Strategic Fire Protection in Historic Buildings

Richard Forrest

 

The introduction of conventional fire doors and partitions can have a disastrous affect on a building's character and historic interest. Risk assessment and the development of a strategic approach to fire safety measures can lead to more sympathetic solutions.

While modern buildings are designed from the outset to allow the occupants to leave quickly and easily in the event of a fire, adapting an historic building is more difficult. Two primary factors must be considered: the protection of persons either living, working or visiting the premises; and the protection of the building fabric and its contents. If the building concerned is also open to the visiting public, the requirement for life safety measures is even greater.

The relative priorities for life safety and property protection will be viewed differently by those involved in the specification or definition of requirements. The fire authorities or the local fire brigade will be primarily concerned with ensuring that optimum standards are achieved for the provision of means of escape and for the inclusion of means for fighting fire. Conservationists on the other hand are primarily concerned with preserving the building fabric without the intrusive effects and loss of fabric that are the inevitable consequences of most standard fire precaution measures.

The building owner or occupier is therefore left in somewhat of a dilemma. What are the legal requirements? What life safety standards should be considered for the current and future usage and occupancy? What provisions should be made to protect the building and its contents against the ravages of fire? What are the risks?

These questions cannot be easily answered. The only requirement in law concerns the provision for life safety and adequate means for escape, not the protection of property. Furthermore, current legislation under the Fire Precautions Act 1971 relates only to those buildings put to a designated use and unless the building incorporates office, shop, factory or hotel use, it is unlikely that the Act and the associated fire certification process will apply. Guides to fire safety standards do exist. These however are based upon prescriptive standards that are founded on generally deemed to satisfy criteria applied to the main factors of design for life safety, such as the provision of exits, protected routes and maximum travel distances. These standards have no real scientific basis, but rather have evolved over time and are considered appropriate for most building types and occupancy.

Fire safety design standards advocated by the current Building Regulations primarily apply to building work only. However they can affect existing buildings where 'material alterations' are proposed that will effectively downgrade existing provisions covered by the regulations, such as means of escape, fire spread and access by the fire services. They also apply where there is a 'material change of use' including conversions to form an hotel, a public building or a dwelling, and the subdivision of a building to form a flat. The Regulations are a relatively recent development responding to modern building techniques and materials. Older buildings may have been constructed with provision for very different methods of transferring air, heat and light around a structure, often in the form of ducts and shafts. With the introduction of more modern building services, the original structure may well have been built over or adapted, creating voids. Recent tragic history has shown, as with the fires at Hampton Court Palace and Windsor Palace, that such voids can contribute to the propagation and rapid spread of fire.

Recent years have seen rapid developments in our understanding of the nature and science of fire. As with any other facet of building development, the specifier no longer needs to rely solely on the prescribed standards for design: a specific tailored solution can be derived to take full advantage of existing features.

Fire precaution issues can be broadly broken down into two very specific categories: 'passive' protection measures which rely on physical barriers to restrict the development or spread of fire; and 'active' fire protection measures including, for example, fire detector and extinguisher systems.

PASSIVE VERSUS ACTIVE MEASURES

Current prescriptive fire safety standards rely very heavily on passive protection, usually involving the enclosure of staircases and corridors leading to final escape points and the provision of fire doors across passageways. In addition to the intrusion that the additional doors and partitions can cause in an historic interior, upgrading original historic features such as doors, walls, floors and ceilings to standards of fire resistance intended for more modern buildings is less than ideal. The usual requirement is to ensure that all elements of a defined fire compartment can resist the passage of fire and smoke for a minimum period of thirty minutes. If the main elements of the construction are masonry or similar then this standard can be achieved relatively easily, with only nominal enhancement to breaches in their integrity caused by pipe runs or cracks for example. However, the main weak spots are the door openings: original doors are rarely able to satisfy the half-hour requirement. In some cases it may be possible to upgrade their resistance using linings and seals with limited affect on their character and interest, but this may not be acceptable where particularly fine work is concerned; in other cases there may be no alternative to their complete replacement if a passive approach to fire protection is to be employed.

Active fire protection measures provide the fire engineer with the potential to offset some of the more onerous passive measures by trading off certain elements of the design process.

RISK ASSESSMENTS AND TRADE-OFFS

The key to a successful solution is the risk assessment, which is carried out as part of a comprehensive fire safety review of the building, to identify the degree of risk to both life and property. The assessment also needs to consider any occupier requirements that may have an effect on fire strategy. For example, where a stately home or a museum is concerned the need to salvage artefacts could well represent an important factor in the final definition of the fire plan for the premises. Having identified and quantified fire risk, the basis of fire safety design must be defined, always taking into account the requirement to satisfy life safety issues, and to balance property protection issues against physical intrusion.

Recent reports commissioned by the Government to enquire into major fires in historic premises have placed strong emphasis on the requirement and importance of structural compartmentation. The fire plan design philosophy should reflect this emphasis by identifying a solution that utilises existing building features. Any existing features which may also be beneficial such as large room volumes and high ceilings should also be identified and incorporated into the plan. The risk analysis should be used to provide fire safety management recommendations aimed to reduce the probability of the outbreak of fire and to minimise the potential effects of fire by reducing or managing the fire load.

