Arup & Partners
Shigematsu, Ellen van Loon y Victor van der Chijs, David Gianotten
Balmond, Ove Arup & Partners
Building and Industry
State Construction and Engineering Corporation
Central Television (CCTV)
1, 234m, Tower 2, 210m
Central Television (CCTV) is the principal state-run broadcaster in China.
Around the turn of the millennium, they
ran 13 channels but had ambitious plans for expansion. They wanted to be able to run
200 channels and compete globally with the likes of CNN, NBC, BBC, and Sky
News. In 2002 they organized an international design competition for a new
headquarters. The requirements of the building were that all the functions of production, management, and
administration to be contained on one site, but not
necessarily one building, in the newly developed Beijing Central Business
competition was won by the team of Rem Koolhaas (OMA) and Arup. The idea behind the concept is that the building combines
administration and offices, news and broadcasting, program production and
services – the entire television process, into a single, continuous loop.
envisioned a building that functioned as a system. The three-dimensional form would
offer CCTV staff the ability to perform the formerly scattered functions of television
production within a “continuous loop” referring to a closed-circuit television.
His solution presented the beginning of an engineering challenge. The building took the form of a three-dimensional continuous
loop, similar to a Mobius strip, formed by a 9-story base structure joining two
50-story high leaning towers which are then joined at their apex via a 13-story
cantilevered “overhang” structure, suspended 36 stories in the sky. This truly new
building shape is exploited to provide the primary structural support system.
Through this, he also merges high-rise
occupancy with the shortened distances of a continuous
concept for the CCTV building faced both design and site issues. From a design perspective,
there is a general instability due to the unusual continuous loop form with
sloping towers and a cantilevered overhang. There is also instability due to
weight. The dead load of the CCTV headquarters is equal to 125,000 tons of
steel. This weight comes from the fact that the building is constructed mainly
from raw steel, including primary structure, internal structure, and transfer
trusses. The live loads are also greater than a regular office building of similar
square footage due to the fact this building is being used for television production.
In addition to people, there will be numerous
studios, broadcasting equipment, and facilities.
site also had its share of issues. Beijing
is seismically active, an earthquake zone, so every building must take that
into consideration. The site also had a shallow subsoil condition meaning there
is high settlement risk for a poured concrete foundation. There is also a large
amount of underground water beneath the site.
the temperature in Beijing changes dramatically from season to season.
Temperatures of over 100 degrees Fahrenheit in the summer and below freezing in
the winter are not uncommon. The engineers had to consider dramatic thermal expansion
and contraction. The average wind speed in Beijing is around 5 mph, and because
of the unique design, it created large
areas to catch the wind and consequently
the wind load is substantial. Another environmental consideration to load was
snow and ice. This is because the cantilever is a large suspended area in the
air, meaning the snow and ice generated throughout the winter could directly
increase the building load.
ENVELOPE AND FAÇADE DETAIL
the design concept and design challenges drove the ultimate concept and execution
of the high-performance building envelope and façade. The innovative shape of the building is utilized to provide
its main structural support and stability system. The form necessitated a
primarily structural steel building for shape and to enhanced seismic
performance. All the structural support elements in the building are steel, apart
from some external columns that are steel-reinforced concrete. The floors are
composite slabs on steel beams.
The diagrid frame system was chosen
due to the fact that it is on average about 20% lighter than a typical moment
steel frame design. It was also necessary due to the unusual overall form and for
the ability to vary the overall density of its grid so that it may better
respond to varying structural loads. This was particularly useful in dealing
with the cantilever section of the building. A diagrid is essential a
triangulate structure with diagonal support beams. The building loads follow
the diagonals of the triangles. Through this load transfer, gravity and lateral
loads can be transferred safely to the ground. Consequentially, the internal
cores transfer only minimal amounts of the gravity load and the floor slabs do
not transfer any lateral loads. Steel is most commonly used in a diagrid system
due to its high compressive and tensile strength and was used in the CCTV building. It is the marriage of
columns, diagonals, and bracings into a single system.
The CCTV’s self-supporting hybrid façade
is made up of a blast-resistant unitized curtain wall that attaches to the
structural steel diagrid. Each diamond in the grid spans between 4 to 10 stories
in height, depending on both the use of space and structural concerns. Every piece
of glass in the unitized curtain wall has a 50% silver frit admixture that both
provides sun shading and is used to attain a matte effect on the exterior view.
This was done to draw attention to minimal exterior cladding and to give the
radical design an unexpectedly understated appearance in the Beijing skyline.
The facades of CCTV portray the
irregular geometry of the building’s steel diagrid structure. The structure at
work within the building is pulled from
the interior to the exterior, rendering the geometry on the face of the
building with cladding. As the façade wraps its way around the Mobius strip structure, it is a constant push
and pulls of structural density, with the
diagrid opening and closing as the load requirements change. It is both a dense
and open grid of diagonals forms the structural system of the building and mirrors
the dispersal of forces that the structure experiences under different load
conditions. Koolhaas has turned the façade
into a simplified, visual manifestation of the structure that lies beneath.
Given the odd form, this building could look unstable. Thanks to this expression
of structure, the building appears sound.
Based on soil analysis, the bearing
capacity of the subsoil at the site of the CCTV headquarters was not deemed
sufficient for a standard poured concrete foundation under the entire
structure. A piled raft foundation was used under the main towers. The piles
are 1200mm in diameter and up to 35m in length following a lengthy iteration
process to determine optimal lengths. Under the remainder of the 9-story base
was a record-setting 40,000 m^2
continuous concrete pour foundation. In addition to regular gravity and lateral
forces acting on the structure, there are significant additional construction
stage forces due to the fact that the building comprises two separate leaning
Towers with cantilever up until the point at which they are joined to become
one structure. The construction had to be sequenced very carefully to deal this
It went as following: Building foundation raft, setting up
the column, using craning to build up the structure, connecting the overhang, completing the overhang structure,
installing the exterior glass.