Chicago’s skyline tells the story of American structural engineering, and few recent chapters are as remarkable as the Vista Tower. Now known as the St. Regis Chicago, this 93-story, approximately 1,200-foot supertall building represents a masterclass in concrete construction by Magellan Development, Studio Gang Architecture, and Magnusson Klemencsic Associates (MKA). McHugh served as general contractor on this demanding project requiring meticulous planning, innovative formwork, and high-performance concrete. The building envelope challenges at this scale are immense, and understanding the principles behind Building Wrap Selection Installation and Performance of Weather resistive barriers becomes essential when constructing at heights where wind and temperature differentials test every material specification.
Pre-construction talks began in 2012, four years ahead of the August 2016 groundbreaking. The team worked out constructability details with Studio Gang, the Architect of Record bKL, and MKA, and determined early on that the project could achieve a three-day construction cycle per floor. This article examines the engineering innovations, concrete technology, and construction methods that made the Vista Tower a landmark achievement in structural concrete building.
Design Innovation: The Frustum Geometry and Structural Challenges
The Frustum Concept
Studio Gang settled on a design using geometric shapes called frustums. In geometry, a frustum is the portion of a solid, normally a cone or pyramid, that lies between two parallel planes cutting through it. For the Vista Tower, these frustums are inverted on top of each other so that the three interconnected towers angle outward and inward as they rise, producing a distinctive rippling silhouette.
Wind Engineering and Occupant Comfort
During the design and engineering phase, supertall buildings undergo extensive wind tunnel testing to study wind shear. Building models are placed in wind tunnels to observe how wind flows around the structure. Engineering changes often follow to further minimize wind shear forces. One result of the Vista Tower testing was the decision to include two floors of damping tanks near the top of the building to help reduce building sway, ensuring occupant comfort even during high wind events.
The Staggering Column Design
An unusual feature of the Vista Tower is that the outside columns stagger from floor to floor, shifting 5 inches from the column below so that there is no vertical load path to the ground. These corner columns move in two directions simultaneously. MKA engineered the transition level floors to handle these complex load transfers. Perimeter columns shift 5 inches outward for 12 to 13 floors, creating a cumulative shift of 5 feet 5 inches. They then shift back toward the center of the building at the next transition level. This intricate geometry required advanced Building Retrofitting Structural Strengthening Methods for Seismic Upgrades principles to be applied at every transition floor.
Foundation Engineering and Concrete Technology
Caisson Foundation System
Max Levin, the project manager for McHugh Concrete, reported that the foundation consists of 124 caissons encased in steel liners and heavily reinforced. Of these, 24 are 10 feet in diameter and extend as long as 115 feet, socketing 6 feet into bedrock. The remaining caissons are bell shaped, flaring out at the bottom to distribute loads over a wider bearing area. This anchors the tower to bedrock and provides the needed stability.
Mat Slab Construction
According to Mike Fields of McHugh, there are two mat slabs, one under each tall tower core structure. The thickness differs for each: 8 feet for the shortest tower and 10 feet for the tallest one. A low-heat 7,000 psi self-consolidating concrete (SCC) mix design was used for the mats. The spread of this mix varied between 14 inches and 26 inches, providing the flow characteristics needed to fill the dense reinforcement cages without segregation. Self-consolidating concrete eliminated the need for internal vibration in the heavily congested mat reinforcement, reducing labor requirements and ensuring uniform consolidation throughout.
High Modulus of Elasticity Concrete
Concrete with a high modulus of elasticity (MOE) is necessary for supertall buildings to increase stiffness and minimize building sway. Occupants feel uncomfortable when structures sway in the wind. Testing high-MOE concrete requires certification and special equipment. Flood Testing Laboratories performed all the testing, with Nick Beristain of Prairie Material reporting approximately 20 mix designs for the project. These included 7, 6, and 5 million psi MOE mixes for different locations in the building, compressive strength mixes of 12,000, 10,000, 8,000, and 6,000 psi, and 7,000 psi lightweight concrete for the hotel floors.
Concrete Mix Design Summary
| Concrete Type | Strength / MOE | Application |
|---|---|---|
| Self-Consolidating (Low Heat) | 7,000 psi | Mat slabs (8 ft and 10 ft thick) |
| High MOE Mix 1 | 7 million psi MOE | Lower structural zones |
| High MOE Mix 2 | 6 million psi MOE | Mid-rise structural zones |
| High MOE Mix 3 | 5 million psi MOE | Upper structural zones |
| High-Strength Compressive | 12,000 psi | Columns and core walls (lower floors) |
| High-Strength Compressive | 10,000 psi | Columns and core walls (mid floors) |
| High-Strength Compressive | 8,000 psi | Columns and core walls (upper floors) |
| Standard Structural | 6,000 psi | General floor slabs |
| Lightweight | 7,000 psi | Hotel floor slabs |
Construction Methods: The Three-Day Floor Cycle
Cycle Sequence
Kevin Morley, McHugh’s concrete superintendent, explained that after the parking garages and lower portions of the building were completed, the Vista team began a three-day cycle floor construction schedule for each tower. One floor was completed every three days per tower, each roughly 21,000 square feet. The pace required meticulous coordination of crews, materials, and equipment.
