Chicago’s iconic Wolf Point site, which has eluded development for several decades, is now the proud home of one of the largest technology companies in the world, Salesforce, Inc. The 60-story, 850-foot-tall, glass and steel office tower is the crown jewel of the three-structure development that delivers office and residential living to this vibrant area of downtown Chicago.
Wolf Point, named after a Potawatomi Native American who once lived on the site, is located at the confluence of the North, South, and Main Branches of the Chicago River. Situated within Chicago’s central business district, the site sat dormant for decades, primarily due to its limited access and a 50% view corridor requirement mandated by the Apparel Mart directly to the north. The site is also at the end of a peninsula, making it susceptible to higher wind pressures coming down the Chicago River. Bounded by water and limited road access, development plans came and went, each hampered by site logistics and constraints.
That all changed in 2012, when private developers Hines Interests LP and Park Holdings Group LLC, the principal investment entity of the Kennedy family, set their ambitions on collectively developing the challenging site. The development team pulled together a strong project team that found solutions to construct this slender office tower, which is sandwiched between existing residential towers to the east and west and located only a few feet from the Chicago River to the south.
Managing Wind in the Windy City
The slender tower located on the site’s southern tip is exposed to substantial winds traveling north and west along the river. This “urban canyon” along the Chicago River framed great views but did not provide the level of wind protection at the Wolf Point site typically afforded by the skyscrapers of the Chicago Loop. The wind funneling between tall buildings along the river can create unusual wind loads on the tower.
Engineers performed wind tunnel studies to determine the best tower orientation on the site and to sculpt the tower’s top to reduce the wind loads optimally. According to the studies, the wind pressures channeling down the river closely resembled an Exposure Category C rather than the more typical Exposure Category B found in Chicago’s urban development. The increased wind loads on the tower required a robust lateral system to resist the horizontal wind forces and associated overturning moments.
A Fortified Lateral System
The active winds at Salesforce Tower Chicago are resisted by a central concrete core lateral system that wraps 26 elevators, exit stairs, restrooms, and primary vertical MEP/FP risers. With an aspect ratio of 20:1, the concrete core alone does not provide enough stiffness to meet the tower’s strength and serviceability requirements. To achieve the required strength and stiffness, a two-story mechanical space at Levels 38 to 40 is used to introduce steel truss outrigger trusses that reach out to perimeter columns to stiffen the concrete core. The outriggers widen the building’s stance to a modest 7:1 aspect ratio.
The large wind forces require each truss to deliver nearly 4,500 kips of horizontal load and 3,000 kips of vertical load to the core. Domestically produced A913 high-strength steel (65 ksi) is used for the various truss members to resolve the loads with smaller member sizes than conventional steel. High-strength steel is also utilized for all gravity columns to further economize the steel tonnage. The use of high-strength steel reduced the project tonnage by approximately 500 tons, resulting in smaller foundations, cost savings to the owner, and lighter “picks” for the tower crane.
A well-conceived bolted connection detail distributes the large outrigger forces through the core/outrigger truss interface. Two 3.5-inch-by-24-inch steel tie-plates are embedded through the core wall, providing continuity between the steel truss chords on each side of the core. These double tie-plates penetrated the core formwork, allowing a double-shear bolted connection through the web of the jumbo W27x539 chord members. The erection team recognized the importance of the extremely tight tolerances required to install these tie-plates. Detailed plans were carried out with the help of robust erection aids, multiple survey shots, and carefully executed ironworker choreography.
A two-story steel belt truss is provided along the perimeter, perpendicular to the outriggers, to distribute outrigger forces across all seven columns on each side of the tower. The steel belt trusses reduce peak demand in each given column and, more importantly, minimize unbalanced transient axial stresses at adjacent perimeter columns. At 60 stories tall, it is critical to evaluate the perimeter column’s elastic shortening during erection to achieve floor levelness in concert with the stiffer concrete core. Traditional gravity-only columns are more highly stressed than outrigger columns that are oversized to provide increased axial stiffness. The belt trusses allow the perimeter columns to be more uniformly stressed and minimize differential shortening between adjacent columns, ensuring tighter tolerances when installing the enclosure system.
