"Inspirational Buildings
No.1"
The Natural Ventilation strategy of 30
St. Mary Axe
by Fergal White
Why 30 St. Mary Axe ?
Although this building was originally lampooned by the British
public, mainly for it's eccentric form, it is now the pride of the
London skyline forming the signature London backdrop for many
movies. Designed by Foster & Associates, it has been hailed as
a miracle of biomimetic design being compared to the Hexactinellid
Euplectella aspergillum, or Venus' Flower Basket, with its "strong,
cylindrical, lattice-like exoskeleton". However, as much as the
glove fits, Foster knew precious nothing of sea sponges during the
design process and it can be safely said that the genius of this
design evolved from hours and hours of hard work. When I first saw
the natural ventilation design strategy for 30 St. Mary Axe I
realised I wasn't thinking hard enough so I signed up for a Masters
in Environmental Design.
fig 1. 30 St Mary Axe - Building in Context
(http://ced.berkeley.edu/courses/fa10/arch244/?p=2110)
Introduction
Swiss Re, as a company, holds sustainability as a core value,
(Shen, 2009) and
having been closely involved with sustainability issues in the
realm of insurance risks (resulting from global climate change),
stipulated the need for an environmentally progressive design
together with a high standard of internal working environment for
staff. (Munro,
2004). Their chosen architect, Norman Foster, had already
established foundations in ecological office design as far back as
the late 1970s after collaboration with Buckminster Fuller produced
the theoretical "Climatroffice" project. These ideas formed the
basis of the strategies used by Norman Foster & Associates for
the Commerzbank building in Frankfurt (completed in May 1997) which
was considered the world's first ecological office tower utilising
innovative architectural strategies to harness natural systems of
lighting and ventilation (Votruba, 2007).
Overall Natural Ventilation Strategy
fig 2. Site Plan showing building footprint on site (Williams, 2002) and
Circular Form sketches (Meguro, 2005)
Foster's concept was to design a tall building so as to maximise
public space at ground floor level and to help the office floors to
have proportions that would be more conducive to natural
ventilation. The tapered circular form is derived from the wind
conditions at the site where the aerodynamic form reduces the wind
loads on the façade while also reducing the pedestrian level winds
as compared to a similar height rectangular plan. The most
significant advantage of the building form is the minimisation of
wind turbulence. Air flowing around the building creates positive
pressure on the windward side and negative pressure on the leeward
side creating a driving force for cross flow ventilation (Mumovic and Santamouris,
2009).
fig 3. Level 26 Typical Floor Plan (Williams, 2002) & Pressure
Differential and Rotating Atria Concepts (Meguro, 2005)
The typical floor plan houses an enclosed structural core and
locates offices around the perimeter. Each floor plate has six
triangular atriums at the perimeter. This circular plan is the key
feature of the ventilation strategy as it is rotated on each floor
by 5̊ to create six spiralling atriums. These atriums, enhanced by
the pressure variation, act as the lungs of the building, providing
natural ventilation with air entering through automatic monitored
opening windows where the positive pressure occurs . A BMS, linked
to exterior weather stations reading temperature and wind speed,
determines when the weather is appropriate so that 800 windows can
open to let in outside air (Barkkume, 2007). These atriums work
out at between two and six floors tall thus preventing updraft
within the building and are distinguishable on the façade by their
darker glazing for glare and solar gain reduction (Shen, 2009).
fig 4. Monitored Opening Windows and Typical Atrium Interior
(http://5osa.tistory.com/tag/Contemporary?page=73)
The natural ventilation of the façade helps to keep the interior
cool by exhausting the hot air within the façade. The façade is
designed as a double skin with a double glazed external layer, 1 to
1.5m cavity, a solar blind and then an internal single glazed
screen. Air intake is located along the junction between triangular
shaped glass panels along the line of the floor plenum through an
oversized louvre blade (to allow for future change of use of
program, ie conference centre use etc) (Meguro, 2005).
fig 5. Section showing ventilated facade and air intake at the
plenum.(Williams,
2002)
The buildings distance from window to core is 15m which is in
excess of the naturally ventilated norm. The design as built has
the interior glass wall left out at the balconies in the atriums.
