When it was decided to build new office space for the Federal Energy Regulatory Commission (FERC) in Washington, D.C., the federal government was determined to do so in a relatively short amount of time. Contracted on a design/build basis, the 11-story, 900,000-square-foot building was completed 21½ months after design began in January of 1994. In spite of tight time constraints, quality was not sacrificed, and the project received a Washington Building Craftsman Award.
The footprint of the building is essentially rectangular with 30-by-30-foot typical bay sizes. The structural framing chosen for this project is conventionally reinforced cast-in-place concrete. A 9½-inch flat slab with 5½-inch drop panels is utilized for the roof and floor framing, as well as for the three parking levels below grade. The precast curtain wall is carried by the slab which is designed for a maximum deflection of 0.25 inch at the perimeter of the building. One-story-deep transfer girders support columns from the upper floors above the courtroom level, and allow 60 feet of column free space in the courtroom.
The lateral forces are carried by a combination of shear walls and frames acting together. A mat foundation which varies from 3- to 4-feet in thickness supports the columns and shear walls.
Normal weight concrete with a specified compressive strength of 4,000 psi is utilized for the floor system. The compressive strength of the concrete for the columns is typically 6,000 psi, with 8,000 psi concrete used for the columns at the transfer girder. The compressive strength of the concrete for the transfer girders is also 8,000 psi.
Due to the location of the courtroom, no expansion joints were permitted in the 540-feet long building. Thus, controlling the thermal shrinkage of the concrete and the thermal expansion and contraction of the building were major design considerations in the project. Measures to control the thermal shrinkage had a significant impact on the construction sequencing.
Above grade, the building was constructed as two separate structures, and a pour strip was utilized at each floor level. To allow for initial shrinkage, the structural engineer specified a five-week interval between pours. The location of construction joints at the pour strips necessitated cold joint details, including one in the transfer girder.
"Mobilization and excavation started at the same time as the design,” said William F. Faschan, LERA’s partner in charge of the project. “Final structural drawings were issued, shop drawings made, and concrete cast on a rapid floor by floor cycle. Only with the team effort of the developer, the designers, and the builder was this schedule possible."
Concrete versus Steel Framing
Structural steel framing was never an option for this project. Due to height restrictions in Washington, D.C., the concrete framing scheme was chosen since it resulted in smaller floor to floor heights, and, thus, a greater number of floors for a given height.
The general contractor said flexibility of the reinforced concrete afforded the opportunity to achieve the accelerated schedule.
Specifying structural steel would have easily added five to six months to the overall schedule due to the time required to develop the mill set, bid competitively, select the supplier, and wait for the mill order.
Greenebaum & Rose Associates, Washington, DC
Kohn Pederson Fox Associates, P.C., New York, New York
Leslie E. Robertson Associates, R.L.L.P., New York, New York
Hyman Construction (now Clark Construction), Bethesda, Maryland