Joe Porada, SE, PE, Kevin Jackson, SE, PE, And Chris Porst, SE 2024-03-02 12:51:40
A new sportsbook adjacent to Wrigley Field showcases steel’s ability to integrate into an existing structure.
THE LATEST ADDITION to the Wrigley Field campus was among the trickiest to execute.
The new DraftKings Sportsbook—which opened for food and beverage service in June 2023—follows the 1060 Project, a multi-year $1 billion restoration and expansion of the Chicago Cubs’ historic home ballpark. The 1060 Project focused mainly on restoring the 100-year-old ballpark, adding fan amenities and expanding the Wrigley Field campus. Building the sportsbook, though, meant adding a structure to the iconic stadium.
The sportsbook is situated on a triangular-shaped site between Wrigley Field to the north and west, Addison Street to the south, and ballpark entry and exit gates along Sheffield Avenue to the east. It’s a three-story structure that packs a lot of functionality into a compact 9,400 sq. ft footprint with 279 tons of structural steel. Its massing and aesthetic matches Wrigley Field’s exterior and was approved by the National Park Service long before it was envisioned as a restaurant or sportsbook, meaning no major alterations could be made to its exterior for its interior transformation.
Structurally designed by Thornton Tomasetti, the sportsbook occupies the ground and second floors. Those floors also include a kitchen, back-of-house circulation, and offices. The east end features a large two-story space anchored by a 2,000 sq. ft wraparound video screen visible with sightlines from each seat in the sportsbook. The second-floor balcony is raked to provide similarly unobstructed sightlines. Hiding below the ground floor is an 18,000-ft3 concrete detention tank.
The roof serves as a terrace extension of Wrigley Field’s upper concourse with additional public space, concessions, and restrooms housed in the mechanical penthouse. The roof structure consists of steel trusses spanning nearly 70 ft north-south over the width of the site.
New Meets Old
One of the project’s primary challenges was coordinating the new sportsbook structure with Wrigley Field. The new spread footing foundations, ground floor slab-on-grade, steel column locations, and steel framing at the expansion joint between the sportsbook and ballpark all required careful coordination with the existing documentation and as-built drawings from the 1060 Project.
Fortunately, the project team at Thornton Tomasetti that worked on the design of the sportsbook also worked on the 1060 Project. That experience helped the team build the sportsbook and detention tank adjacent to the ballpark foundations with varying bottom-of-footing elevations without undermining the existing structure.
Structural steel was a natural choice for several reasons. Primarily, the triangular site introduced building geometry challenges that steel framing could overcome. With the ballpark directly adjacent to the sportsbook, the building columns were set back from the edge of the slab, creating a 10-ft cantilever to the north. Additionally, the two-story seating space featuring a raked balcony required 17-ft cantilevers to the south off the same columns. Structural steel achieved the double cantilever with limited deflection and vibration.
The sportsbook was designed shortly after the Chicago Building Code was revamped to require seismic design considerations for all structures. Although the site’s location defined the building as moderate seismic risk in SDC B, the building’s triangular geometry caused the structure to be torsionally irregular.
Furthermore, the setback from the existing ballpark triggered a vertical irregularity in the seismic force resisting system, and the large floor opening at the second-floor balcony resulted in diaphragm discontinuity and weak story irregularities. The low self-weight of structural steel and limited seismic risk in Chicago helped the team design for those structural irregularities. Steel’s low self-weight also allowed the foundations to be shallow spread footings and avoid more expensive deep foundations.
Structural steel also allowed the framing to be delivered, staged, and erected in the middle of the 2022 baseball season. Deliveries were made and staged on the east end of the site along Sheffield Avenue, and contractor Pepper Construction orchestrated careful scheduling of these deliveries to avoid overcrowding the confined site and conflicting with Cubs games.
