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Ventilation strategies for atria

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technical & practice: A new guide from the BRE provides basic instruction on controlling smoke and fires in the tricky world of building atria

Natural Ventilation in Atria for Environment and Smoke Control is an introductory guide to the advantages of using atria as part of natural ventilation strategies in buildings. It points out that while opportunities exist, with many office buildings having atria, there is a relatively low uptake of naturally ventilated atrium buildings. It suggests that one barrier is the perceived conflict between natural ventilation strategies and fire-safety strategies. However, it may also be that naturally ventilated office buildings are not as easy to let as air-conditioned ones. The guide is applicable to buildings other than offices. It is one of the results from research by the BRE, Colt and Ove Arup and Partners' project funded under the DETR Partners in Innovation scheme.

Given concerns over links between fire safety and natural ventilation, this guide covers both areas, pointing out that the perception that atrium buildings higher than three storeys cannot be naturally ventilated due to constraints of fire safety is mistaken.

This focus on the dual aspects of natural ventilation and fire safety suggests the need for integrated design; indeed the guide warns about not making appropriate strategic decisions early on - principally, it would lead to more expensive, less aesthetic and less functional compromises.

The guide summarises the principles of natural ventilation and reminds designers about the importance of the different strategies needed for winter and summer conditions.

In winter the key issue is the control of indoor air quality whereas during the summer it is mainly for the control of internal temperature.

With fire safety, the atrium design strategy revolves around controlling the creation, movement and venting of smoke to enable building occupants to escape, as well as allowing fire-services access. The guide also acknowledges that fire-fighting activities are also about minimising damage to property and the consequential losses. It is an issue that the Loss Prevention Council pursues with some vigour because of the cost to insurers of fire damage.

There is a range of seven smoke-control strategies outlined, from 'do nothing' to smoke-exhaust ventilation from each storey separately and including atrium depressurisation. The 'do nothing' strategy would only be appropriate where it takes longer to fill the atrium with smoke to a dangerous level, than it takes for the occupants to escape. Although this might seem a viable strategy, proving it is another matter. It relies on quantitative predictions of fire growth and occupant escape times.

The difficulty with this is that predicting the growth of fire is hard during its early stages. Coupled with this is the difficulty in accurately predicting the time needed to evacuate people from a building. Atrium depressurisation may be easier to contend with. Leaky atrium facades are an obvious weakness when it comes to preventing the spread of smoke or hot gasses from the atrium to adjacent spaces. One strategy, other than sealing the gaps, is to use natural depressurisation. Rather like natural ventilation it relies on the 'stack effect', the hot air causing the smoke to vent at the top of the atrium.

The similarity between the concepts of natural ventilation and atrium depressurisation means that in principle the strategies can be mixed, which guide discusses in more detail. In atrium depressurisation smoke is prevented from leaking from the atrium into adjacent spaces using similar principles to natural ventilation.

A computer model has been developed which allows designers to calculate the natural and fire ventilation openings required. It uses existing published equations to enable designers to calculate inlet openings with and without wind and roof vent sizes with and without fire. A copy will be available from the BRE web site (www.bre.co.uk) in March. This report acknowledges the limitations of the model.

Principally that it applies to stereotypical buildings. It is useful for testing the design during the early stages of a project, but will need to be backed up by expert advice for more detailed calculations.

The report outlines the model and input and output information and provides two worked examples, one for a 10-storey atrium offices and the other for a four-storey offices.

This publication is a useful starting point for designers. It also complements another recent BRE publication, Design Methodologies for Smoke and Heat Exhaust Ventilation. Aimed principally at fire engineers, this report summarises current advice and it covers the principles of smoke and heat ventilation in detail, giving equations and calculations.

Natural Ventilation in Atria for Environment and Smoke Control: an Introductory Guide. C.

Williams, E Perera, H Morgan, R Harrison, B Caplen, A Ferguson. CRC. £25, tel 020 7505 6622

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