Cira Centre South
STRUCTURAL SYSTEMS
Description:
FMC Tower is composed of two different structural systems. It is often talked about as being “two buildings, one on top of the other”. The lower portion of the tower, floors 1 through 26, is office space. The structural system used for these floors consists of a strong central concrete core with steel framing surrounding it. The floors themselves are constructed using 4,000 psi lightweight concrete on metal deck. In this structural system, loads are transferred horizontally through the floor to the steel beams, which then transfer the load horizontally to the steel columns and the concrete core. The concrete core and the columns then transmit the load vertically to the foundations. Figure 1 is a typical structural plan for the office floors which shows the horizontal load transfer in the bottom left corner. The upper floors, floors 29 through 46, are residential and are constructed using cast-in-place reinforced 6,000 psi concrete. Similarly to the office floors below, the load on these upper floors is transferred horizontally through the floor slab to the columns and the concrete core, which then transmit the load vertically. Figure 2 is a typical structural plan for the residential floors that shows the reinforcement detailing in the concrete slab. At floor 47, the first mechanical floor, the tower transitions back to a steel frame and concrete core system for ease of construction. Floor 27, which is about 25 feet tall, is a transfer floor. This floor transfers the load from the upper portion of the building to the lower portion of the building using a series of complex trusses. Figure 3 depicts several perspective renderings of the transfer floor. This floor was required because the columns in the upper residential portion of the tower do not align with the columns in the lower portion of the tower. The trusses provide a way for the load from these upper columns to reach the lower columns of the tower, and eventually the foundations.
Figure 1
Figure 2
Figure 3
Reason for System Selection:
A central core system was used for the tower because the core, which is composed of 8,000 psi concrete, provides a large amount of strength to the building. This strength is important not only because of the large live and dead loads associated with skyscrapers, but also because of the large wind loads that the tower will experience. This core helps to combat these large loads, providing stability to the structure without adding much girth.
As for the surrounding structure, the lower office floors have a taller floor to floor height and much larger open spaces than the upper residential floors. Steel framing supports these lower floors because of its high tensile strength, resulting in smaller columns and beams with larger spans than would be required for a concrete frame. This allows for greater floor area for given amount of structural framing, essential for leasing productivity.
The upper residential floors use concrete frame for cost effectiveness. Residential areas require smaller spans and can have more columns due to their living uses as opposed to the large areas needed by office spaces. Use of concrete also reduces the need for high load cranes to lift steel to the high floors.
Foundation:
The foundation of FMC Tower is composed of concrete caissons, the deepest of which is embedded 18 feet into the ground, extending 10 feet into bedrock. These extensive lengths are necessary to support the very high height to footprint ratio of the highrise.
The subsurface conditions beneath the building most likely consist of first urban fill, then other various soils which have been deemed fit for use as foundation soils, then bedrock.
A major issue during foundation construction was water infiltration during drilling. Because the project site is located very close to the Schuylkill River, the groundwater table is higher than usual. Holes drilled for the caissons closer to the riverside eventually filled with water and inspection of these caisson holes required scuba diver certified geotechnical engineers to complete.
Innovative Construction:
The concrete core of FMC Tower is currently being constructed using a jump form system, which can be seen in the image above. This system consists of self-climbing formwork and working platforms that are lifted from floor to floor by hydraulic jacks. It can raise the central core three floors in each setup and allows core construction to be completed multiple levels ahead of floor construction.
By using this system, work can be done with reduced reliance on cranes. It also provides significant speed and cost advantages for high-rise construction. Another advantage to using a jump form is its engineered nature. Formwork can be quickly and precisely adjusted so buildings with repetitive outside formscan be placed easily. In certain cases, it makes it possible to complete the elevator lift motor room during construction, enabling the building to become lift-operational.