The Metropolitan Tower is a $52 million cast-in-place concrete building with 24 stories of luxury apartments over a seven-story parking podium and a single level of below-grade parking. Its long, cantilevered balconies situated around the perimeter provide unobstructed views of downtown. The project’s floor system for both the parking structure and apartment tower consists of 7.5-inch-thick post-tensioned flat plates with spans ranging from 27 to 30 feet. Typical columns not part of the lateral-force-resisting system are 24 inches square for the full height, a layout developed to maximize forming efficiency while streamlining rebar placement.

A combination of shear walls and special moment-resisting frames provide lateral resistance, with the shear walls carrying 70 percent of the seismic forces. Shear walls are 24 inches thick for the full building height and are located around elevator and stair cores. At the top of the parking structure on the eighth floor, wall lengths are reduced to account for the decrease in lateral forces.

One of the project’s several design features is found at the foundation level 20 feet below grade. The building is supported on a floating foundation mat of variable thickness, which eliminated the need for caissons. 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 performed a three-dimensional soil-structure interaction analysis to verify the design. The foundation uses about 5,500 cubic yards of concrete and is reinforced with Grade 75 bars with a yield strength of 75,000 psi.

High-strength concrete is also used on the project. In the columns, 10,000 psi concrete goes up to level 20, and 6,000 psi mix is above that. Concrete for shear walls is 8,000 psi to level 20 and 6,000 psi above.

Both the 8,000 and 10,000 psi concrete were specified at 90 days to allow for more curing time, and fly ash was used to assist in long-term strength gain. To make column placement easier, maximum aggregate size was specified at .5-inch and a superplasticizer used.

Normal weight concrete with a strength of 7,000 psi was used for the post-tensioned floors. Using that compressive strength achieved compatibility with those found in the vertical elements in the lower 20 floors. That eliminated the need to “puddle” high-strength concrete around the perimeter of the vertical elements — significantly decreasing construction time and associated labor costs.

Using concrete over steel in the building’s construction allowed shorter floor-to-floor heights, saving more money. Also, since the bottom of the slabs serve as the ceilings for the apartments below, the need for an architectural ceiling was eliminated. Finally, concrete doesn’t need additional fireproofing like steel does.