Early in 2002, Craig McNay was having lunch with his old friend Bill Gerdes, a steel fabricator from Quincy, Ill., when he mentioned that he was serving as the owner's rep for a Miglin Properties project in Chicago's River North neighborhood.
McNay, chairman and CEO of Chicago's CMC Organization, told Gerdes that his new project, a 209-room hotel for the Staybridge Suites brand, would most likely utilize concrete construction. “Everything being thrown up in Chicago was concrete,” says McNay. “Miglin was looking for something new and a little different that would stand out in the city.”
For several hours the two friends discussed various construction methods, and by the end of the lunch, Gerdes had persuaded McNay to seriously consider steel construction, using a staggered truss framing system. Later, Gerdes brought McNay to the American Institute of Steel Construction's Chicago headquarters for a quick tutorial at the AISC's Steel Solutions Center.
The fundamentals of staggered steel truss frame construction—a method never before used in Chicago—involve a series of floor-height trusses (nine-feet, three-inches for this project) erected in a staggered pattern that span the width between two exterior columns and act as a cantilever beam when subjected to lateral loads.
The system's major advantage is speed of construction: the trusses are fabricated offsite and arrive ready to erect in any weather. They're also lightweight, which reduces the amount of foundation work necessary for a project. And they can be used in conjunction with concrete plank flooring, so the hotel would get both a semi-finished floor and ceiling in one installation. Finally, the system offered design flexibility and open floor plates, thanks to a significant reduction in interior columns.
Those advantages were enough to overcome any hesitation Miglin Properties might have had about being the first developer in Chicago to utilize the system.
The AISC introduced McNay to Structural Affiliates International, a Nashville-based structural engineering firm with considerable experience working with staggered steel truss systems. The association also hosted a training seminar to explain the system to all contractors bidding on the project. Structural Affiliates International joined the Building Team in spring 2002, well before an architect was hired.
“The owner wanted this to be a unique project, and I knew once we picked an architect, everything would fall into place,” recalls McNay.
The developer's short list of architects was finally winnowed down to Valerio Dewalt Train Associates, a Chicago-based firm, which joined the project in early 2003. Their first task: figure out what the staggered truss system could do.
“It's the first building we did with a staggered truss system,” says David Jennerjahn, principal at Valerio Dewalt Train. “We had this basic system, and we wanted to be creative with it while still being structurally responsible.”
Structural Affiliates International helped demystify the system, working closely with the architects on basic design parameters, budgeting, and efficiencies (they wound up specifying 115 trusses, totaling 815 tons of structural steel). “At first it seemed exotic, but it really isn't,” says Jennerjahn. “It's simply a different way of thinking how a structure goes together.”
During the design phase, the architects approached Structural Affiliates International about the possibility of a cantilevered truss system. “The structural engineer was very intrigued,” says Jennerjahn. “No one had asked him that before.”
Initially, the architects wanted to create two separate cantilevers, but load limitations bound them to just one, so they created a 17-story high-rise (with a four-story poured-in-place concrete garage base) whose south façade juts out seven feet, six inches on floors nine through 13. The resulting offset on the north façade creates the illusion of a cantilever on floors 14-17.
“The cantilevered design is a direct result of the staggered truss system,” says Jennerjahn. Some of the steel trusses were left exposed, reminiscent of Chicago's SOM-designed John Hancock Center and the city's many historic bridges. “We wanted to celebrate the structure instead of having this unique system buried inside the building,” says Matthew Dumich, project architect at Valerio Dewalt Train. “We wanted it to be part of the design, an expression of the building.”
Further enhancing the building's muscular structural design is its metal skin; when the project is completed this fall, the north and south façades will be sheathed in lightweight insulated aluminum composite panels, while the east and west façades will be glazed.
“It's so different from its neighboring painted concrete condo buildings,” says Dumich. Which is exactly what Miglin Properties wanted.