 | PRACTICE ENGINEERING THE WATER CUBE
| A swimming pool made of bubbles. Tristram Carfrae of Arup outlines the engineering factors which led to the competition-winning Beijing Olympic Swimming Centre, by PTW, China State Construction and Engineering Co and Arup. |
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The Water Cube seen
adjacent to Herzog and
de Meuron’s Olympic
Stadium.
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Plateau’s
geometry of soap
bubbles.
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Structure of
Weaire Phelan foam.
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Concept design using
Weaire Phelan foam.
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Competition model.
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CAD model of the
structural system.
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Sketch study of
The Water Cube acting
as a greenhouse,
showing the vented
cavity.
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Concept
facade detail.
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The
entry showing detail
of the facade design.
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Cladding the
building during the
construction process.
All images courtesy
PTW + CSCEC + Arup.
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STRUCTURAL CONCEPT. In early 2003 the
Municipality of Beijing announced a limited design
competition for the 2008 Olympic Swimming Centre.
Our consortium of China State Construction and
Engineering Co (CSCEC), PTW Architects and Arup
was shortlisted as one of ten international teams.
Unusually, we started the competition process
by outlining to the architects what we wanted to
achieve technically in terms of different engineering
disciplines. Based on our previous experience, the
solution that solved the most technical issues was an
insulated greenhouse with diffuse natural light, and
the main steel structure housed in a cavity, isolated
from both the outside and the corrosive pool
atmosphere. We even opined that ETFE cladding
would be an efficient means of construction – it
would use minimal material and remove the need for
a secondary structure, while providing better
insulation than single glazing.
Meanwhile, the architectural planning team
calculated that, to fit all the desired facilities, the
entire square site would have to be used. We also
became aware of the winning design for the Olympic
Stadium – our neighbour – the fantastic Herzog and
de Meuron/Arup curvaceous, red “bird’s nest”.
Suddenly it seemed our swimming centre should be
a blue box. Thus, the “Water Cube” was born.
The outstanding issue was: what form should
the structure take and what would the resulting
cladding pattern look like? The design team all
preferred the notion of a continuous skin that
covered the walls and the roof. And we knew that
it needed to be out of the ordinary to win this
prestigious commission. Early explorations led to
the question: what structural topology fills
three-dimensional space uniformly, other than the
somewhat prosaic triangulated space frame?
I assumed there would be many examples of this
in nature, from living cells to mineral crystals. But,
in fact, this seemingly innocuous question does not
have a straightforward answer. After exploring
various possibilities we started to look at work on
soap bubbles, firstly by the eighteenth-century
Belgian scientist Plateau. However, it was the highly
efficient solution to the question of how soap
bubbles connect, arrived at a century later by
Professor Weaire and his research assistant
Dr Phelan at Trinity College, Dublin, that provided
us with the answer for the Water Cube. The curious
thing about Weaire Phelan foam is that, despite its
complete regularity, when viewed at an arbitrary
angle it appears to be random and organic.
To construct the geometry of the structure of
our building, we start with an infinite array of
foam (oriented in a particular way) and then carve
out a block equal to the size of our building –
177 x 177 x 31 cubic metres. The three major
internal volumes are subtracted from this foam block
and the result is the geometry of the structure. The
structure is then clad with ETFE pillows inside and
out to achieve the desired organic look and to work
as an efficient insulated greenhouse.
So, in searching for the most efficient way of
subdividing space, we found a structure based on
the geometry of soap bubbles, and clad with plastic
pillows that look like bubbles. And inside, all the
water of a swimming centre! We were confident that
we had a winning scheme; our next challenge was to
convey the idea accurately to the judges.
We decided to build an accurate physical model
of all 22,000 structural elements and 4,000
(different) cladding panels. The only way to do this
seemed to be Rapid Prototyping machinery,
commonly used in the manufacturing and
automobile industries. It took us many weeks to
learn enough about the CAD modelling and the data
translation required just to make the structural
model. With two days left, the structural model was
flown from Melbourne to Beijing, where it was
joined to a handmade plastic skin (we just couldn’t
draw all the different pillow shapes in time), and the
model was complete. In July 2003, we were
announced as the winners of the competition and
awarded the design commission.
Now came the tricky question – does it work?
We had spent all our energy during the competition
in making the model and had not had a chance to
analyse the structure. In fact, it proved impossible to
manually select the size of all the structural elements
and obtain a structure that would stand up. So we
developed new software to automatically select the
member sizes through an iterative optimization
process. The result was remarkably efficient.
In time, our whole production process became
automated: one programme generated the entire
geometry from scratch, based on Weaire Phelan
foam and the size and shape of the building; the
structural optimization process sized all the
steelwork members and their connections; a
purpose-written script converted the structural
analysis wire frame model into an accurate threedimensional
solid CAD model; and construction
drawings and schedules were automatically
produced from the three-dimensional model. By the
end of the design phase, it took less than a week to
generate a whole new set of construction documents
after a major change to the building size or shape.
The foam structure is a true space frame in that
all the members are framed into the nodes. This
might seem inefficient in a country that does not
experience major earthquakes, but it is a perfect
energy-absorbing structure for seismically active
Beijing. We decided to make the structure from
simple circular tubes welded to spherical nodes at
each end to simplify the fabrication process.
There are 4,000 ETFE bubbles making up the
Water Cube cladding, with some as large as nine
metres across. The roof is made from seven different
bubbles; the walls from fifteen bubbles, which are
repeated throughout. Despite this repetition, the eye
perceives a random pattern. ETFE, or Ethylene Tetrafluoro-
ethylene, is a tough, durable plastic closely
related to PTFE (Teflon). It transmits more UV light
than glass and cleans itself with every rain shower.
