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Thornton Tomasetti provided both structural engineering and façade engineering services for the new 415,000-square-foot Global Hub — the new home of the Kellogg School of Management at Northwestern University in Evanston, Ill. The Global Hub consists of four, six-story concrete “lofts” positioned around two vertically stacked central atriums. The ground floor features a three-story atrium known as the Collaboration Plaza. This space is surrounded by a network of pedestrian bridges, cantilevered balconies, and monumental stairways, and is accented by a “floating” collaboration room at the top of the space.

Directly above the Collaboration Plaza is another two-story atrium known as the Faculty Summit, which also has interconnecting monumental stairs and flanking seminar rooms. Additional features of the Global Hub include large, column-free flexible classrooms, the two-story White Auditorium, light courts at each loft, extensive exterior cantilevered terraces and canopies, and a fitness center. A structural steel penthouse sits atop each loft, contains mechanical and HVAC equipment, and forms the roof of the Faculty Summit.

Amid numerous structural challenges, the dissimilar column grids of the lower-floor classrooms and upper-floor offices presented the opportunity for a unique structural solution. A hidden network of post-tensioned concrete beams was developed to support the 169 transfer columns needed to realize the dramatic setbacks and overhangs prominent throughout the building. Thornton Tomasetti addressed this and other design challenges without sacrificing budget or architectural intent.

The five sets of monumental stairs required sophisticated analysis and structural steel solutions. The Collaboration Plaza’s Communication Stair is a curved single span of 55 feet. Two additional sets of curved stairs stretch 35 feet each at the Faculty Summit and two sets of 34-foot-wide Spanish Steps serve as gathering spaces in the Collaboration Plaza. Creative and technical solutions assured openness while adhering to structural considerations of stability and acceptable vibration limits.

Thornton Tomasetti used BIM to coordinate and document the design. This rendering (top or left) from the model shows the main entrance looking east. Thornton Tomasetti’s Façade Engineering team used a building envelope system comprising a segmented, bespoke unitized curtain wall (bottom or right). A glass fin of variable depth was used to further accentuate the building’s form. Images: Thornton Tomasetti

The complex nature of the building’s geometry necessitated special attention to structural performance for occupant comfort. The main design considerations for the meeting and study rooms, which are situated between concrete lofts, included minimizing vibration and creating spaces that foster deep and collaborative work. The architectural vision was achieved by the fusion of concrete and steel framing solutions — a marriage requiring careful planning of connection details and material compatibility.

For example, the architect’s concept of a floating meeting space was realized by suspended steel framing enclosed by full-height glazing. In collaboration with Thornton Tomasetti’s façade engineering practice, the thoughtful analysis of the base structure and back-up steel considered both the long-term deflection of cantilevered concrete floor systems and the differential deflections at multistory wall systems.

One of the architectural features of the building is a “floating” collaboration room, which is suspended from the underside of the fourth-floor framing between the northwest and northeast lofts. The room is stabilized against sway in the north and south directions by diagonal bracing concealed in the sidewalls of the box, while inverted moment frames are used for stability in the east and west directions. Photo: Thornton Tomasetti

To create the signature lofts and harbors, the façade engineering team worked with the client to identify various strategies for curved enclosures. Ultimately, the design team opted for a segmented, bespoke unitized curtainwall on the main body of the building. This system includes a glass fin of variable depth, which further accentuates the building’s form. The system was designed for a wide variety of loading scenarios due to the variable building radii and fin depth. To enhance occupant comfort and reduce burden on the MEP system, triple-glazed insulated glass units and thermally broken mullions combat the extreme winter lakeside temperatures.

The auditorium space in the southeast loft, along with some of the harbor façades, called for tall-span systems. The façade team selected a minimal structural steel-supported curtainwall that is both elegant and maintains the high weather performance of traditional curtainwalls.Thornton Tomasetti’s façade and structural practices collaborated to identify slender HSS sections that met the structural and aesthetic requirements.

Other façade systems of note include the metal panel rain-screen system cladding the canopies and the structural glass entry vestibule, where curved glass panels are the self-supporting enclosure wall.

The design team featured 20 specialty design consultants. This collaborative team innovatively delivered on the project’s aspirational vision, which complements Kellogg’s long-term goals of remaining one of the world’s top graduate business schools. Thornton Tomasetti worked with specialty consultants to implement sustainable design elements, such as in-slab radiant tubing for passive climate control and raised access flooring.

The building’s exterior features dynamic canopies that accentuate the curves of the structure, specifically at the roof and entryways. Wind tunnel testing generated load data that accurately captured the effects of lakeside winds and irregular geometry on the canopies and wall systems. Thornton Tomasetti worked with the client to develop a canopy form that supported the client’s architectural vision while maintaining structural efficiency.

Information provided by Thornton Tomasetti (www.thorntontomasetti.com).