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Theme: fi re resistance

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Still a relatively young discipline, fire engineering is increasingly being used by architects to formulate appliable and life-saving fire safety strategies for their buildings

Fire engineering offers architects the advantage of flexibility in the design of a building.

The use of a range of measures and analysis of the geometry of a building, allows for a more efficient design that has a reasonable standard of fire safety.

Although Approved Document B (Fire Safety) of the Building Regulations (ADB) recognises the value of fire engineering for the design of large buildings, fire engineering is still a relatively young discipline, and continues to develop as an engineering field.

However, the past two years have seen the appearance of a number of British Standards that reflect its growing maturity. The most important is BS7974, 'Application of fire-safety engineering principles to the design of buildings - code of practice', which describes the principles of fire engineering and their application. It describes the methodology for the design of a fire-engineered strategy and includes calculation methods and equations for different aspects of building design.

One thing to note is that fire engineering is not a theoretical discipline. Reference to BS7974 can suggest that developing a fire strategy is a highly theoretical scientific exercise - it is not. It is all about designing a safe building.

The codes recognise that fire engineering is the only way to provide an effective fire strategy for large complicated buildings, such as airports and shopping centres.

However, fire engineering can also provide solutions for small, less complicated buildings. In the past few years, for example, there has been a substantial increase in the use of fire engineering in residential buildings. This is particularly so where flats are part of a mixed-use development, or where there is an atrium. For example, in the common areas of flats, the results of research into the effectiveness of smoke venting of fire-fighting shafts can be applied to provide more efficient smoke control with a smoke chimney, reducing impact on net floor area.

Further research is being carried out on the effectiveness of smoke venting in residential buildings, and the results are likely to influence future guidance.

Fire-engineered solutions are also being supported by continuing developments in materials and construction techniques, which provide protection against fire and smoke spread within buildings.

Computer software continues to develop to allow more complicated modelling, and with increased use, the results are becoming more acceptable to the approving authorities.

Dealing with atria Fire engineering is used increasingly in publicly accessible atrium buildings. Many modern education buildings are designed with atria, and they are incorporated into hospitals, leisure buildings and courts. The atria link different storeys, and could increase the risk of smoke and fire spreading through the building. Sprinklers and smoke control are commonly used as part of a fire-engineered solution to deal with atria, as they will control a fire and prevent the build-up of heat and smoke within the building. They protect escape routes, minimise the possibility of fire spread through and between buildings, and make fire fighting easier.

In a tall building, or a large volume multistorey building such as a shopping centre, sprinklers will almost certainly be needed.

ADB recommends that the sprinkler installation should meet the life safety standards.

This would prevent the use of concealed heads, which may pose problems for ceiling heights and aesthetics. However, research has shown that in certain circumstances, concealed heads can provide an appropriate standard of protection, and it may be possible to develop a solution to allow their use.

The next step is to assess the impact of the atrium on the fire strategy. For a phasedevacuation building, a simple solution to fire and smoke spread is to enclose the atrium with fire-resistant glass. One alternative could be an open atrium with fire curtains to provide fire separation. These combine the advantages of smoke curtains and fire shutters: a relatively small bulk (the housing may be as small as 200 x 200mm), smoke retardation, fire resistance for integrity, and, in some cases, a level of insulation protection. Another alternative is to enclose the atrium in toughened glass and to provide smoke control in the atrium, typically with smoke vents in the atrium roof.

One point to bear in mind with ETFE is that, although it degrades at high temperatures, it will still contain smoke at temperatures that are harmful. Smoke at temperatures above 60°C can burn, and even cooler smoke can lead to loss of visibility that would prevent escape. Therefore, ETFE's use as a fire-safety measure is not appropriate in all buildings.

If the geometry of the atrium is complicated, conventional smoke modelling may not be appropriate, and more sophisticated computational fluid dynamics models may be needed. These allow modelling of smoke movement using the fundamental principles of fluid mechanics, and can result in more efficient design of smoke control.

The diagram (above) shows the results from modelling smoke spread within a smoke-vented atrium in a college in Ireland.

The model shows that smoke does not compromise escape routes around the balcony.

Automatic fire detection and alarms will also be needed to ensure that the large numbers of people are made aware of a fire quickly and are directed to make their escape. Conventional point detectors would be adequate, although a sophisticated analogue addressable system would probably be needed to allow investigation of an alarm and to minimise disruption due to false alarms.

If an existing building is being redeveloped, there may be problems installing new fire-safety measures. In historic buildings, consideration might be given to a radio firealarm system. This involves having manual call points and detectors linked to the alarm panel by radio signal. Radio fire alarms allow automatic fire detection to be installed in historic buildings, without destroying the fabric they are intended to protect.

The client may prefer an aspirating smoke-detection system to point detectors.

