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Nashville
May Grow Up With a 1,057-Ft Tower
7/31/2006
By
Nadine M. Post
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Stand
Out. Tower, if built, would be the tallest in the
U.S. outside Chicago and New York City.
(Photo courtesy of Paradigm Productions) |
With approval for a redesigned high-rise in
its pocket, the developer of what may become Nashville’s tallest
building and the tallest tower in the U.S. outside Chicago and New
York City, is moving full-steam ahead. The plan is to break ground
in January, after financing is secured for the 1,057-ft-tall, hotel-residential
condominium tower.
The goal is to have financing wrapped up in
November. But before that happens, the developer must lock in a
hotel operator and an equity investor. “So far, we are funding
the estimated $300-million project out of pocket,” says Ted
Kromer, development director for Giarratana Development LLC, Nashville.
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| Outriggered.
A concrete core braced by outriggers to perimeter supercolumns
minimized structure on the tower’s faces. |
Animation
Windows Media (560 KB)
Quicktime (396 KB) |
Signature Tower is Giarratana’s second
try for the site. The first building, which had office space as
well as a hotel and residential units, was rejected by the city
because, with punched windows, it looked too much like other buildings,
says Kromer.
After the rebuff, the developer switched architects
and started from scratch. Smallwood, Reynolds, Stewart, Stewart
Associates Inc., Atlanta, came up with a more contemporary look,
with large windows and a spire.
There was also a change of use and a footprint
reduced to a 137-ft square with recessed, articulated corners for
balconies. The size is more efficient for residential units, says
Kromer. But it meant the tower had to be taller to have enough units
to make the numbers work. So, the building grew up.
The irregular perimeter reduces turbulence
by breaking up the wind. But it adds to cost because it greatly
increases the surface area of the window glass. The upside is that
the units, with better views, are more valuable, and that will offset
the cost premium for the glass, says Kromer.
The owner’s wish for unobstructed views
drove the structural concept, currently in the design development
phase. “No perimeter frames were allowed,” Timothy R.
Santi, senior project manager for the structural engineer, the Atlanta
office of Walter P. Moore.
To keep the perimeter as free of structure
as possible, the engineer devised a lateral load-resisting system
in reinforced concrete that relies on an outrigger- braced core.
At three levels—14, 40 and 68—eight outrigger walls span
33 ft from core corners via a column 7 ft away to rectilinear supercolumns
in line with the outriggers at the tower’s perimeter. In plan,
the configuration creates a tick-tack-toe board. The lower two outriggers
are 32.5 ft tall and the hat outrigger is 14 ft tall.
To accommodate differential shortening due
to gravity loads between the core and the columns 7 ft away at the
two lower outrigger levels, the engineer devised a 7-ft-wide “vertical
pour strip.” Each strip contains two steel plates. Initially,
one plate is connected to the outboard column and the other is connected
to the core.
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| Loaded.
Brighter colors show more downward deflection of floor under
gravity bed . (Rendering top left above and below courtesy of
Walter P. Moore and Associates, Inc.) |
Disconnect
WPM’s strategy is to keep the columns
and core divorced for as long as possible. “They will be locked
in at the last minute,” says Santi.
The lower pour strips will be cast when the
intermediate outriggers are cast. The upper pour strips will be
cast when the hat outriggers are cast, says the engineer. The infill
concrete will connect the outriggers to the core, further engaging
the lateral system.
The temporary disconnect allows the columns
and core to shrink independently as they are loaded during the construction
phase. Typically, engineers use brute force in the form of extra
reinforcing steel to handle the differential shortening so that
the contractor can cast the elements all at once, says Santi.
The tower’s 200-ft-tall spire is expected
to have a damper to reduce accelerations in the uppermost occupied
levels. The damper will stiffen the building 15% to 20%, says the
engineer.
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| Divorced.
Vertical strip is cast later to accomodate different settlement.
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Tight Site
The design calls for a nine-story parking
garage under and around the tower. Tight site constraints and high-fracture
bedrock kept the engineer from bracing the tower near the street
level. Instead, the tower cantilevers from the bottom of the excavation.
Though the building above grade is only 1,057 ft, the vertical cantilever
is 1,167 ft. The tower is “extremely slender,” says the
engineer. It has an 8.6:1 aspect ratio from the structural base
to the spire’s top.
The Atlanta office of Turner Construction
Co., the general contractor, plans to line drill and blast through
80 ft of rock to create the 100-ft-deep hole for the parking structure.
Care must be taken not to disturb three churches on three sides
of the site. “It’s a tight site, with no laydown area,”
says David Butler, Turner’s vice president.
Once out of the ground, Turner has a strategy
to minimize crew sizes and hoist traffic congestion and to eliminate
the need for a second tower crane. Instead of dividing the 18,700-sq-ft
footprint into two pour areas, Turner will create work quadrants,
each with four teams with a specific task. Crews will be on a four-day
rotation.
The smaller pours are a “big advantage
because they allow us to stagger the shifts in each quadrant, minimize
crew size and reduce the number of workers trying to get to the
top of the building at the same time every morning,” says Butler.
The outrigger floors, with their hefty walls,
are a different story. Butler expects them to take seven days each,
instead of four.
Construction of the top of the building is
expected to be a challenge. “It will be difficult to get the
tower crane high enough above the structure to set the steel spire,”
says Butler. Details of the phase have yet to be worked out because
much of the top of the building is still in the planning stage,
he adds.
If all goes as planned, construction would
be completed in 2009. That would put Nashville on the “supertall”
building map, however unintentionally.
“We didn’t start out to make the
tower particularly tall,” says Kromer. “There was a cost
savings in reducing the area of the floor plate.”
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