The three-day cycle followed this sequence:
- Day 1 (Concrete Placement): Concrete is placed for columns and floor slab simultaneously. Workers must be able to walk on the floor approximately three hours after placement to begin layout work, column forming, and floor forming for the next level.
- Day 2 (Reinforcement and Rough-In): Placing rebar, post-tension tendons, structural steel members when needed, rough-in electrical and plumbing work, and edge form installation.
- Day 3 (Final Preparation and Pour): Completion of all reinforcement, form inspection, and concrete placement for the next floor. The cycle repeats immediately.
Shear Wall and Core Construction
The shear walls for the Vista Tower are as thick as 54 inches of concrete at the base of the structure, tapering to 18 inches near the top. Core construction precedes floors by at least one story using self-climbing systems that lift hydraulically without crane involvement.
Kevin Morley notes that shear walls on the outside of the building, also called silo walls on this project, move in and out with the shape of the building. These walls are sloped to match the facade and are not stepped from floor to floor. Self-rising forms from Doka handled this complex geometry. Proper Bedroom Humidity Building Envelope Best Practices and Weatherstripping principles became relevant here as the building envelope had to accommodate both the geometric complexity and the extreme height-induced environmental loads.
Column to Floor Interface
Column and floor concrete use different mix designs but are placed simultaneously. To ensure there is no confusion, concrete for the columns is placed just ahead of the floor placement. It is delivered in buckets by the tower crane and overflows the top of the column forms. Floor concrete is placed by pump lines. This simultaneous placement requires precise timing and communication between the crane operator, pump operator, and concrete crew. At the time of reporting, 81,000 cubic yards of concrete (10,000 truckloads) had been delivered to the jobsite.
Formwork Systems, BIM, and Project Takeaways
Advanced Formwork Solutions
McHugh involved Doka USA early in the process, recognizing that formwork technology would be critical to achieving the three-day cycle. Deian Ivanov, Doka’s Midwest Account Manager, noted that one of the challenging aspects was the constantly changing floor dimensions. Floor dimensions vary floor to floor because the building angles outward or inward at every level, meaning form requirements change for each floor.
Three primary formwork systems were deployed:
- Core Forms (Super Climbing System): Four hydraulic cylinders lift the inside and outside forms from one floor level to the next. A platform anchored to the top of the forms holds a concrete placing boom that automatically lifts when the forms are raised. Workers can store rebar on the formwork, improving productivity.
- Shear-Wall (Silo Wall) Forms: McHugh used Doka’s SKE 50 and 100 self-climbing form systems. The outside wall forms are hydraulically lifted, while the inside closeup forms are crane-set by McHugh. These follow the angle of the building.
- Safety and Screening: SKE self-climbing scaffold platforms with Doka’s X-Bright screening system surround the top floors. This prevents tools and materials from falling, provides shelter from wind during concrete placement, and encloses floors for temporary heating.
- Floor Deck System: McHugh used Peri’s MultiFlex decking system with their MultiProp system to form all the floor slabs. This modular system allowed rapid setup and stripping to maintain the tight cycle times.
Building Information Modeling
Building Information Modeling (BIM) matches three-dimensional drawings with databases that provide detailed information about every construction element. BIM is now standard practice, and larger projects employ full-time people to manage BIM drawings on site. McHugh used Tekla software for their BIM work because of its ability to handle complex reinforcement detailing and clash detection. Morley noted that jobsites may soon go paperless due to this technology, though Levin admitted he still appreciates paper plans for some field work.
Key Differentiators for Super-Tall Concrete Construction
Compared to high-rise construction, supertall buildings pose greater challenges in several areas. Engineers must focus much more on wind shear and occupant comfort. Building sway during construction requires contractors to invest in and adopt advanced layout technology. Keeping costs in line is more difficult, and every decision has amplified consequences at scale.
There are three primary reasons why structural concrete supertall buildings have become cost competitive with other structural methods:
- The development of highly efficient concrete pumping systems that can move material vertically over 1,000 feet without segregation.
- Advances in cost-productive technical forming systems such as self-climbing formwork that reduce crane dependency and speed cycle times.
- Concrete mix designs that can meet all the performance requirements including high MOE, high strength, self-consolidation, and low heat generation simultaneously.
Final Building Program
When complete, the Vista Tower includes 5 floors of parking, a 12-story hotel, 81 floors of apartments and condominiums, 3 mechanical levels, and 2 floors of damping tanks to help reduce building sway. This mix of uses demonstrates the versatility of concrete construction.
The lessons from Vista Tower apply across project scales. The emphasis on pre-construction planning, advanced formwork, high-performance concrete mixes, and the application of Building Science in Action Key Takeaways From the industry’s best practices demonstrate that successful construction depends on understanding the interaction between materials, methods, and design intent at every level. Whether building a home or a supertall tower, the fundamentals of moisture management and structural integrity remain the same. The Vista Tower stands as proof that when these fundamentals are executed with precision and innovation, the results can redefine a skyline.