Walking on Air

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Floating high above the sea of historic mid- and high-rises framing Grant Park in Chicago’s South Loop neighborhood, 1000M—Helmut Jahn’s final and tallest Chicago tower project—is a masterclass in found space. The 788-foot, 73-story luxury apartment tower at 1000 South Michigan Avenue opened in June 2024. Magnusson Klemencic Associates’ (MKA) straight, vertical, and orthogonal design of the tower’s initial 10 floors provides stability to the more structurally complex, space-reclaiming features above, including a cantilever that extends over a neighboring building and incrementally increasing floorplates that create an expanding twist in the building with height. Dual-purpose water tanks located on the tower’s rooftop level serve to dampen the structural response and provide the fire suppression water reservoir.

The development team behind 1000M had the luxury of owning two adjacent sites—1000 South Michigan, where the tower sits, and 1006 South Michigan, home of the eight-story Leightner Building. The 1000M site, only 100 feet wide, abuts that of the Leightner Building, its neighbor to the south. Tearing down the smaller building to increase 1000M’s footprint was not viable due to Leightner’s landmark-protected status. Instead, the development team set its sights on the air rights above and capturing space at height.

Jahn proposed a unique solution to widen the tower above the Leightner and create more space: a south-facade cantilever that slopes gently over the 100-foot Lightner Building, claiming air rights to maximize leasable floor area and unit count. MKA designed an all-concrete structure to suit, with primarily flat plate floors and lateral stability provided by a central core buttressed at its base.

McHugh Construction, its in-house concrete subcontractor McHugh Concrete, and concrete formwork supplier Peri Formwork Systems collaborated to build the 18-foot, MKA-designed sloped building extension, continuing each floor farther south over the floor below via sloping columns between Levels 11 and 20. At Level 11, a 19-foot “fire lid” slab extends over the neighboring Leightner Building, serving as a four-hour fire separation, protecting the 1000M building from potential heat from the roof of the Leightner Building should a fire break out.

At Level 20, where the steeply sloping southern columns resume a vertical path, 1000M’s appearance changes. The building’s lower floors, an aesthetic nod to the Historic Michigan Boulevard District and buildings of 200-foot scale, give way to a slender, carved tower designed to look at a distance like a separate building, complete with differentiating color and sheen.

Unlike the rectangular shape of the building’s base, the upper tower’s form begins in plan with symmetrically curved east and west faces and a flat aspect to the north and south. Diagonal planes then emerge from these straight north and south facades as each floor’s perimeter columns “walk” away from the center of the building to further expand the tower. This expansion continues at each of the top 50 stories of 1000M, growing the exterior bay dimensions from 30 to 43 feet and increasing the individual floorplate area by 900 square feet.

To accommodate the lengthening spans, MKA’s engineers designed a series of increasingly thicker flat slabs to maintain a flat soffit and avoid costly and intrusive beams extending beneath the slab.

As is common in the greater Chicago area, suitable soil bearing to support such a tall tower is hidden below layers of fill and soft clay. To reach these soils, large diameter belled caissons were drilled to depths of approximately 75 feet to support elements primarily resisting gravity forces. Due to the narrow site and inherently narrow stance of the lateral system on the foundation, caissons resisting the building overturning were drilled and socketed into bedrock at a depth of approximately 100 feet to achieve their required capacities. The use of deep foundations was also advantageous in bypassing the existing nearby foundations, ensuring the 1000M structure did not surcharge the adjacent properties.

To utilize as much width as possible on the narrow site, the tower is supported by a continuous caisson-supported mat that carries all the tower columns. This solution engages the total dead weight of the building to minimize net uplift demands on the deep foundations, thereby reducing the depth, size, and quantity of expensive rock-socketed caissons.

A building of this height naturally captures enormous lateral wind pressures acting on its surface. Of equal importance was resolving the massive gravity-induced lateral forces created by the steeply sloped building columns, which induce significant horizontal thrusts on the building at the top and bottom of their slope extents. Resistance to these additional forces is provided by the same structural system that resists the wind forces.

A core-only solution to resolving the totality of the lateral forces was infeasible due to a tower height-to-core width aspect ratio of over 20:1. MKA considered two solutions to augment the core: buttress walls or an outrigger system, both of which engage the exterior of the building to provide a much greater lateral stance.

An outrigger system was deemed too disruptive to the regularity of both the typical floor plan layout and the construction rhythm. Instead, the central concrete core engages with internal buttress walls aligned with core end walls and demise between apartments. The buttress walls extend to mid-height in the building and are end-capped by six-foot-square super-columns for the lowest 10 stories. The south-side sloping columns are tied to this system via heavy bands of high-strength reinforcing bars within the concrete floor slabs. As these heavy bands take up a large volume within the slabs and compete for space in the grid of post-tensioning tendons, a full-size mock-up was created to verify these elements could all be placed within a very tight area while maintaining the necessary configuration.

While the core and buttress systems were sized to ensure the building had all the necessary lateral strength, wind tunnel testing revealed the additional challenge of maintaining occupant comfort during significant wind events. The frequencies at which tall towers sway can resonate with occupants’ senses and lead to motion sickness. This effect is measured in minimal levels of acceleration, with an industry-accepted limit for residential occupancy at 18 milli-g during a 10-year storm. Rather than resolving the issue by adding stiffness through costly and space-taking increases in the size of the shear walls or super-columns, MKA employed two east-west oriented tuned liquid sloshing damper tanks strategically located on 1000M’s rooftop. Together, these dampers contain more than 33,000 gallons of water. These water tanks were tuned to the frequency of the building by careful measurement of as-built building motions and filling the tanks to a specific depth in relation to tank length, creating wave action matching those frequencies. The number and proportions of tanks were selected to activate sufficient mass to impart meaningful changes to the building’s response to wind excitations. To minimize the impacts of the massive tanks on the roof slab and overall concrete volume, the tanks abut the central core shear walls. This solution utilizes the shear walls themselves as part of the tank enclosure while also relying on the core stiffness to cantilever the tank boxes off the core and stiffen the roof span condition.

In another example of found space at 1000M, the damper tanks double as the building’s fire water reservoir. In the event of a fire, the damper tanks will drain to provide water for firefighting crews. Unlike other damping solutions, a general contractor can build liquid sloshing dampers using materials and crews already mobilized on site. Rebar is tied, walls are formed and cast, and waterproofing liner is installed as it would in a rooftop pool. A case study of NEMA Chicago tower, a similar residential high-rise, found that adding sloshing dampers for a construction cost of $1.4 million saved just under $5 million relative to the next best wind management solution.
1000M is a study in shaking off the constraints of site. As large city lots grow scarce, consider taking advantage of found space using smart engineering solutions that do not massively disrupt the flow of construction. As evidenced by 1000M, small incremental changes can add up to large benefits. ■