The new Centene campus has an eye-catching design with a sleek, modern aesthetic. The forested site inspired a nature-focused concept that feels like working in a treehouse; floating volumes overlook the tree canopy, an outdoor amphitheater is tucked under a cantilever, and infrastructure integrates seamlessly with the architecture and the serene environment.

The design looks elegant and effortless, but all designers know that delivering simplicity can be deceptively difficult. The Centene design was no exception, given the fast-track schedule that had site elements being constructed as the design continued to evolve. The pace required constant communication among all involved: the client, the architects from LS3P, the structural engineers from Uzun+Case, and the contractor Clayco, along with a host of subcontractors, fabricators, and vendors. The construction schedule had  subcontractors working around the clock in multiple shifts, requiring real-time communication and design coordination among all parties.  It was clear from the beginning that this campus would be something special, and the all-hands-on-deck, highly collaborative process made it possible for the client’s vision to take shape as the design continued to evolve.

Cantilevers within Cantilevers

A cantilever – a building element that appears to float unsupported- always requires careful structural consideration. A multistory concrete cantilever of nearly 60 feet, with internal cantilevered “treehouse” meeting rooms and suspended stairs that appear to float in space, is an altogether unusual proposition. The structural engineers at Uzun+Case were undaunted by the challenge. A steel structure would have been a relatively simple solution; however, the design team wanted to maintain the architectural vocabulary of the rest of the building and create the cantilever out of exposed concrete. Another common method of supporting a large cantilever is to suspend it from structure above, but the tight construction sequence and order of operations made that option infeasible as well.

In tandem with LS3P, the Uzun+Case team studied multiple schemes before arriving at the structural solution, which included a framing plan which is unique within the market. In this system, junior beams spaced 15’ and post tensioned girders allow for increased flexibility for floor penetrations. In conjunction with the columns and beams, the system capitalizes on frame action from the concrete to create an efficient 25” structural depth. Diagonal bracing at every bay in the cantilever minimizes the forces acting on any particular component while adding a stunning design element. The structural system has built-in redundancies for maximum stability – particularly  important during the construction phase with shoring in place. Instead of relying on a typical back-span, the braces transfer the overturing force to the vertical shear wall.  For the “treehouse” meeting areas, the structure is cantilevered out from the main floor plate as a solid slab, then post-tensioned for strength.

Bespoke and Beautiful Structural Elements

Pulling off this elegant structural feat required custom structural elements, innovative systems, and (literal) cutting-edge fabrication. The graceful round braces are a slender 10” diameter, with the same dimensions on every floor; 10” round bars, however, pushed the limits of the fabricator’s ability, as this was the largest diameter solid steel bar they had ever used. A laser cutter eventually did the job. The brace connections required further innovation; Uzun+Case collaborated with Clayco to embed steel columns within the cantilever’s concrete columns to join with the steel braces, with pre-planned holes for post tensioning. LS3P coordinated on the design of minimal round gusset plates which complement the architectural aesthetic. The concrete shear walls also required significant coordination for large openings for duct work and other penetrations which had to be carefully orchestrated.

Accounting for Movement

The structural engineers had an additional challenge to solve: movement. Concrete is a nonlinear material which initially compresses, then creeps over time. The design had to accommodate this change over time while also allowing for possible deflection when the shoring underneath the construction was carefully removed. Careful design and construction accounted for this deflection, and the element most impacted- the curtainwall – was designed in concert with the required concrete tolerances for smooth installation. The curtainwall at the cantilever couldn’t be installed until the shoring was removed; installers began at one corner and worked carefully around the cantilever, and the operation was a success. The predicted cantilever deflection was 1.5”, and measurements made during and after construction confirmed that number, giving added confidence to the team that the structure was behaving as designed.

A New Spin on the Parking Structure

The idea of cantilevered feature elements from the office building extends elsewhere on the Centene campus through implementation strategies within the design of the parking garages. The 750,000 SF garages accommodate 2,100 parking spaces and also feature cantilevered ornamental  exterior stairs and escalators, which are typically only found in large-scale entertainment and transportation venues, and rarely if ever in constructed with precast concrete. Uzun Case was originally slated to design the foundations, but screening elements, cantilevered stairs, and even cascading escalators hung 15’ off of the structure were added to the project scope. Every addition had to be factored into the structural load, and to intensify the challenge, the topography included a 50’ elevation change across the footprint of the garages, which necessitated multiple 30’ retaining walls surrounding the parking structures.

Precast concrete construction was selected early on as the best option for meeting the aggressive schedule. The design uses 36’ interior bays, with steel escalator and stair supports cantilevered off of the exterior of the building. Escalator length had to correspond to module of the bays, particularly challenging where optimal layouts for the two did not necessarily align. To support the hanging exterior stairs and escalators, additional columns woven into the precast design transfer load from floor to floor while vertical steel truss members concealed within the building cladding transfer load to the precast columns and create the appearance of a floating structure. Exposed architectural board formed concrete gives the walls a tactile wood-grain appearance found throughout the campus, and a few fully integrated 30’ tall waterfalls exist within the retaining walls surrounding the garages.

Materials Matter:  

Because the exposed concrete was key to the aesthetic as well as the structure, the board formed concrete was key to several prominent locations. Getting this material right required extensive coordination between LS3P and Uzun+Case to design structural elements that would also be a highly visible finished product.  Additional reinforcement within the walls helps to minimize cracking, though the irregularities of a concrete finish are part of its appeal to add texture and visual interest to a monolithic surface. Tie holes and joints are carefully integrated into the design and formwork before pouring, and a self-consolidating concrete mix helped to streamline the installation process.

Structuring the Site

In addition to the ingenuity required for the office and parking components of the project, the 80-acre site contains many elements requiring structural innovation, including several which were added during the construction phase.  The site features over 5,000 linear feet of cast-in-place retaining wall, and the complexity of the structural design meant that interactions between upper footings and lower footings in certain locations needed to be carefully considered. Landscape elements, from an expansive exterior deck with organic geometry to interior landscape features requiring boulders, water features, and other heavy loads, added further structural challenges.

The canopy between the parking garages is also a custom design. A typical canopy is built with separate structure and cladding; at Centene, this canopy bridges directly between the office building and parking garage structures. With an aluminum frame supporting amoeba-like openings on top of a steel frame with 4’ deep girders, these intertwined elements had to accommodate up to 9” of horizontal movement across an unusually large span.  A butterfly canopy below the larger drop-off canopy integrates directly with the canopy and garages structures, and provides additional shelter from the elements for pedestrians walking into the office building.

The team reached the topping out stage one year after project design kick-off! Planning for a million square feet of construction on a complex 80-acre site with steep topography would have been a challenge on any timeline, but the project team agrees that the commitment to authentic collaboration on the part of every entity involved is what kept the project on track to meet this important milestone. Design additions, complex structural elements, large-scale infrastructure, and an overarching desire to create “the best campus on the East Coast” motivated everyone to bring their best to the project; that effort is evident in the resulting world-class design.