the 12-foot area of the balconies. Two hoists for the first 40 floors and two for the top half of the building will also be attached by bridges to the formwork system.
Another deep pour
The marshy lakefront land will require a deep concrete mat to enable the tower to achieve its 822-foot height. The Building Team will pour 2,500 yards of concrete into the former rail yard site where Aqua will break ground this spring. The Building Team also will sink more than 300 caissons into the site, while working around more than 1,100 linear feet of underground freight tunnels.
One advantage that McHugh does have is that Studio/Gang is a firm believer in the use of building information modeling (BIM). Thus, construction teams will be able to share construction documents and plans via PDAs and on-site computers. The shape of each of Aqua's concrete floor slabs, for instance, will be plotted via global positioning satellite coordinates extrapolated from the project's CAD files. “We'll have the CAD files so we can take any measurements from there,” Alexander said.
Tenants on site
It is anticipated that a number of tenant uses will come online before the Aqua project is completed in 2009. Construction workers will still be completing the upper floors when the hotel opens on floors 4-19. The apartments on floors 20-53 are planned to open early, too.
Alexander noted that the Trump International Tower Chicago will also open its lower floors before completion. “Safety is the biggest concern, of course,” he said. “Beyond that, moving construction crews around tenants is challenging,” with vertical transportation from only 24 elevators and four hoists in the whole building. There is also a need to control dust, noise, and water during construction. “It's something we've done before for tall building projects like this,” said Alexander. “I think it's going to become more common for these projects. This is the way they are being built now.”