The previous section demonstrated that the potential use of atria to successfully address social, economic and environmental issues has made a compelling case for their incorporation in buildings.
The focus of this thesis is to examine the daylight potential of atria as it is recognised to be one of the key aspects of the atrium form, contributing to a building’s aesthetics, experience and environment.
Atria allow for the adjoining spaces to have larger windows to admit daylight without considerable heat losses or heat gains. This potentially increases the amount of occupied space that can be naturally lit, and replaces artificial lighting which is typically the primary
cause of energy consumption in commercial and office buildings, particularly when its associated cooling load is considered.
Within an atrium, daylighting helps to define the atrium space and animate it. In the adjoining spaces, daylighting can improve illuminance distributions, thereby reducing the problem of brightness imbalance which can occur in unilaterally glazed rooms in deep plan buildings, and reduce the use of artificial lighting. If the requirement is to save energy in adjoining spaces by displacing electric lighting, then even higher levels of daylight may be required in the atrium, bringing with it the risk of glare and unwanted heat gain and/or loss consequently resulting in an increase in the energy consumption. The successful design of an atrium is therefore a fine balance between interdependent factors such as daylighting, heating, cooling and ventilation, as well as taking into consideration aesthetic and functional aspects.
Goncalves (2007) highlighted the use of different types of atria as one of the key strategies that have led to the improved daylighting in office buildings.
The Lloyds register of Shipping in London by Richard Rogers has two glazed atria slotted between the radiating 14 storey office wings allowing daylight to penetrate the office spaces, providing views in and out of the building and acting as a thermal buffer between the offices and their external environment. Atrium’s glass balustrades, and glazed and light opaque atrium facades and the floor act as light reflectors enhancing the lighting conditions and lending transparency to the atria (Figure 1:18) (Rogers Stirk Harbour + Partners, 2011).
In the Century Tower in Tokyo by Foster and Partners, the strategy of a top and side lit atrium with highly reflective surfaces combined with shallow adjoining floor plates and column free double height office spaces with suspended mezzanine floors that are open to the atrium create a day-lit environment (Figure 1:19) (Foster, 1992).
Figure 1:18 Atrium in the Lloyds Register of Shipping, London. (Photo by: Katsuhisa
Kida / FOTOTECA)
Figure 1:19 Century Tower in Tokyo by Foster and Partners
(http://www.fosterandpartners.com/Projects/04 09/Default.aspx)
The Evelina Children's Hospital in London, by Hopkins Architects, is characterised by wards on its one side and the giant roof of the atrium which is essentially a big, curve of glazing that meets the atrium floor (Figure 1:20). This arrangement, along with the use of highly reflective surfaces brings daylight to the atrium and importantly into the adjoining ward spaces.
Figure 1:20 Evelina Children’s Hospital, London by Hopkins Architects (http://www.hopkins.co.uk/projects/6,9/)
The Swiss RE HQ in London, designed by Fosters and Partners, is a circular plan 41 storeys building which has six triangular shaped atria carved out from the plan’s edges on each floor; these are joined vertically and spiral up the facade with a five degree shift on each floor (Figure 1:21). These triangular atria create six rectangular modules of office spaces; the atria are enclosed at every sixth floor and are essentially social/meeting spaces (Zukowsky and Thorne, 2000). The tower’s diagonally braced structural envelope creates column-free floor spaces and enables a fully glazed facade, which along with the atria enable daylight penetration and views (Foster and Partners, 2011).
Figure 1:21 30 St Mary Axe, London
(http://www.fosterandpartners.com/Projects/1004/Default.aspx)
The Heron Tower, in London, implements atria in its design by vertically sub-dividing the 36 storey building into three storey separate ‘blocks’/villages, where each village is connected by a three storey atrium space on its glazed north elevation (Figure 1:22). The two upper floors on each sub-divided ‘block’ are recessed at the centre allowing daylight to flood the internal spaces and reduce their dependency on artificial lighting. The considered orientation of the building and the atria cuts out any need for solar shading. This solution addresses the problem of unequal daylight distribution characteristic of tall atrium spaces (Slavid, 2006).
Figure 1:22 Three Storey Atria in the Heron Tower, London (Photo by: Kohn Pedersen Fox Associates PC)
1.5.1
Atrium Design Parameters
Daylight performance of an atrium is complex and depends on the predominant sky conditions in which the building stands, the nature of its roof and fenestration system, atrium orientation and geometry, design of the atrium facades including reflectance of its walls (glazed and opaque areas) and floor surfaces, and the characteristics of the adjoining spaces.
Climate and the sky conditions have a great influence on the way light behaves in an atrium. Consequently varied approaches are adopted to suit the different climatic conditions. For the temperate climate of Britain and the rest of Northern Europe, daylighting expectations are based on overcast skies. The ideal atrium in these circumstances is largely top-lit, and with a clear, unobstructed glazed roof to achieve the maximum transmission of light. The roof configuration not only dictates how much light enters the atrium but can affect its direction in a significant way. The fenestration system will control the intensity and spatial distribution of light entering the atrium. The net transmittance of the fenestration will vary with glazing system, geometry, glazing orientation and type, shading system and the illuminance conditions.
Although it is the transmittance of the roof structure that determines how much daylight enters the atrium, it is the design of the atrium wall surfaces and their reflectance properties that dictate how daylight is distributed about the atrium and its adjoining spaces, which this study examines in a four-sided, top lit, square shaped atrium. In addition to the atrium boundaries, the size of an atrium and its configuration, known as the atrium type, can affect the amount of daylight that penetrates it and its distribution. In general, the shallower and wider the atrium space, the better the contribution of direct daylight to the adjoining spaces.
Although the daylight potential of an atrium has been recognised widely, atrium buildings have a tendency to not utilise daylight successfully in spaces adjoining the atria. Daylight levels within the atrium space are generally sufficiently high. However, this may not be the case for spaces adjoining the atrium, where daylight varies significantly with every floor level. Rooms on the top floors can be over-lit and suffer from glare while daylight levels on the lower floors can be low, particularly in tall/deep atria. One of the key parameters which plays a fundamental role in the way in which light is distributed within the atrium and its adjoining spaces is the atrium facades that this study aims to investigate. A brief review of this subject area is outlined in the next section from which the thesis aims are drawn.