Inspirational Buildings No.1

"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.

London-Gherkin-building

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

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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).

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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).

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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).

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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.

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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.

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