The Proscenium is a 24-story, 540,000-square-foot Class A office building located at the corner of 14th and Peachtree Streets in the Midtown section of Atlanta, Georgia. Trammel Crow, the building’s developer, sought a design that was "state of the art—at the lowest possible cost." This directive and the building’s urban site were the stimuli for a creative design in reinforced concrete that yielded a unique structure, including the signature spire at the top of the building. Adjacent to the building is a 10-story, 325,000-square-foot precast parking structure.
It was determined early in the developmental stage that approximately $250,000 could be saved if the building’s floor to floor height was 12 feet 6 inches rather than the 13 feet 0 inches that is customary in the Atlanta market. This savings would result from reducing the lengths of the vertical elements of the building, most notably the facade. In order to accomplish this, the structural depth of the floor system could be no more than 16 inches, which was a challenge for the structural engineers, since there was a desire for a 43-foot clear span from the building’s service core to the exterior wall.
Typical bays in the buildings are 30-by-43-feet. In the 30 foot direction, 14-by-16-inch post tensioned "junior beams," spaced at approximately 14 feet on center, are utilized. This relatively large spacing was provided in order to minimize the amount of floor area affected by post-tensioning so that future openings in the floor could be easily accommodated. It also resulted in cost savings by maximizing the span of the 5-inch slab, which is the minimum thickness prescribed by the Code of Practice for Fire Safety in Buildings, and by minimizing the number of members and amount of side formwork. The post-tensioned members in the 43-foot direction are also 16–inches deep and range in width from 36- to 42-inches. At some of the columns, it was necessary to provide tapered haunches for stress and/or deflection. These haunches project at most 10 feet out from the column centerline and 14 inches below the bottom of the beam; utilizing the haunches did not increase the floor to floor height and did not interfere with any of the utilities.
The haunches had the added benefit of stiffening the frame for lateral load resistance. As a result, it was possible to eliminate shear walls for all but the lowest three floors, which is unusual for a 24-story building. This resulted in a considerable reduction in the cost of the building’s frame, which is approximately $12 per square foot. Had shear walls been provided for the full height of the building, overturning due to lateral loads would have resulted in net tension at the base. The dual shearwall/frame system that was used resulted in a more efficient distribution of the building’s lateral forces, eliminating the uplift problem.
The concrete strength is 5,000 psi for the floor members and ranges from 10,000 psi at the base of the building to 5,000 psi at the upper levels for the columns and walls.
Concrete framing for this office building and parking structure resulted in considerable savings for the owner. Additionally, building tenants are pleased due to the lower build-out costs attributed to the reduced height of vertical elements in their spaces.
Trammell Crow Co., Atlanta, Georgia
Thompson, Ventulett, Stainback & Associates, Atlanta, Georgia
Uzun & Case Engineers, Atlanta, Georgia
General/Concrete Contractor: Year:
Hardin Construction, Atlanta, Georgia