Each pillow is permanently inflated by a low power
pump. This internal air pressure transforms a
0.2-millimetre-thick plastic into a cladding panel
capable of spanning relatively large distances. In
pillow form, ETFE is also a better insulator than
glass and, when equipped with frit patterns for
shading, it achieves the desired greenhouse effect.
FIRE. ETFE is an amazing material, but it is also
combustible, and China has a prescriptive building
code. For this innovative material to be used, Arup
addressed concerns about its performance in fire, the
potential fire scenarios, and the consequences for
safety. The greatest attribute of this material is that it
shrinks away from a fire, thus “self-venting” and
letting smoke out of the building.
The Chinese code also requires that structures be
fire-rated. To achieve this, a fire spray or a
fire-resistant intumescent paint would need to coat
the structure. With 90 kilometres of steel elements,
comprising 6.8 hectares surface area of steel, these
options were not feasible. A complex combination of
structural and fire engineering analysis
demonstrated that for worst-case fire scenarios, the
structure would continue to carry the loads without
failure, and therefore did not require fire protection
to the steel.
Egress presented further issues under this code.
An estimated 20,000 people are expected to use the
building at any one time during the Olympics. The
Chinese code required the equivalent of two sides of
the building, or 200 metres, of exit doors. Not only
would this have significantly impacted the look of
the building, but it would also have been a security
issue. Using the international guidelines for sporting
venues, and through detailed analysis of egress and
circulation, the number of exits was significantly
reduced. A computer model called FDS (Fire
Dynamic Simulator) was used to analyse how smoke
and heat would spread through the building, and the
performance of different smoke exhaust rates to keep
smoke away from people as they exit.
Appropriate fire safety systems like sprinklers
and smoke exhaust were incorporated, allowing the
more open and familiar circulation routes to be used
for egress.
This is the first time that such a major public
building in China has been designed using a
performance-based approach to fire engineering.
ENERGY. To maximize energy efficiency, the Water
Cube acts as a greenhouse. The ETFE cushions allow
high levels of natural daylight into the building and
harness the sun to passively heat the building and
pool water. This sustainable concept reduces the
energy consumption of the leisure pool hall by an
estimated 30 percent.
The system generates an effective negative
U value, a net energy gain to the building. Thermal
mass heat storage (in both the swimming pool water
and heavy surfaces surrounding the pool) ensures
that solar heating during the day is offset by
overnight cooling.
Variation in shading of the facades ensures that
fabric heat loads are minimized in summer but
maximized in winter, when the solar heat gain is
most beneficial. This is achieved by patterning the
various layers of the facade with translucent painted
frit and by ventilating the heat out of the cavity in
summer, and containing it in winter. The location
and pattern of these translucent elements respond to
the daylight and thermal requirements of the various
building uses adjacent to the facade.
The energy consumption of the large pool halls
is greatly reduced by using the displacement
ventilation principle in the mechanical systems.
The concept of stratification is critical to achieving
high passive solar heat gains without generating
large space cooling loads. Allowing stratification of
air in these large spaces, the mechanical system only
has to provide cooling to the occupied spaces. This
can reduce effective cooling loads by a factor of 10.
In summertime, non-pool and office areas will
use airconditioning to be kept around 23°C. The heat
rejection of the airconditioning will then be used to
heat the pools. The leisure pool must be kept at
around 30°C; the competition pool at around 28°C.
Swimming pool centres are corrosive
environments if not designed properly, so air
distribution is critical. Because of the unique facade,
surface temperature and air movement are essential
for preventing condensation. To achieve this, nozzles
will be located around the perimeter of the building
to supply air up the walls. During winter, thermal
buoyancy will lift warm air to the top; in summer, it
will be the reverse, with cooler air being supplied to
compensate for heat gain. This cooler air will not
have the same buoyancy force, so it won’t go all the
way to the roof and only condition the lower part of
the building. This stratification of the air will reduce
the energy consumption of the building.
This smart building was designed to have the
ability to create a responsive, comfortable
environment. For example, spectator seating areas
will be airconditioned separately by an
under-seating supply system which will only
operate during events, preventing wastage.
For us, the swimming centre is a pure delight,
not only because it solves all the technical issues in
one fell swoop, but also because of the wonderful,
and somewhat coincidental, fact that a building full
of water should be made from a box of bubbles.
PROFESSOR TRISTRAM CARFRAE IS A STRUCTURAL ENGINEER
WITH ARUP. HE IS RECOGNIZED AS A LEADING DESIGNER OF
SPORTING STADIUMS AND LIGHTWEIGHT LONG-SPAN
STRUCTURES. HE IS A FELLOW OF THE ACADEMY OF
TECHNOLOGICAL SCIENCES AND ENGINEERS AND WAS NAMED
AUSTRALIAN PROFESSIONAL ENGINEER OF THE YEAR IN 2001.
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FURTHER INFORMATION
Arup
Tristram Carfrae
T 02 9320 9320
F 02 9320 9321
E contact.aus@arup.com.au
W www.arup.com.au
PTW Architects
T 02 9232 5877
F 02 9221 4139
E info@ptw.com.au
W www.ptw.com.au
Vector Foiltec
Manufacturer of the ETFE
material used.
China
T +86 10 6495 8049
F +86 1350 107 6538
E jm@murphyyang.com
W www.vector-foiltec.com
Australia
T 03 9555 0727
F 03 9553 2204
E info@vfaust.com.au
W www.vector-foiltec.com
WATER CUBE, BEIJING
DESIGN CONSORTIUM
Consortium leader China
State Construction and
Engineering Corporation
Architect PTW Architects
and China State Construction
International Shenzhen
Design Consulting Co.
Engineer Arup and China
State Construction and
Engineering Corporation.
Project manager Three
Gorges Corporation. Client
Beijing State-Owned Assets
Management Co.
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