20 C O N T E X T 1 1 3 : M A R C H 2 0 1 0 Potentially longer travel times means that the travel distances stipulated in ADB might be increased while maintaining comparable life safety levels. Alternatively, narrower door widths might be acceptable, given that the slower flow rates are compensated for by increased safe egress time. Further benefits might be gained from enhanced detection systems (over what would be recommended as a minimum standard) and/or higher standards of building management which would reduce pre-movement times of occupants. To weigh up options such as these, it is necessary to determine, for example, the expected rate of fire growth within the property. This requires specialist knowledge of fire growth and heat release. Fire growth rate enables a user ‘risk profile’ to be calculated which is used to determine the minimum life safety recommendations. Are such evaluations and decisions within the capability of building disciplines outside the realm of fire engineering? Notwithstanding the benefits of the BS 9999 approach, it still remains prescriptive as it places limits on the extended (or reduced) life safety measures it offers. How useful can this document be when dealing with an existing building, which may have a mix of narrow exits, high ceilings and unusual construction? A fire safety engineered solution identifies the significance of specific risks and takes full advantage of any inherent benefits which may exist in each individual case. By taking a wider view in reaching its site-specific recommendations, other factors such as a building’s relative isolation to the fire service, prevailing winds, and exposure or proximity to other risks can also be considered. When dealing with an existing building undergoing material alteration, shoe-horning it into either ADB or Andrew Forecast is senior fire engineer for Trenton Consultants. BS 9999 does not necessarily allow full advantage to be taken of any constructional or spatial conditions that the building offers. Conversely, the inherent fire risks may be greater than allowed for within the codes of practice due to obscure or unconventional methods of construction. For example, the existence of significant hidden voids would need to be addressed directly to ensure that life safety is not compromised. In many historic buildings invasive and visually obtrusive life safety systems (such as those relating to detection and suppression) are unacceptable, and alternative technology or bespoke measures need to be considered and designed. These approaches are beyond the scope of current codes of practice but may be acceptable, on the basis of proven performance, as part of a fire strategy developed and justified under the guidance of an experienced fire engineer. In conclusion, BS 9999 is a useful way of providing a more risk-specific fire safety assessment than ADB. However, the limitations of the code and in the knowledge and experience of its intended users should be borne in mind when thinking about its application to historic buildings. Although BS 9999 can be used to assess existing buildings, making fundamental changes in line with the guidelines may be partly or wholly impracticable. The complexity of fire safety issues in historic buildings will often warrant the engagement of a fire engineer to develop a fire strategy that takes full advantage of the existing construction and design, and minimises its impact on the significance of the building. In addition, the fire engineer will be able to take account of risks to property and artefacts, factors which are beyond the scope of codes of practice that are primarily concerned with life safety. Damping down after a fire at Cowgate in Edinburgh’s old town in 2002. Buildings destroyed included the Edinburgh Fringe venue the Gilded Balloon and much of the University of Edinburgh’s school of informatics. (Photo: © Crown Copyright: RCAHMS. Licensor www. rcahms.gov.uk)
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