Between prescriptive guidance and fire safety engineering lies a hybrid approach to fire safety in buildings, such as that found in BS 9999. As Dave Berry explains, the standard provides a structured approach to risk based design where varying physical and human factors can be taken into account.

With the demise of the BS 5588 series of standards, technical guidance on fire safety in the UK, in all but residential buildings,  is now primarily provided at three different levels. These are:

General approach: The majority of new and altered buildings usually follow the ‘prescriptive' guidance contained in documents published by the relevant government departments, for instance Approved Document B in England & Wales (ADB).

Advanced approach: The relatively recent change in the UK to fire safety legislation now firmly based on a risk assessment process has highlighted the need for guidance that provides a structured approach to risk-based design, allowing designers and enforcers to take account of varying physical and human factors that affect the risk. BS 9999 – Code of Practice for the Design Management and Use of Buildings – fits firmly in this ‘middle' level.

Fire safety engineering: Fire safety engineering provides an alternative approach to that provided in ‘prescriptive' guidance and can often be the only practical way to achieve a satisfactory standard of fire safety in all or part of a building. BS 7974 and its accompanying ‘Published Documents' provide comprehensive guidance in this developing area.

These three levels of guidance permit a design approach to be adopted that corresponds to the complexity of the building and to the degree of flexibility required.

Prescriptive problem?
Current ‘prescriptive' guides such as ADB do attempt to reflect risk, albeit in a rather simplistic form. For example, ADB refers to the main occupancy of the building to determine generic levels of risk, and classifies those generic levels using the term ‘Purpose Groups'.  These Purpose Groups are: 

• Residential
• Office
• Shop and Commercial
• Assembly and Recreation
• Industrial
• Storage and Other

However, premises that fall into the same Purpose Group do not necessarily reflect the same level of risk. To illustrate this point we can look at the case of two ‘new' factories approved under the Building Regulations process following their completion but prior to their occupation. They are two identical ‘speculative' factory units with the means of escape (MOE) following ADB ‘guidance' and both were provided with a manual electrical fire alarm system. Yet the fire and rescue service subsequently considered that factory A should (i) install a fully automatic fire alarm, and (ii) provide additional means of escape doors; whilst in factory B they have (i) ‘allowed' the fire alarm provided to be replaced with a manual sounder and (ii) ‘allowed' a number of the MOE doors to be blocked up! How could this be the case? The answer is because they considered that the risk is different in each case.

In factory A the process involves encapsulating petroleum in a ‘plastic' container. Petroleum product is ‘piped' round factory, plastic containers formed using hot process and containers then filled and sealed using a hot process.

In factory B the process involves making ‘decorative' concrete blocks. The materials used are sand, cement, water and the only machinery is electric powered concrete mixers. Concrete blocks are formed in wooden formers and the process is almost entirely ‘wet'.

Risk based guidance – BS 9999
The overall concept of BS 9999 is to more properly reflect the true nature of the risk, and to allow the designer and enforcing bodies to balance that risk by taking into account the package of fire precautions that is proposed or provided. The intention is that the package of fire precautions provided for a building should reflect: the nature of the use of the building; the occupants; the processes; the materials stored and used; and the fire safety management provided. These characteristics are then used as the basis for categorising buildings by their ‘risk profiles', a process which provides a basis against which the risk to occupants can be assessed and the appropriate level of fire precautions determined. A risk profile primarily comprises two main elements: occupancy characteristics and fire growth rates.

Occupancy characteristics are based on the groupings originally developed for BS 5588: Part 7 – Atria Code. They are:
A - Occupants who are awake and familiar with the building
B - Occupants who are awake and unfamiliar with the building
C - Occupants who are likely to be asleep; these are then split into three sub-categories:
       Ci - Long-term individual occupancy (individual flats without 24 hr maintenance and management control on site)
       Cii - Long-term managed occupancy (serviced flats, halls of residence, boarding schools, etc)
      Ciii - Short-term occupancy (hotels)

The fire growth rate is the rate at which it is estimated that a fire will grow. Here it should be emphasised that a building with a high fire load density will not necessarily have a rapid fire growth rate, and conversely a low fire load density will not necessarily have a slow fire growth rate. Guidance on this is provided in PD 7974-1, which is a Published Document in support of BS 7974 (Fire Safety Engineering). There are four bands of fire growth; these together with examples are shown in table 1.

