Bank of America Tower

Shaping the System: Lessons from Beta Testing to Improve LEED

Bank of America Tower is the first project in the United States—and the highest-rated in the world at the time of its certification—to achieve LEED v4 Platinum Core and Shell Certification. It was one of 100 projects to pilot LEED v4 preceding the formal launch, however, due to the philosophy and values of the project’s developer, Skanska Commercial; contractor, Skanska USA Building; and architect Gensler, the project elected to pursue the newest, and substantially more rigorous, version of LEED. Bank of America Tower was one of if not the first large-scale U.S. commercial development to use Whole Building Life Cycle Assessment (WBLCA) to optimize the environmental impacts of the project’s structural system.

A Shift in Strategy
The pursuit of the WBLCA credit, and a focus on reducing embodied carbon, caused a significant shift in the approach the design team used to evaluate and compare structural and enclosure assemblies. Achieving reductions in embodied carbon was a key project goal and was considered, along with cost and schedule, during both the design and construction phases.

A Collaborative Approach
Early-stage LCA studies, using the Athena Impact Estimator for Buildings, showed that more than 75% of the embodied carbon of the structure and enclosure was from the concrete framing. Nearly 35% of the project’s concrete volume was in the foundation elements and 40% in the horizontal framing. These studies, and the design team’s understanding that portland cement—the binder used in concrete—is responsible for the majority of concrete’s embodied carbon, led to an aggressive approach to cement minimization.

It is standard for a structural engineer to specify concrete strength at 28 days. However, because concrete gains strength over time, mixes with high levels of supplementary cementitious materials (SCMs), will typically achieve higher, long-term strengths. To reduce cement content, the design team studied when structural components required their design compressive strength and did not arbitrarily specify concrete strengths at 28 days. For example, the design team specified 90-day strength for the mat foundation.

Many structural engineers permit SCMs in their concrete mixes, but for Bank of America Tower, the design team mandated SCMs to reduce cement content. The concrete mixes in Bank of America Tower required up to 55% fly ash replacement. Design phase communication and iteration between the design team, general contractor, concrete subcontractor, and concrete supplier was crucial to achieving the reduced cement content mixes.

Following Through During Construction
Despite the close collaboration during the design and early procurement phases, communication between the design and construction teams during the full Construction Administration phase was crucial to achieving the reduced cement content mixes. The project experienced a brief hold following the construction of the podium.

Upon restarting, the schedule became a key driver, and the concrete contractor proposed an alternate, faster method for forming the concrete core walls. Instead of forming the core walls on top of each floor, the core wall form work would be supported by the most recently placed section of the core.

This process, called jump forming, increased the early stage strength requirements for the core. Since the initial revised mix to permit jump forming substantially increased the embodied carbon, the design and construction teams collaborated to refine the analysis establishing the early stage strength requirements to enable a lower carbon concrete mix for the walls.

This demonstrates that the structural engineer must remain diligent in the Construction Administration phase to ensure the sustainability goals of the concrete mixes are met. This diligence resulted in a core and shell with significantly less embodied carbon than compared to common construction practices. Even more noteworthy, the mixes developed for Bank of America Tower have since been used on other projects in the region.

EXTERNAL PROJECT TEAM
Owner: Skanska Commercial Development
Architect: Gensler
Structural Engineer: Walter P Moore

INTERNAL PROJECT TEAM
Principal in Charge: Dennis Wittry
Project Manager / Structural Engineer: Fernando Torrealva
Sustainability Specialist: Dirk Kestner
Structural Design Manager: Asif Wahidi
Parking Consultant: Brian Lozano
Civil Engineer: Charlie Penland / Daniel Falkenstine