|April 16, 2014|
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|CHICAGO – The Young Professionals Committee of the Council on Tall Buildings and Urban Habitat (CTBUH) facilitated a roundtable discussion on Chicago’s under-construction mixed-use development 455 North Park Drive, at Crown Hall, Illinois Institute of Technology. The team behind 455 North Park emphasized the importance of early coordination among consultants at the concept and schematic design phases.
Presenters included Joe Fox, AIA, LEED AP BD+C of Solomon Cordwell Buenz, Robbie Chung, PE, LEED AP, of Environmental Systems Design and Matt Streid, SE, of Magnusson Klemencic Associates.
Fox provided an overview of the $204 million project, a 173-meter, 54 -story tower containing 400 hotel rooms, 398 residential apartments and a 230-space parking garage on a site just north of the Chicago River in the popular Streeterville area.
The project faced numerous challenges, including a sloping site that complicated service-vehicle access, protected view corridors to the Tribune Tower, and relatively unstable soil, due to its location on the filled-in far reaches of the Ogden Slip, a shipping channel. Before schematic design could begin, negotiating permission to build involved a six-month process with reviews by neighborhood groups, the Chicago Plan Commission, and the Chicago Planning Department.
|Proposed vision for 455 North Park Drive|
The team looked at least six scheme comparisons, weighing the number of dwellings against the hotel units and public program, resulting in floor-area-ratios of 8.02 to 15.24.
The team reviewed at least three basic orientations. A review of view corridors and adjacencies determined the best orientation was a tower + “leg” (podium) combination aligned on the site’s north-south axis. The “L” shape of the project’s plan was selected because it reduced the costs of the structural frame, construction time, and elevators, while giving the hotel owners the large floor plates they desired, Fox said.
Concrete framing was used for the main tower, while long-span steel was used for the ballrooms in the podium, so as to offer column-free space.
|Attendees at the presentation.|
|The original scheme also involved a public park that would have connected the project to the Chicago River by way of an elliptical staircase. This was later scrapped due to budget constraints, Fox said.|
Many of these changes occurred in the initial conceptual and schematic design phases, which the presenters said was typical for a project of this scale. It was nearly a year before major consultants, including structural engineer Magnusson Klemencic Associates, were hired.
“We rely on SCB and their experience in building high-rises to set up the basic column grid and core locations so that we’re in a good starting position,” Streid said. “It was important to get on the same page with our structural assumptions and stick with slab thicknesses, MEP service locations, and stairways as we moved forward.”
Likewise, early coordination with contractor Bovis Lend Lease helped ensure the project would be buildable from the outset, starting with the basement, where a typical 5-inch (127 mm) slab-on-grade was not an option due to the poor soil condition of the filled-in waterway. Instead, the team had to design the slab as if it were a bridge or floor higher up in the building, treating the poor soil location as a void to be crossed. That meant a thickness of 12 inches (305 mm), driving up costs.
|Proposed entrance for 455 North Park Drive|
|“Lend Lease was brought in early on in the concept design phase, almost in a design-build role, which was very helpful, because they were able to flesh out ideas with us,” Streid said.|
Those ideas and assumptions were further tested in a wind tunnel, which MKA performs on any building over 200 feet (61 meters). The design evolution involved rejecting some undesirable structural schemes, including one that would have required protrusions into some of the residential and hotel units. The structural engineering team had to bear in mind three objectives: serviceability, specifically, the effect of structural stress on finishes and plumbing; occupancy comfort, controlling for unpleasant accelerations due to wind; and strength, in this case, assuring the integrity of link beams without compromising the desired ceiling heights in corridors.
The wind-tunnel testing had to be completed at least six weeks before issuing applications for permits to lay foundations, Streid said, which meant that the testing had to be frozen at a given point in time so the design team could coordinate technical drawings. No subsequent design change could reduce the core wall dimensions submitted to the wind-tunnel test by more than 10%.
|Rendered view of 455 North Park Drive within the city context.|
|Coordination with MEP was also a key component in realizing the project. The requirements of the MEP system often determine structural choices, Streid explained. For instance, while office building floors are often in steel in order to accommodate heavy ductwork above the finished ceiling and below the floor slab, typical residential high-rises use flat-plate concrete, the shallowest floor system, to minimize building heights. In Chicago this presented a particular challenge, because local code required cast-iron fittings up to six inches’ (152 mm) depth, leaving very little tolerance in an eight-inch (203-mm) post-tensioned slab that also had to accommodate reinforcing bars. The requirements for hotel rooms differed from the residential units, adding more complexities.|
The 455 North Park design team illustrated in great detail the level of coordination required to make such a project successful. It was unsurprising to learn that the total project lifespan is estimated at 49 months, or just over four years. The project was begun in the first quarter of 2011. Occupancy is scheduled for the second quarter of 2015, Fox said.