The occupancy characteristic and the fire growth rate are now brought together to determine the risk profile for the building, or part of the building, concerned. The occupancy characteristic is identified by either the letter A, B or C; and the fire growth rate by the numbers 1 to 4. This results in the risk profiles which are shown in table 2.

Building design guidance
The Code of Practice provides general guidance on identifying and analysing the fire risk and the appropriate provision of fire precautions during the design, construction, in use and management phases of a building. It provides that guidance in a number of sections, including ones that specifically address:

• Designing so that a building can be constructed
• Managing fire safety
• Designing means of escape
• Access and facilities for firefighting
• Designing the building structure (load-bearing and non-load-bearing elements)
• Special risk protection
• Managing occupied buildings

Where appropriate, the guidance specifically addresses the needs of less able bodied people including: mobility impaired people; wheelchair users; people who are deaf or hard of hearing; blind and partially sighted people; people with cognitive disabilities; the use of personal emergency evacuation plans; the use of refuges; evacuation using lifts; evacuation using stairways; test evacuations for people requiring assistance; and fire instruction and training.

The Code of Practice also includes the management of fire safety as an integral part of the guidance. General and specific guidance is provided covering:

• Planning for changes in risk profile
• Resources and authority
• Staffing levels
• Fire training
• Work control
• Communications procedures
• Maintenance and testing of fire safety systems
• Liaison with the fire and rescue service
• Contingency planning

There are three benchmarks of management levels:

• Level 1 giving the highest level of management
• Level 2 giving a normally acceptable level of management
• Level 3 giving a very basic level of management

Against each of the areas identified above examples are given of the attributes that would help establish the level of management being provided. A table is provided which provides guidance as to the levels of risk management that are considered appropriate for each risk profile. The management levels required for each different risk profile are shown in table 3.

Guidance is given on designing the building structure and covers: the elements of a structure; fire resistance; compartmentation; openings; materials and finishes; external fire spread; ancillary accommodation to the main use of the building; and engineering services.

Guidance is also given on the provision of access and facilities for firefighting, and covers such areas as: firefighting shafts; firefighting stairs and lifts; vehicle access; water supplies for fire and rescue service use; heat and smoke control; and communications systems for fire and rescue service use.

An illustration: travel distances
The different approach provided by BS 9999 can be illustrated by considering how guidance for means of escape travel distances is provided. The standard initially establishes a package of minimum fire protection measures, including travel distances. The maximum travel distance when minimum fire protection measures are provided is given against the appropriate risk profile, as shown in table 4.

Using an A2 risk profile as an illustration, a two-way travel distance of 55m is allowed; by fitting sprinklers, the risk is lowered and the risk profile is changed to A1, so the maximum length of two-way travel allowed is increased to 65m.

Depending on the type of occupancy and level of management within the building, the provision of an automatic detection and alarm system, primarily utilising smoke detectors and incorporating an informative warning system such as a voice alarm, thereby increasing the time available, might allow longer travel distances and narrower doors. Where a clear benefit resulting from the addition of detection and warning systems is demonstrated and is appropriate to the circumstances, a 15% increase in allowable travel distance and a 15% reduction in door width, corridor width and stair width can be applied.

For rooms with high ceilings, it is permissible to increase the travel distance and decrease the door width, corridor width and stair width, provided that the increase/reduction is no more than the percentages given in the standard and that the entire escape route, with the exception of corridors and lobbies, has a high ceiling. The high ceiling forms a reservoir for smoke, allowing more time for the occupiers to escape before the corridors/lobbies are affected. This flexibility is only appropriate when a full account is taken of the risk presented. i.e. position, height and nature of fire load.

However the guidance also establishes maximum travel distances when additional fire protection measures are provided (table 5). So no matter what additional measures you provide, there is an absolute limit beyond which the standard does not allow you to stray.

Summary
With BS 9999 it is not a case of just replacing the ‘numbers' in the old standards with new ones. This really is a case of a different philosophy – a totally different approach to previous standards. For some it will take some getting used to, but for many it is already providing them with room to manoeuvre in a structured environment. 

Dave Berry was vice chairman of the BSI committee responsible for BS 9999, is vice chairman of the Fire Industry Association's  risk assessment council, and chairman of Fire & Risk Management Support Services Ltd.