Site Logistics and Below-Grade Challenges
The surrounding river and limited site access created challenging logistics for the construction. Salesforce Tower was the last of three structures to be built within the development. Also, it was in the most precarious location—sandwiched between tall residential towers to both the east and west and an existing garage to the north.
Fortunately, the design team of the east residential tower anticipated this challenge by designing a two-story haul road within its below-grade parking structure to connect the only site access road to the tower’s future loading dock. This loading dock served as the primary layout space for contractors, with all deliveries, including steel beams and trusses, passing through the below-grade haul road. The tower’s loading dock, which was eventually covered by a public plaza, remained open to the sky throughout construction, allowing the tower crane to pick materials directly from the truck beds and lift them to the working deck.
When the building topped out and the tower crane was dismantled, the loading dock was covered to finalize the plaza construction. The expansive public plaza ties the streets, river, tower lobby, and retail together. The heavy plaza loads, comprised of deep soil and public assembly loading, are supported by a series of long-span concrete beams that clear-span the column-free loading dock below.
It’s All About the Views
The typical floor plates are configured to maximize the views above the river, and the tower’s base is shaped to maintain the required view corridors through the site.
The primary floor plate steps inward below Level 7, then again below Level 4 to open views through the site required from the north. These setbacks result in 80-foot-tall perimeter columns with only one discrete point of bracing at Level 4.
The final setback defines the two-story lobby as a column-free space, with Level 3 hanging from Level 4. The lobby’s defining architectural feature—a downward cantilevered glass fin wall — hangs from Level 3 to provide a thin glass wall that envelopes the first floor’s entire perimeter. A construction analysis was implemented to predict the movement of the multi-tiered hanging support relative to self-weight and the future build-out of the space. The large, cantilevered slab edge supporting the glass fins was soft-shored around the perimeter, leaving an intentional gap to account for the anticipated deflection while maintaining a constant, level elevation around the perimeter to bottom-out the slab edge as the concrete was placed. This maintained the stringent elevational tolerances necessary for the glass fin wall installation.
Attention to Detail
A critical component to reducing high-rise structural costs is to design an optimized typical floor plate, as the benefits or mistakes are repeated throughout the height of the tower. At Salesforce Tower, the architect introduced several re-entrant corners around the typical plan to maximize perimeter glass area, creating space for 12 corner offices, 10 of which are column-free. The column-free corners provide architectural benefits, but also eliminate the need for two columns at each floor level. The columns are replaced with carefully tuned cantilevers coordinated with the mechanical distribution system. The project team worked closely to economize potentially costly edge details and standardized the myriad of corner and notch conditions essential to the building’s iconic form.
The project team designed and implemented a rigorous surveying and monitoring program for the structure. The contractor, erector, and structural engineer established a direct line of communication to expedite the review of the frequent survey results with three rounds of data per tier (two floors). This allowed the team to create a quick feedback loop on column shortening that was continually calibrated with staged analysis modeling using ETABS results that were post-processed in Excel. This data informed the column shim adjustments necessary to react to the erection, column shortening, and differential settlement variables around the building perimeter to confirm the structure was within tolerances.
Salesforce Tower Chicago topped-out in May of 2022 and is on track to open later this year. Open communication between the design, construction, and ownership teams was key to maintaining the project schedule on this challenging site and the tower’s ultimate successful completion.
Project Team
Design Architect: Pelli Clarke & Partners
Architect of Record: HKS
Structural Engineer: Magnusson Klemencic Associates
MEP Engineer: Alvine Engineering
General Contractor: Walsh Group
Concrete Subcontractor: Walsh Group
Steel Fabricator: Cives Steel Company
Steel Erector: Danny’s Construction Company
Connection Engineer: Drucker Zajdel Structural Engineers