Paramount Apartments
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Performance & Safety > Paramount Apartments  Completed
in late 2001, the Paramount Apartments is the tallest concrete building
in a region of high seismic activity in the U.S. At an overall height
of 420 ft (128 m), this 39-story building includes 486 apartment units
and has a total area of 660,000 sq ft (61,316 sq m). 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 sq ft (2,880 sq m).
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 4th floor. Parking for almost
300 cars is located on the north side of the building on floors 3 to 7,
with residential units on the south side on floors 4 and above. The 8th
floor serves as a podium above which typical 13,700-sq-ft (1,273-sq m)
residential floors begin. The building steps back at the 34th floor to
the high tower, where the floor areas are 9,900 sq ft (920 sq m).
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 4 floors consists
of a conventionally reinforced
9-in. (230-mm) -thick flat plate with a specified compressive strength
of 4,000 psi (25 MPa). Typical spans are 24 ft (7.3 m), and floor
heights average 14 ft-6 in. (4.4 m). Where parking occurs in the
northern portion of the building, the framing consists of precast
concrete beams spanning up to 43 ft (13.1 m) and a 6-in. (150-mm)
-thick one-way slab spanning 18 ft (5.5 m). The depth of the structural
system at the parking floors is 2 ft-6 in. (0.8 m), and the story
height is 9 ft-8 in. (3.0 m); 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 6-1/2-in. (165-mm) -thick post-tensioned
flat plates with spans ranging from 24 ft (7.3 m) to a maximum of
27 ft (8.2 m). To achieve a compressive strength of 3,000 psi (20
MPa) in 3 days for stressing, a 5,000-psi (35-MPa) 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 ft-4 in. (2.5 m)
clear story height.
 Column
sizes vary to fit the apartment layouts. Typical column sizes are 24 in.
by 30 in. (610 mm by 762 mm) up to level 20 with a specified compressive
strength at 56 days of 8,000 psi (55 MPa). Above level 20, the columns
are 24 in. by 24 in. (610 mm by 610 mm) with a strength of 6,000 psi (40
MPa).
The foundation
of the building is a concrete mat, which is 5 ft (1.5 m) thick under
the tower and 3 ft (0.9 m) thick under the remainder of the structure.
The mat thickness was kept to a minimum to maintain about a 5-ft
(1.5-m) 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 (520 MPa) and is post-tensioned in
both directions under the tower. A 4,000-psi (25-MPa) mix was specified
for the mat, and basement retaining walls utilize 3,000-psi (20-MPa)
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
8th 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 in. by 36 in. (914 mm by 914 mm) up to the 20th level
and are 24 in. by 36 in. (610 mm by 914 mm) above that level. Concrete
strengths for the frame columns are similar to those for non-frame
columns. Typical frame beams are 24 in. by 36 in. (610 mm by 914
mm) with a specified compressive strength of 5,000 psi (35 MPa),
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 UBC.
 Construction
Data
This project is the first significant application of the 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 8th floor averaged 5 days per floor,
with the top 8 floors being assembled at a pace of 4 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.
Credits
Owner: Third and Mission Associates, LLC
Design/Builder: Pankow Residential Builders II, LP
Structural Engineer: Robert Englekirk Consulting Structural Engineers,
Inc.
Executive Architect: Kwan Henmi Architecture/Planning, Inc.
Design Architect: Elkus/Manfredi Architects, Ltd.
Design Architect (Interiors): Ismael Leyva Architect, PC
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