Fire and smoke control in naturally ventilated buildings

Abstract

There is a developing interest in achieving low-energy, naturally ventilated, non-domestic buildings in significant numbers over a relatively short period that is driven by the government's commitment to achieve reductions in carbon dioxide emissions and building owners' interests in reducing life cycle costs. The presumption is that this is possible within the current regulatory context. However, design for natural ventilation of public buildings is still innovative, at least within the regulatory framework. Two case studies involving the implementation of natural ventilation schemes indicate the concern and barriers raised by local authority Building Control departments, the fire authorities and various prospective insurers in response to design proposals. The design strategies devised fell outside the provisions of the prevailing codes and regulations in their approach to ensuring safe and effective fire control and smoke clearance using natural ventilation. The destabilizing effects of the current regulatory system impact negatively on the use of natural ventilation and hinder innovation. Schemes proposing to incorporate natural ventilation may be rejected through risk- and value-engineering exercises because of perceived uncertainties about compliance with current fire regulations and codes, and the time and cost implications of embarking on an exercise to prove at least equivalence in the creation of a safe environment.

Depuis relativement peu de temps, on s'intéresse de plus en plus à la construction en grands nombres de bâtiments à usage non domestiques, à faible consommation d'énergie et à ventilation naturelle; cette situation est le fait de l'engagement des pouvoirs publics à réduire les émissions d'oxyde de carbone et de l'intérêt des propriétaires à abaisser les coûts des cycles de vie. On part de l'hypothèse que cela est possible dans le contexte réglementaire actuel. Toutefois, la conception de systèmes de ventilation naturelle pour les bâtiments publics reste novatrice, au moins dans le cadre réglementaire. Deux études de cas portant sur la mise en œuvre de systèmes de ventilation naturelle font apparaître les préoccupations et les obstacles dressés par les autorités locales chargées de régir des bâtiments, les services de lutte contre l'incendie et divers assureurs prospectifs en réponse à des propositions de conception. Les stratégies de conception élaborées ne sont pas conformes aux dispositions des codes et des réglementations qui prévalent actuellement dans leur manière d'aborder avec efficacité la lutte contre les incendies et les dégagements de fumées en utilisant la ventilation naturelle. Les effets déstabilisants du système réglementaire actuel ont un impact négatif sur l'utilisation d'une ventilation naturelle et empêchent l'innovation. Certains systèmes proposant d'incorporer la ventilation naturelle risquent d'être rejetés, après évaluation technique des risques et des coûts du fait des incertitudes perçues au niveau de la conformité avec les codes et les réglementations actuelles en matière de lutte contre l'incendie ainsi que des aspects ‘temps’ et ‘coût’ d'un exercice destiné à prouver au moins l'équivalence dans la création d'un environnement sûr.

Keywords:

• fire and smoke modelling
• fire engineering
• fire regulations
• governance
• innovation
• natural ventilation
• performance-based regulation
• regulation
• sustainability
• UK
• modélisation des incendies et de la fumée
• systèmes anti-incendie
• réglementation en matière d'incendie
• gouvernance
• innovation
• ventilation naturelle
• réglementation basée sur les performances
• réglementation
• durabilité
• Royaume-Uni

Acknowledgements

The author thanks Dr Frank Mills, Eur Ing; F. A. Mills, BSc (Hons) CEng FCIBSE MIMechE MASHRAE of Environmental Design Services; and G. Dacey, Fire Safety Officer, University of Cambridge. In addition the following organizations played an important role in the research required to achieve the two buildings described: the Institute of Energy and Sustainable Development De Montfort University, Leicester and the BP Institute for Multiphase Flow at the University of Cambridge.

Miles Owarish, M10 Fire Consultancy Ltd., PO Box 138, St. Albans, Hertsfordshire, AL1 5LP (email: [email protected]) undertook the work described in this paper while working as Principal Consultant, Warrington Fire Consulting Ltd.

