The Fred & Pamela Buffett Cancer Center at the University of Nebraska
By Michael Kuhse
A collaboration of the University of Nebraska Medical Center (UNMC) and the Nebraska Medical Center (NMC), the $284 million Fred & Pamela Buffett Cancer Center is the largest project in the university’s history and the largest public-private partnership in Nebraska’s history. The project capitalized on $50 million in state funding, $35 million from the City of Omaha, and $5 million from Douglas County, with the remainder covered by private donations.
The L-shaped building rises in two towers: a ten-story, 252,000-square-foot cancer research tower with 98 laboratories; and an eight-story, 325,000-square-foot, 108-bed hospital and inpatient tower. Researchers work in laboratories just steps away from clinicians caring for patients. The proximity means opportunities for collaboration, communication, and problem-solving abound, and that patients receive advanced treatments based on the latest research findings.
Additionally, the building accommodates a pair of fueling-capable heliports on the roof, each large enough to support a large military helicopter. The facility was also designed to be LEED-certifiable.
Structural System Overview
Concrete has been the structural material of choice for UNMC and NMC for years, and Fred & Pamela Buffett Cancer Center is no exception. The entire structure utilizes over 25,000 cubic yards of structural concrete as its primary structural system. Concrete was selected for a number of reasons:
- UNMC prefers concrete due to its inherent fire resistance, and the client prefers not to have friable fireproofing.
- The lab spaces have restrictive vibration criteria, and the mass and stiffness of concrete makes it easier to satisfy the performance requirements.
- The continuity in monolithic concrete construction provides a moment frame for lateral resistance at minimal cost.
- The concrete structure made it easier to match the floor-to-floor heights of the adjacent existing building.
- It was simpler to cantilever the floors back to meet the existing structures on two sides of the building with a concrete structure.
- For radiation resistance, the linear accelerator vaults require massive concrete walls (up to seven feet thick) and roofs. Integrating these concrete vaults into a concrete structure was far easier than if the structure had been steel.
- Ten-story cancer research building and eight-story inpatient care and surgery building.
- Strict vibration limits in the research spaces.
- “Skip-joist” and girder systems are used to resist gravity loads.
- Moment frames in both directions are used to resist lateral loads.
- Drilled piers serve as the building foundations.
- Typical concrete strength is 4,000 psi, with the columns utilizing concrete strengths from 4,000 psi to 6,000 psi.
The structural team solved hundreds of problems on this project, but this article will focus on just a few of the highlights.
Like most major construction projects today, compressing the design and construction schedule dictated early design bid packages. An early foundation and concrete frame package for the cancer center was issued for construction, which meant that construction was well underway before the architectural, mechanical, and electrical designs were complete.
To meet the aggressive schedule, the design team sat as a special project team within the HDR Omaha office, which facilitated closer coordination and faster information sharing. All of the design disciplines modeled their work in Revit, so that the project could be coordinated in three dimensions. The Revit models were also regularly shared with the contractor, Kiewit Building Group (KBG), so KBG could track the progress of the design, update its construction schedules, and adjust its cost estimates. KBG also exported the Revit model into Navisworks for the sub-contractors to use for coordinating their work.
While it is true that Nebraska is not a high seismic region, the Occupancy Category of this building, along with the ASCE 7 ground accelerations and soil classification, placed the cancer center in Seismic Design Category (SDC) C. The structural forces were manageable, but the bigger implication of being classified as SDC C is that ASCE 7 requires seismic bracing for all critical mechanical and electrical systems. Seismic equipment/system bracing had the potential to add millions of dollars to the project and would add to the complexity of above-ceiling coordination.
The seismic accelerations were only slightly above the line between SDC B and C, so Terracon Consultants were hired as the geotechnical engineer to perform a site-specific ground acceleration analysis. The results of the site-specific analysis reduced the ground accelerations, and permitted the buildings to be re-classified as SDC B. Thus, seismic bracing of the mechanical and electrical systems was no longer required by code.
Flooring adhesives changed from an oil-based adhesive to a water-based adhesive about ten years ago to be more environmentally friendly. Although better for the environment, water-based adhesives can deteriorate if too much water vapor remains in the concrete floor slab. This problem is exacerbated on fast-track projects such as the cancer center, because there is less time for the concrete floor slab to “dry out” between when the concrete is placed and when the flooring is installed.
The Buffett Cancer Center has 90,000 square feet of concrete slabs that incorporate Aridus rapid-drying concrete. Aridus is a proprietary concrete mix design supplied by Ready Mix Concrete that optimizes the concrete mix to reduce the amount of water required for hydration. This project is the first large-scale use of Aridus concrete in Nebraska, and its use eliminated the vapor transmission concerns that lead to flooring adhesion issues and allowed for acceleration of the construction schedule.
Linear Accelerator Vaults
The cancer center includes four linear accelerator vaults, which require massive concrete to shield the spaces outside the linear accelerator vaults from the high-dose radiation used for cancer treatments inside the vault. The density and thickness of the concrete vault walls and roof are critical in providing the required radiation shielding.
The available thickness of the vault roof slabs in the cancer center was limited due to the floor-to-floor height of the building. Thinner vault walls were also desirable to provide for an improved architectural floor plan.
To provide thinner vault walls and roof, 1,900 cubic yards of higher-density concrete was used in combination with lead blocks in order to achieve the required radiation shielding. The concrete density was increased using 390 pounds per cubic yard of Rad Ban aggregate added to the concrete mix. This combination allowed for the vaults to fit within the required floor-to-floor height.
One of the many unique spaces within the Fred & Pamela Buffett Cancer Center is a healing garden and sanctuary space, which was added to the project after construction was well underway. A donor came forward with funding for artist Dale Chihuly to create the rooftop healing garden and sanctuary space. The sanctuary is meant to be a quiet and contemplative space filled with Chihuly artwork, and its sculptural form is an integral component of the art.
The floor/roof that was to become the sanctuary and healing garden had not yet been poured when the concept was developed. Additional reinforcement could be placed in the slabs for the revised loading; however, the columns and, more importantly, the corbels supporting the framing at a building expansion joint were already poured and could not easily be strengthened. As a result, the capacity of these existing support elements limited the load that could be added.
Further complicating the addition of the sanctuary space was the organic shape of the design. The design started with hand-sketched floor plans from Chihuly that were converted into a Rhino model by HDR. Once Chihuly approved the plan in the Rhino model, the Rhino model was exported into Revit to create the geometry used for the construction documents. The plan coordinates of the columns were exported from the Rhino model and imported into Ram Advanse to generate the structural analysis model. Once the columns were imported into Ram Advanse, roof framing and loads were added to complete the analytical model.
The roof is framed with metal roof deck on wide-flange steel beams supported by three concrete columns located in the interior. HSS columns double as vertical mullions for the exterior glass walls around the perimeter. The roof loads were more uniformly distributed onto the concrete slab below by using the mullions as load-bearing elements spaced at the width of the exterior glass panels.
The Fred & Pamela Buffett Cancer Center was completed on time and within budget. Perhaps more importantly, the cancer center achieved the highest designation as a Comprehensive Cancer Center from the National Cancer Institute – one of only 41 in the United States.
Michael Kuhse, P.E., S.E recently retired after working more than forty years as a senior structural engineer for HDR in Omaha, Nebraska. Although he has worked on numerous project types all over the United States, Michael specialized in large-scale healthcare projects.