Aspirating smoke detectors operate at very low levels of smoke concentration and can detect fires before they start flaming. This has made the system particularly valuable in areas where a fire may cause significant disruption of business. However, they also offer advantages in buildings with high airflows, such as shopping centres, tall buildings where smoke might take a long time to reach and trigger a point detector, and in dusty or dirty environments such as warehouses, airport baggage-handling areas and enclosed public car parks.

An aspirating system works by drawing air into a small-bore pipe and then into a sampling chamber, which acts as the detector. The sample is analysed and the results 'interpreted' by computer software.

In theory the systems can be 1,000 times more sensitive than point detectors, but in practice, in most buildings, the sensitivity is reduced to typically 10-200 times that of a point detector.

The system is also relatively unobtrusive;

its main physical manifestation is the narrow bore tube, which carries air to the central sampling and analysis point. This can be hidden in ceilings, or run along steelwork.

Fire-safety measures can also be used to provide a more efficient design for the number and size of escape stairs, and thus increase the overall efficiency of the building design. The growing acceptability of fire engineering also allows more flexible use of common building installations. There has been much talk of using lifts as part of the escape route in tall offices, where there is a high ratio of lift capacity to occupant numbers. However, it is also possible to use escalators and lifts to form part of the means of escape from public areas of other buildings, where they can be upgraded to compensate for extended travel distances and small shortfalls in stair capacity.

With large compartments, there can be severe restrictions on the unprotected areas of external elevations. Because atria link a number of storeys, they increase the area of the elevation that must be considered. A fireengineered solution to the atrium strategy can allow large glazed elevations. However, there is a very strong technical case for arguing that a properly designed sprinkler system will prevent fire spread across the site boundary (this is accepted in the codes for Scotland).

Fire-fighting access also needs to be considered. In a tall building, this will require fire-fighting cores. An important part of the design of these cores is the control of smoke movement into them. Until recently, for an internal core, this would have needed pressurisation or large shafts running the full height of the building. However, recent research has confirmed earlier theory that a chimney serving only the stair lobbies provides a better standard of safety than required under current recommendations.

Insurance issues Up until now, many buildings have been designed to meet the requirements of the Building Regulations, with little input from insurers in terms of fire-safety measures. Designers rarely make use of the Loss Prevention Council's (LPC) Design Guide for the Fire Protection of Buildings. But the insurance market has become much tougher in the past few years, with large increases in insurance premiums. There has also been greater involvement by the insurance industry in the specification of fire strategies for buildings, with particular reference to fireresistance periods and to active fire-safety measures. The industry is seeking to get the LPC design guide updated and is pushing for it to be used more. Implications could include increased specification of sprinklers - even if not needed for life safety - and greater periods of fire resistance for structure and compartmentation (which could lead to aesthetic and practical problems in design and construction).

Insurers now more commonly require the installation of sprinklers in warehouse and industrial buildings, where a fire not only causes loss of the building but also the contents, as well as the high cost of disruption to business. And although sprinklers may not be required for life safety in low-rise offices, they may be a requirement of insurers, especially where the offices are ancillary to another use.

Insurers are also prohibiting the use of certain materials or construction types. An example is the use of composite cladding panels (sandwich panels), where insurers may prohibit combustible insulation in certain applications despite the fact that their use would comply with Building Regulations.

Human factors The recent and proposed British Standards reflect the increasing maturity of fire engineering as a discipline. They summarise recently developed best practice, incorporate experience of the use of fire-safety systems and include the results of research carried out in the past decade. This development should allow even more innovative design and flexible buildings.

One thing to bear in mind is that, while there has been much research into fire physics and combustion products, there are elements of unpredictability in fire. Some aspects of fire engineering are not as well understood as they might be.

The area of human behaviour still requires research. The main purpose of fire safety in buildings is, after all, to get people out of a building that is on fire. Public buildings are usually designed with two circulation routes: the normal routes and emergencyonly escape routes. In commercial buildings, this may be reasonable, as staff will be aware of, and use, secondary routes. But in a public building, experience shows that unfamiliar emergency-only routes are rarely used. Is this the most efficient way to design a building? If not, then how are we to recognise and safeguard the use of normal routes? Fire engineering can offer a solution, but more research is needed to allow us to understand human behaviour better and so improve the efficiency of escape routes in buildings.

Fire engineering is intended to produce a safe building. While the incorporation of innovative solutions and new products can provide a high standard of safety and allow architectural and operational aspirations to be met, a healthy degree of scepticism from the design team and approving authorities is no bad thing.

Have your say If you wish to have a say in the future of fire-safety design of buildings, there is an opportunity to take part in a review of ADB.

The intention is to review fully the scope and recommendations. An advanced consultation exercise is being undertaken, with an online questionnaire available at www. bre.

co. uk/frs/adb. This largely deals with the user's requirements of ADB, and users can say what improvements could be made to it. The review is an opportunity to bring ADB up to date with current building trends and best practice, and to reflect user's needs better.

Gordon Garrad is technical director of fire-engineering consultancy Jeremy Gardner Associates

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