Major deliveries and site activity were limited on game days, on which Pepper was required to shrink the site by temporarily restoring the pedestrian sidewalk for fans along the site’s southern edge. The steel erection was divided into four sequences, beginning with the framing for the two-story space at the east end, followed by the mechanical penthouse above, and finishing with the west end.
Pepper used a 260-ton crane to set the two easternmost trusses, a 240-ton crane to set the remainder of the two-story space and mechanical penthouse, and a 60-ton crane for the west end.
Truss Design and Vibration Considerations
The design of the steel roof trusses incorporated a sloping bottom chord to maintain the sightlines at the top of the video screen. While the lengths of these five roof trusses varied from 40 to 70 ft due to the triangular site, the kink point of each bottom chord remained fixed 30 ft from the truss’ south end to preserve these views. The video screen was hung either directly from these trusses or from wide flange framing spanning between the trusses. These locations, along with the bracing of the truss bottom chords, were coordinated with the video screen manufacturer and architectural team.
Since the trusses are part of the lateral system, the bottom chords experience compression, and bracing the bottom chord was a critical component of the truss design. Double-angle shapes comprise the chord and web members of the trusses, and the connections are bolted to be consistent with the existing trusses throughout the ballpark. Like the second floor, the roof trusses cantilever towards the ballpark at their north end to allow the setback of the foundations.
The connection design of the various bracing, moment, hanging, truss, and shear connections was completed by Thornton Tomasetti’s Kansas City, Mo., office, which made for seamless coordination between the structural design team in Chicago and the connection design team in Kansas City.
Two areas of the steel structure were particularly sensitive to vibration: the cantilevered balcony on the second floor and the roof trusses above supporting the occupiable roof deck.
Vibration of both areas was evaluated with a finite element model created using the recommendations in AISC Design Guide 11: Vibrations of Steel-Framed Structural Systems due to Human Activity. Since the roof trusses and the columns supporting the cantilevered second-floor balcony were part of the typical moment frame on each column line, these two areas were included together in the FEM model.
The steel framing was modeled as frame elements, and the floor slab was incorporated using area elements. Including the slab in the model allowed for realistic mass and mode shapes to be considered and accounted for the composite action between the steel framing and composite slab.
The composite action was quantified by calculating property modifiers that were applied to the frame elements in the model. The property modifiers were determined for strong axis bending using the ratio of transformed stiffness to bare beam stiffness. The transformed stiffness considered the fully composite section moment of inertia, because it’s reasonable to assume the full composite action can be developed under small vibration loads. Property modifiers were also applied to the area elements to account for the one-way span behavior of the composite slab.
After properly configuring the model, a modal analysis evaluated the second-floor balcony, and a steady-state function was defined to analyze the roof trusses. Since the balcony is an indoor seating space, its framing was designed for walking excitation and is classified as a restaurant and public space with seating, which limits RMS acceleration to 0.70% of gravity.
On the other hand, the roof trusses are subject to rhythmic excitation because they support outdoor space filled with excited baseball fans. Thus, the design activity was classified as a lively concert or sporting event space, which has a higher corresponding RMS acceleration limit of 3.5% of gravity. The balcony framing and roof trusses met those criteria.
Using structural steel to overcome the site’s geometric constraints while achieving long, cantilevered spans with limited deflection and vibration helped the Cubs complete a unique addition to the Wrigley Field campus that complements the renovations made over the last decade. By way of several intentional design decisions, the DraftKings Sportsbook has been incorporated into Wrigleyville’s fabric while maintaining the ballpark’s historic appeal and aesthetic.
Owner
Wrigley Field Holdings, LLC
General Contractor
Pepper Construction
Architect
Gensler
Structural and Connection Engineer
Thornton Tomasetti
Fabricator and Detailer
LeJeune Steel
Joe Porada (jporada@thorntontomasetti.com) is a project engineer, Kevin Jackson (kjackson@thorntontomasetti.com) is an associate principal, and Chris Porst (cporst@thorntontomasetti.com) is a senior project engineer, all with Thornton Tomasetti.
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