Completed in late 2001, the Paramount Apartments, San Francisco, California, is the tallest concrete building in a region of high seismic activity in the United States. At an overall height of 420 feet, this 39-story building includes 486 apartment units and has a total area of 660,000 square feet). The construction cost is approximately $90 million. The lower eight floors and basement level accommodate a variety of functions with a maximum per floor area of 31,000 square feet).
Retail space occupies most of the first and second floors. Residential amenities include a leasing office and business center on the third floor, and a fitness center and outdoor pool on the fourth floor. Parking for almost 300 cars is located on the north side of the building on floors three toseven, with residential units on the south side on floors four and above. The eighth floor serves as a podium above which typical 13,700-feet residential floors begin. The building steps back at the 34th floor to the high tower, where the floor areas are 9,900 square feet.
A combination of cast-in-place and precast concrete is used to meet the functional and aesthetic demands of the project. A precast concrete perimeter moment frame serves as both the lateral force resisting system and the architectural facade. The architect was able to utilize the high quality and flexibility of precast concrete to create an exciting design. The beams on the south face of the tower frame a true curve. White concrete is used to differentiate the central tower element from the ends or shoulders of the building, which are dark gray concrete. Sandblasting, reveals, and bullnosing all add to the complexity and interest of the facade treatment. By combining the structural frame and facade into one component, the contractor was able to save over $4 million and to reduce the construction schedule by several months.
Structural Framing System
The framing for the southern portion of the first four floors consists of a conventionally reinforced
9-inch thick flat plate with a specified compressive strength of 4,000 psi. Typical spans are 24 feet, and floor heights average 14 feet 6 inches. Where parking occurs in the northern portion of the building, the framing consists of precast concrete beams spanning up to 43 feet and a 6 inch thick one-way slab spanning 18 feet. The depth of the structural system at the parking floors is 2 feet 6 inches, and the story height is 9 feet 8 inches; this helped to maximize the number of floors without increasing the height of the podium. An all-valet parking system, with access from the basement by two auto elevators, eliminated the drive ramp and enabled the same total number of parking spaces with two fewer floors.
The typical tower floor framing consists of 6 ½ inches thick post-tensioned flat plates with spans ranging from 24 feet to a maximum of 27 feet. To achieve a compressive strength of 3,000 psi in three days for stressing, a 5,000-psi mix was specified for the slabs. Stressing through the perimeter precast frame was not possible, so an internal stressing program was developed. To accommodate the apartment layouts, cables were placed uniformly in the north-south direction and banded in the east-west direction. The flat plate floor system allowed the bottom of the slab to be used as the ceiling and provided a comfortable 8 foot 4 inch clear story height.
Column sizes vary to fit the apartment layouts. Typical column sizes are24-by-30-inches up to level 20 with a specified compressive strength at 56 days of 8,000 psi. Above level 20, the columns are 24-by-24-inches with a strength of 6,000 psi.
The foundation of the building is a concrete mat, which is 5 feet thick under the tower and 3 feet thick under the remainder of the structure. The mat thickness was kept to a minimum to maintain about a 5-foot buffer between the bottom of the mat and the water table. The mat is reinforced with Grade 75 reinforcing bars with a specified yield strength of 75,000 psi and is post-tensioned in both directions under the tower. A 4,000-psi mix was specified for the mat, and basement retaining walls utilize 3,000-psi shotcrete.
Lateral Force Resisting System
Two types of lateral force resisting systems are utilized in the project. Below the 8th floor podium, a combination of structural walls and cast-in-place special moment frames is utilized. Varying slab elevations and required occupancy separations between parking and living spaces facilitated the use of structural walls at these levels. Above the eighth floor, a perimeter precast moment frame was developed using both the hybrid beam system and the Dywidag ductile connector (DDC) system®.
The hybrid beam system is the predominant framing system utilized, while the DDC system is used at short frames, which occur at re-entrant corners of the building where the effective post-tensioning force required by the hybrid beam system could not be developed. Typical frame columns are 36-by-36 Inches up to the 20th level and are 24-by-36-inches above that level. Concrete strengths for the frame columns are similar to those for non-frame columns. Typical frame beams are 24-by-36-inches with a specified compressive strength of 5,000 psi, and are set flush to the outside face of the columns.
The hybrid beam system consists of concentric post-tensioned cables anchored at both ends of the frame. The clamping force creates a friction force between the beams and columns, which transfers the shear demands. Mild reinforcing steel — the straining of which provides the necessary energy dissipation during a seismic event — is placed at the top and bottom of the beam through the joint and is grouted in place. These bars are also wrapped (debonded) in a region adjacent to the column to reduce inelastic strain demands and to force all post-yield rotation to occur at the beam-column interface. By limiting post-yield rotations to the joint, damage to the system is minimized. An additional benefit of the hybrid beam system is the restoring force provided by the elastic post-tensioned cables.
The geometry of the building required biaxial stressing at several corner columns. The columns could not effectively accommodate two tendon anchors, so an around the corner stressing detail was developed. The tendons are threaded through a curved pipe that is anchored in both directions at the back face of the column.
The DDC system uses dywidag ductile rods cast into the column. A high-strength thread bar is screwed into the rods and coupled at the beam centerline. All inelastic action occurs in the ductile rods, which causes system deformations to occur at the beam-column interface. In this project, a precast face shell serves as a form for the beam that is cast in place with the slab.
Column confinement is provided by Baugrid welded wire grids. These grids met the tight tolerances required for bar placement. Columns are placed in two-story lifts and are spliced at the top of the beam using NMB Splice Sleeves, which are approved as Type 2 connectors by the United Brotherhood of Carpenters and Joiners (UBC).
This project is the first significant application of the International Conference of Building Officials (ICBO)-approved Precast Hybrid Moment Resistant Frame. With over 2,200 precast pieces cast by Pankow in an off-site casting yard, significant scheduling savings were achieved on the tight downtown site. The floor to floor construction schedule for the project above the eighth floor averaged five days per floor, with the top eight floors being assembled at a pace of four days per floor.
Concrete Versus Steel Framing
Concrete framing was chosen for this project for the following reasons:
- Steel framing would have resulted in higher floor to floor heights, which would have added to the overall cost.
- Additional costs and construction time would have been required for the exterior cladding of a steel frame, as well as for dropped ceilings in all spaces with steel floor framing.
- Ongoing uncertainties about steel moment frame connections prevailed after recent earthquakes.
- Lead time for structural steel would have added months to the project.
The Paramount Apartments demonstrate the versatility of concrete, and especially precast concrete, as a building system. Significant schedule and cost savings were achieved by using precast concrete for both the lateral force resisting system and building facade. The architect, through the use of color, aggregate, texture, and varied reveals, created a unique and aesthetically pleasing building utilizing concrete.
Third and Mission Associates, LLC
Pankow Residential Builders II, LP, San Francisco, California
Robert Englekirk Consulting Structural Engineers, Inc., Los Angeles, California
Kwan Henmi Architecture/Planning, Inc., San Francisco, California
Elkus/Manfredi Architects, Ltd., Boston, Masssachusetts
Design Architect (Interiors):
Ismael Leyva Architect, PC, New York, New York