Madison Concrete Construction had the pleasure of being part of a unique and important project in Philadelphia—the FMC Tower which, when completed, will be the city’s first true vertical neighborhood.
The tower is distinctive because it is composed of two separate structural systems. It is essentially two buildings stacked on top of each other—a 22-story concrete-framed residential apartment building constructed on top of a 28-story steel-frame office building.
The concrete-framed residential over steel-framed office design was chosen because it was not as costly as a completely steel-framed structural system and did not require as long a construction schedule as a concrete-framed building. It also offered more flexibility in column locations and reduced the need for high-load cranes to lift steel to the upper floors. A major challenge in building for two different structural systems, however, came in the form of load transfer.
The lower portion of the FMC Tower is designed to accommodate office space. The structural system for these floors is a strong, central concrete core with steel framing surrounding it. We constructed the floors using 4,000-psi lightweight concrete on metal deck. Loads are transferred horizontally through the floor to the steel beams. The beams then transfer the load horizontally to the steel columns and the concrete core. The concrete core and the columns then transfer the load vertically to the foundations.
The upper portion, the apartment structure, is located off-center from the rest of the building and contains more columns to accommodate living spaces. The upper floors are constructed using cast-in-place reinforced 6,000-psi concrete, and the columns are not aligned with columns in the lower part of the tower. This presented a challenge in transferring load from the top structural system to the bottom.
The solution was constructing the 27th floor to function as a transfer floor. The space is 25 feet tall and it shifts the load from the upper portion of the building to the lower portion of the building via a series of complex trusses. This method enables the upper column load to reach the lower columns of the tower as well as the foundation.
Using a hydraulic (self-climbing) formwork system allowed us to complete core construction multiple levels ahead of floor construction. We completed the final pour and demobilization of the cast-in-place concrete core formwork in August 2015, and the 28th-floor transfer level in December 2015. The tower is expected to be completed and ready for occupancy by mid-2016. We look forward to seeing such a fantastic building and community in use.
Have you worked on a project with separate structural systems? We would love to hear about it. We would also love to hear from you about your structural engineering challenges and successes and any questions you might have. Submit your questions to Rick Stone at email@example.com.