Notes

1. The following is the current regulatory context in the UK. (1) Part B of the Approved Documents, the Building Regulations: Approved Document B ‘Fire Safety’ 2002, is a ‘deemed to satisfy’ document published by the Office of the Deputy Prime Minister as advice and guidance on meeting the functional requirements of Part B of Schedule 1 of the Building Regulations 2000 (Office of the Deputy Prime Minister, 2002). In the UK, Building Control permission is granted by Local Authority Building Control Officers in response to a full Building Control Application. Fire Officers have become consultants to Building Control in this process and all communications are directed through Building Control. (2) BS 5588. Fire Precautions in the Design, Construction and Use of Buildings. Part 7: Code of Practice for the Incorporation of Atria in Buildings (AMD 10546) (BSI, 1997a), as amended 1999 and 2004. The BS 5588 set of codes are a set of ‘deemed to satisfy’ documents that relate to different types and aspects of buildings. These are generally used as an alternative to the Approved Documents or as a supplement where they do not contain adequately detailed recommendations and/or guidance.

2. The following definitions relating to fire issues are used. (1) Compartmentation: fire-resisting and smoke-retardant construction used to subdivide a building or large space into smaller spaces for the purpose of containing a fire and the products of combustion, and limiting the potential risk to life and damage to a building in the event of a fire occurring. (2) Fire damper: a mechanical or electromechanical device used to close off a route for the spread of smoke or fire. These devices are typically located within ductwork or other large openings where they pass through compartment walls/floors or other fire-resisting elements of construction. Dampers can be actuated by heat, smoke or a combination of both. (3) Risk management: a strategy of identifying the potential sources of ignition and the combustible materials in their vicinity and actively seeking methods to minimize the likelihood of a fire occurring by reducing the potential sources of ignition and/or making the remaining sources of ignition as safe as reasonably practicable while also minimizing the amount of combustible materials stored in areas where they may be exposed to the possibility of ignition. The amount of investment in managing the risk should be commensurate with the perceived risks to life and the building. (4) Value engineering: a formalized process of budget review to extract a maximum value. (5) Buoyancy-driven flow: flow driven by density/temperature differences. (6) Stack effect: the exploitation of the variation of barometric pressure by height-driven flow. (7) Flash-over: the point in the development of a fire at which the heat energy radiated downwards from the smoke/hot gas layer at the ceiling level is great enough to induce the spontaneous ignition of all or most of the other combustible materials within the room or space. This typically occurs when the temperature of the hot gas later reaches 600–650°C depending on the amount of ventilation within the space and the type of materials therein. (8) Atrium: a space within a building, not necessarily vertically aligned, passing through one or more structural floors (BS 5588 Part 7; BSI 1997). However, BS 5588 Part 7 also notes that it is not applicable to buildings of only two storeys in which the occupants are likely to be awake. Further, Approved Document B recommends that BS 5588 Part 7 needs not be used where the atrium does not pass through a compartment floor. (9) Fire-rated construction: an element of construction (wall, floor, door, etc.) that has been tested in accordance with the relevant part of BS 476 (BSI, 1997b) and shown to be capable of resisting the passage of fire for a designated period. (10) Mechanical smoke extraction: a system of fans and ductwork designed to draw smoke from a space at a predetermined rate in order to maintain a certain condition within that space. (11) Automatic fire detection: a system capable of monitoring specific characteristics (temperature, particulate density, toxicity) of an environment and measuring those characteristics that exceed the predetermined criteria. A warning is given by the system to indicate that the fire is likely to be in progress. (12) Smoke reservoir: a space designed to contain smoke within a defined region, preventing it from spreading out and affecting larger portions of the building. (13) T-squared fire: an idealized equation used for modelling the growth of a fire's heat output against time. A constant, based on the materials and expected characteristics of the fire, is multiplied by time squared (T 2) to determine the heat output.

3. See http://www.fire.nist.gov/fds