The effects of fire in or near buildings are mitigated by fire precautions required by national regulations and codes of practice. Over the past three decades building regulations have moved away from comprehensive prescriptive regulations to functional regulations supported by detailed technical guidance. It is generally accepted that existing prescriptive guidance, such as Approved Document B (AD B), is not suited to large, less compartmented and more complex buildings and is not suited to buildings which are refurbished when many of the features required simultaneously by prescription do not exist, for instance in single staircase buildings with only one obvious escape route.
While the present combination of regulations and guidance has served well, and will continue to be appropriate for simple buildings, there remains a pressing need for wider acceptance of Fire Safety Engineering (FSE) - see definition below. This should include performance-based principles, acceptance criteria, and calculation tools which permit the use of time-based calculations to address the important relationship between the time required for escape (which is determined by people behaviour, internal planning of the building and fire defence systems) and the time available for escape (which is determined by the human tenability of the fire effluent for each person at his or her location in the building). It seems generally true that performance based guidance is weakest in means of escape and the challenge here is to gain further information on the time it takes for people to get away from danger in, for instance, shops, public assembly and institutional buildings where large numbers of people, including disabled people, may be present. To accomplish this goal a methodology based on sound engineering principles, which employs calculation tools and expert judgement, is needed. This methodology is called fire safety engineering (FSE)
Fire Safety Engineering may be defined as the application of engineering principles, rules and expert judgement, based on a knowledge of human behaviour and a scientific understanding of the phenomena of fire and its effects, to:
save life, protect property and preserve the environment and heritage
quantify the hazards and risk of fire and its effects
The task in FSE is daunting and exciting due to the many imponderables in fire. Will the fire be accidental or malicious? In which room will the fire occur? What will be the item first ignited? Will doors be open or closed? What will be the design fire? Will the sprinklers operate and will they control the fire? How soon will occupants be aware of the fire and how rational will be their escape behaviour? These kinds of questions cannot be quantified in deterministic design but can be accounted for in probabilistic design if appropriate statistics are available. It is the availability of such statistics which is currently limiting probabilistic design quite apart from the need for all members of the design and enforcement team to be appropriately educated in quantitative risk assessment. In deterministic design, calculations are made assuming a limited range of plausible worst-case fire scenarios, and often accompanied by a large measure of engineering judgement and experience. An example of the deterministic approach and methodology can be seen in the first paper listed in My Fire Safety Publications under Fire Safety Engineering - click here to see
UK building regulations are functional, not prescriptive. For England and Wales the official guidance on how you satisfy the regulations is in a government document, Approved Document B 'Fire Safety' (AD B), which states that the prescriptive guidance in the AD does not have to be used and it allows a fire safety engineering approach to be used. It is similar in Ireland and Scotland. Technical guidance on fire safety engineering is given in a set of recent BSI Publication Drafts in the series PD 7974 under the umbrella of BS 7974: 2001 Application of fire safety engineering principles to the design of buildings. This series replaces BSI DD 240 with which I was very much involved. Shortly to be published, there is BS 9999 which will sit between Approved Document B and the PD 7974 series in terms of technical complexity.
Computer models are used for calculating smoke fill times, sometimes using computational fluid dynamics (CFD) for very large and complex building geometries. However, in all my audits of the work of fire safety engineers, 2-zone plume equations have been employed for smoke fill type problems using a spreadsheet such as Excel for number-crunching and making parametric studies. Computer-aided numerical models are sometimes used for people-evacuation modelling of complex large spaces. Computer models are not often used for calculations of structural fire resistance except when it is possible to show that steelwork may be used with less fire protection or none at all. Simple computer models are used for predicting thermal radiation hazards to nearby buildings, but not often. In my experience most consulting fire engineers are not making probabilistic analyses for ordinary buildings - such analyses are confined to nuclear power stations, petrochemical plant and sometimes in hospitals. Interestingly, in the UK we are free to use whatever technical guidance is available and appropriate (which may come from ISO, CEN, or BSI or even a professional body within the UK (such as CIBSE on smoke management) or elsewhere (such as NFPA standards), but this variation and freedom then makes it difficult for the checking and approving authorities.
I have been closely associated with fire safety engineering from its beginnings in the UK and was a member of the small group contracted by government (DTI) to prepare the first British Standard DD240 on the subject. I was also fortunate to get Personal Promotion in BRE to focus on FSE and have over the past 15 years decades published several papers on FSE- click here to see. I have been an active member of BSI committee FSH24 Fire Safety Engineering and also remain very active in the International Standards Organisation TC 92 SC4 on FSE.
Gordon Cooke, Fire Safety Consultant, London