The acceptability of design solutions which make full use of the existing fabric with the minimum alteration will ultimately depend on the length of time taken to evacuate the building safely, under fire conditions, and assuming worst case occupation conditions.

In some small historic buildings which contain a staircase within one single space, with all rooms opening directly off it, further compartmentalisation may be unnecessary. Upgrading the doors and walls to provide half hour fire protection can be avoided by introducing an air pressurisation system. In the event of a fire occurring in one of the rooms, the system is activated, rapidly pressurising the escape route and forcing smoke to leave the building through vents introduced in the rooms. However the system has limited application as it can only be used where the staircase is self contained, and due to the problems associated with accommodating the large amount of plant required.

Property protection issues may rely on automatic fire suppression, but where a suitable system cannot be installed perhaps due to the appearance of sprinklers, the time interval between fire initiation and detection becomes very important. It must be recognised that all large fires start in a small way and if detected and subsequently tackled in the incipient stages can be prevented from developing. Where there is no staff in permanent, 24-hour attendance who are trained to deal with an emergency and equipped with first aid fire fighting facilities at the building, early detection will at least ensure the earliest attendance by the fire brigade. 

The most effective automatic means to provide fire protection to any premises is to introduce a fire suppression system. The major benefit of a suppression installation is that not only does it detect a fire, it tackles it at a very early stage. The most common form of fire suppression system is the water sprinkler. Recent environmental issues have restricted the use of alternatives due to effects of certain suppression gases on the ozone layer. Water suppression research has developed rapidly and has given rise to a selection of sprinkler and water spray systems that can be configured to provide ultimate protection against fire growth and spread and also against accidental activation. It is perhaps worthwhile considering that only the sprinkler heads in the immediate vicinity of a fire would activate and then would apply only a fraction of the water that an attending fire service would apply to a developing fire.

RECENT DEVELOPMENTS IN AFD

Should fire suppression systems be considered inappropriate, the installation of Automatic Fire Detection (AFD) can provide the vital design definition of time interval between fire initiation and detection. AFD systems which are incorporated with a fire alarm system have been developed such that a variety of options exist as to operation, appearance and potential for concealment. Traditional yoghurt pot ionisation point detectors are now giving away to low profile optical single point units. These tend to be less obvious and can be coloured by the manufacturer to match surroundings. Optical beam detectors can be installed with effect to large ceiling areas where a proliferation of point detectors is undesirable. The units placed with a beam source and receiver at either end of a room potentially concealed at high level can provide adequate protection with nominal intrusion.

Where such detection devices are to be avoided at all costs and where risk or the design strategy dictates a requirement for AFD, aspirating or air sampling systems may prove a viable option that can be concealed. An aspirating system consists of a series of small diameter flexible pipes with holes along its length. A sampling unit, positioned remotely draws air from the room space into the sampling unit chamber, the unit detects the presence of smoke particles and the alarm is raised. The pipework can be concealed behind the ceiling structure, perhaps installed from above and only very small penetrations in the ceiling fabric need be made at intervals to allow the air to be drawn and sampled. This technique has been used extensively in buildings having very fine decorations and where the requirement for AFD is proved.

In the design and installation of AFD systems for historic and listed buildings, the identification of the best cable routes is as important (if not more so) as the installation of the unit itself. Where it is not possible to use existing cableways a strategy for chasing and concealment should be agreed, which takes into account the need to minimise damage to fine finishes, not least because repairs by a specialist craftsman can be expensive. An alternative is to avoid hard wiring between units by specifying a radio linked system, in which each unit transmits data by radio frequency to a receiver. However, in the author's experience with such installations, the benefits of reduced wiring can be offset by larger, more intrusive units required to incorporate radio equipment; by the cost of maintaining the system and replacing batteries; and by the effectiveness and reliability of signal integrity.

In summary, as part of the development of a fire plan strategy for any historic or listed building, the definition of risk, occupiers' priorities and conservation issues are paramount. The fire strategy consists of various contributory elements including the natural or existing building features and the degree to which more onerous passive upgrading can be offset by the introduction of active protection measures. The ultimate active protection measure must be the introduction of comprehensive fire suppression. This represents the only measure to actually tackle the growth and development of a fire. In respecting the potential difficulty in achieving fire suppression installation, the introduction of AFD enables full advantage to be taken in design to meet the assessed risk and to minimise passive protection measures. Furthermore, any alterations to the original fabric which are unavoidable should be reversible, allowing the element affected to be returned to its original condition. Other than shallow chasing to accommodate wiring circuits, no structural or fabric intrusion need necessarily be made.

Finally, in quantifying and qualifying fire risk, the building owner should never lose sight of the difference between risk and hazard. In other words, if in the unlikely event the worst should happen, what would be the consequences?

 

 

This article is reproduced from The Building Conservation Directory, 1996

Author

RICHARD FORREST is the Director of Engineering Services with Lawrence Webster Forrest. Lawrence Webster Forrest are leading fire engineering consultants who specialise in providing fire safety advice to owners and occupiers of historic and listed buildings. He has been responsible for the design of fire strategy and fire safety systems in buildings such as The British Museum, The Natural History Museum and the Palace of Westminster.

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