This allows the fresh air to penetrate the entire office floor
without mechanical assistance. Air warmed by occupants and
equipment rises up the chimney-like light wells (Russell, 2004).
Hilson Moran Partnership, the service engineering firm responsible
for modelling the building at design stage, used CFD to assess
natural ventilation potential. Based on a number of early CFD
results, a team decision was made to adopt a "mixed mode"
ventilation design, which would not be common for a high rise
building of this type. Local air-handling units have been used in
place of a centralised system which allows mixed-mode use to be
isolated by zone and by floor (Kitson, 2003).
Impact on building energy use and occupant
comfort
Natural ventilation of 30 St Mary Axe reduces but does not
negate the need for conventional air conditioning (Mumovic and Santamouris,
2009). The effectiveness of the natural ventilation system can
be measured by identifying how often the building does not require
mechanical cooling, however, during the course of the literature
review no energy consumption data (predicted or post-occupancy)
relating specifically to natural ventilation could be found.
fig 6. The predicted energy performance of the Swiss Re
building.(Source. Hilson Moran)(Meguro, 2005)
Hilson Moran determined that natural ventilation may be used for
up to 40% of the year, and when conditions become unacceptable, the
building would be sealed to the outside and go into either a
cooling or heating mode of operation (Kitson, 2003). Hilson Moran have stated
"Energy consumption will be approximately 150kWh/m2. This
represents a savings of up to 50% when compared with a
traditionally serviced commercial building of similar type and
size". Hilson Moran are comparing the building to standards
published by the Building Research Establishment in this case (Meguro, 2005).
Post-Occupancy Update
The Swiss Re company only occupy half of the 40 storey building
and are one of the only companies to utilise the natural
ventilation system. They have already decided to lower the
temperature at which the air conditioning kicks in from 26 to 24
degrees after employees complained of stuffiness. It was
found that tenants other than Swiss Re have opted for the year
round air conditioning package, not attracted to the savings, and
opting for comfort. These tenants have also built partitions, to
maintain confidentiality, which would block the natural flow of air
anyway. Additionally the top floors of the building are too high to
allow natural ventilation and air conditioning had to be mandatory
(Shen, 2009) . No
Post-Occupancy figures are available to confirm what effect these
realities have on the original energy saving predictions.
REFERENCES
BARKKUME, A. 2007. Innovative Building Skins: Double Glass
Wall Ventilated Facade. New Jersey Institute of
Technology.
BUCHANAN, P. 2007. The Tower: An Anachronism Awaiting Rebirth?
Havard Design Magazine.
KITSON, M. 2003. Swiss Re Headquarters Inside and Out.
Fluent News, XII, S2.
LOEWEN, L. J. & SUEDFELD, P. 1992. COGNITIVE AND AROUSAL
EFFECTS OF MASKING OFFICE NOISE. Environment and Behavior,
24, 381-395.
MEGURO, W. 2005. Beyond Blue and Red Arrows. MSc
Architectural Studies, MIT.
MUMOVIC, D. & SANTAMOURIS, M. 2009. A handbook of
sustainable building design and engineering: an integrated approach
to energy, health and operational performance, Earthscan.
MUNRO, D. 2004. Swiss Re's Building, London. Nyheter Om
Stålbyggnad, 36-43.
RUSSELL, J. 2004. Foster's "Towering Innuendo" is a Big,
Eco-Friendly Hit. Architectural Record,
192, 218-227.
SHEN, Y. 2009. Buildings: The Gherkin, London. Masters
of Environmental Studies, University of Pennsylvania.
VOTRUBA, M. 2007. Commerzbank Norman Foster Associates.
MArch, University of Waterloo.
WILLIAMS, A. 2002. Round Peg in a square hole. The
Architects' Journal, 25-35.