When completed in July of 2001 at a total construction cost of $52 million, the Metropolitan Tower Apartments will be an elegant addition to the skyline of Seattle, WA. At an overall height of 320 ft (97.5 m), the project includes 24 stories of luxury apartments over a 7-story parking structure podium and one level of below grade parking. Catering to high-tech residents, the building is fully wired for a closed circuit television/intercom system and the Internet. Residents will also enjoy the following amenities, which are located on Level 8 just above the parking structure: pool, spa, health club, conference rooms, party rooms, library and outdoor garden. Other noteworthy features include the unique shape of the building, especially at the rounded prow, and the long, contilevered balconies situated around the perimeter that provide an unobstructed view of the surroundings. A concrete frame provided the most economical solution for this project, satisfying both architectural and structural requirements, including resistance to potentially large seismic forces.

Structural Framing System

The floor system for both the parking structure and the apartment tower consists of 7.5-in. (190-mm) thick post-tensioned flat plates with spans ranging from 27 ft (8.2 m) to 30 ft (9.1 m). Typical columns that are not part of the lateral-force-resisting system are 24 in. (610 mm) square for the full height of the building.

Located in Seismic Zone 3, the structure is designed for a combination of gravity and lateral forces in accordance with the 1997 Uniform Building Code. Lateral resistance is provided by a combination of shearwalls and special moment-resisting frames (i.e., a dual system), with approximately 70 percent of the seismic forces carried by the shearwalls. This type of lateral-force-resisting system is both well balanced and highly redundant, providing the necessary strength and ductility required for the prescribed level of seismic risk. The shearwalls, which are 24 in. (610 mm) thick for the full height of the building, are located around the elevator and stair cores. At the top of the parking structure (8th floor), the lengths of the walls are reduced to account for the decrease in lateral forces.

Special moment-resisting frames, which are situated at the ends of the building and parallel to the corridor, consist of 24 in. by 42 in. (610 mm by 1,370 mm) and 30 in. by 42 in. (760 mm by 1,370 mm) columns for the full height of the building, and 24-in. (610-mm) side by 22-in. (560-mm) deep beams. The relatively shallow beam depth was dictated by the need to maximize the clear height of the units, while preserving a typical floor-to-floor height of 9 ft - 1 in. (2.8 m).

Maintaining constant wall thickness and column sizes for the full building height not only enhanced constructability, but also helped reduce overall time to completion. Concrete strength and reinforcing quantities are reduced over the height of the building to account for the smaller loads.

One of the many innovative design features in this project occurs at the foundation level that is 20 ft (6.1 m) below grade. The building is supported on a floating foundation mat of variable thickness, which eliminated the need for caissons on a site that would have otherwise required them. By covering the entire site with the mat and tying it to the perimeter walls, settlements were minimized and construction costs were significantly lowered. The structural engineer preformed a three-dimensional soil-structure interaction analysis to verify the design. The foundation utilizes approximately 5,500 cu yds (4,205 m3) of concrete and is reinforced with Grade 75 reinforcing bars with a specified yield strength of 75,000 psi (520 MPa).

Concrete Specifications

High-strength concrete is utilized for this project. The concrete compressive strength for the columns is 10,000 psi (70 MPa) up to level 20 and is 6,000 psi (40 MPa) above that level. Similarly, the concrete for the shearwalls is 8,000 psi (55 MPa) to level 20 and 6,000 psi (40 MPa) above. Both the 10,000 psi (70 MPa) and the 8,000 psi (55 MPa) mixes were specified at 90 days to allow for more curing time, and fly ash was used to assist in long-term strength gain. To facilitate placement, the concrete mix for the columns was specified with a maximum aggregate size of 0.5 in. (15 mm) and a superplasticizer.

Normal weight concrete with a specified compressive strength of 7,000 psi (45 MPa) at 56 days is utilized for the post-tensioned floors. Specifying this compressive strength for the floors achieves compatibility with the compressive strengths of the high-strength concrete utilized in the vertical elements in the lower 20 stores of the structure. Eliminating the need to "puddle" high-strength concrete around the perimeter of the vertical elements significantly decreased both construction time and associated labor costs. For stressing purposes, the concrete in the floor members was specified with a compressive strength of 3,000 psi (20 MPa) at 3 days.

Concrete Versus Steel Framing

Cast-in-place concrete was the only logical choice for the structural frame. The floor-to-floor height was significantly reduced when compared to a structural steel option, which resulted in significant savings in the vertical components of the building, i.e., façade, HVAC, electrical, plumbing, and vertical transportation systems. Utilizing the underside of the slab as the finished ceiling eliminated the need for an architectural ceiling, and also resulted in substantial cost savings. Furthermore, the inherent fire resistance of concrete provides the code-prescribed fire-resistance ratings without the need for additional fireproofing. The residents will enjoy quiet living space, which is also automatically provided by concrete structures.

In short, the Metropolitan Tower Apartments is a perfect example of the beauty, functionality, and overall economy that can be achieved by specifying concrete framing.

Credits

Owner: Continental-Bentall, LLC, Bellevue, WA
Structural Engineer: Cary Kopczynski & Company, Bellevue, WA
Architect: Callison Architecture, Seattle, WA
General Contractor: Mortenson, Bellevue, WA
Concrete Supplier: Stoneway Concrete, Renton, WA

 
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