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Rising to the Residential Challenge | Lawrence Webster Forrest
Traditionally, fire safety in residential buildings has been achieved by following the prescriptive building standards and codes of practice. This type of design has resulted in stereotype layouts in which all conform to similar design restrictions. Therefore, a common criticism of residential buildings from developers is that they are all fundamentally the same. This is far from the case, but there is certainly a tendency toward a standard form of layout by following the prescriptive guidance. Influences on residential buildings have led to substantial demands from developers to build more complex, larger and taller residential buildings in order to differentiate their development from the neighbouring buildings. This has resulted in traditional designs being classified as restrictive and inappropriate for high profile developments.
This bulletin describes fire safety challenges in residential buildings and how fire engineering approaches can ensure that the risk presented by the proposals is as low as practicable and ensure an adequate level of fire safety is achieved.
Escape Stair and Common Corridor
The accepted premise in the UK is that for any floor over 4.5m above access level, egress without the use of a protected escape stair is not practicable. In accordance with Approved Document B of the Building Regulations [1], the maximum travel distance in the common corridor is 7.5m (measured from the furthest flat entrance door to the escape stair) and a 1.5 square metre automatic opening vent is typically provided at each end of the common corridor and an openable vent to the stair. Hence, a second stairway is required where the length of the common corridor exceeds 7.5m. Economically, the construction of a second stairway can be critical for schemes in metropolitan cities such as London. Hence, the construction of a single escape stair in residential buildings is naturally attractive to developers as the generation of more efficient building designs is achieved.
Minor extensions may be possible to agree without any additional fire safety measures being provided. However, significant extensions would require a fire engineered solution. In order to justify an extended single direction travel distance, a fire engineered approach can be incorporated in parts of the building where corridor length exceeds the prescriptive distance. One possible fire engineered design includes a ‘push-pull’ system, a mechanical extract and natural supply ducts at opposite ends of a corridor. The application of this type of system is intended to remove limitations on distance of travel and travel distances up to 20m in a single direction have been agreed in the past as part of a fire engineered solution.
This fire engineered solution comprises supply and extracts ducts on each floor to control the smoke flow and limit smoke spread. Backup fans and backup power supplies are required and should be provided to the fans associated to each duct. In addition, smoke detectors should be installed within the common corridor to activate the fire engineered system in the event of fire.
During the evacuation stage while occupants from the fire origin flat are evacuating, air is supplied from the inlet point at one end of the common corridor and extracted at the other end. This has the effect of preventing smoke from entering neighbouring flats and the single stairway. However, the system alters (i.e. both fans extract smoke) during fire service intervention and the principal is to keep the corridor smoke free so that fire service can approach the flat of fire origin. In order to achieve this, the supply fan is put into reverse by the fire brigade and smoke is extracted from the common corridor. Make up air comes from the open door from the single escape stairway. To assist this arrangement, the door from the stair needs to be designed to open into the common corridor at each level, without impeding occupant egress or fire brigade access.
In comparison to a code compliant layout for common corridors, the equipment cost associated with the fire engineered solution will impact on the building cost. However, a cost/benefit analysis should be undertaken and the cost of the system is often outweighed by the additional space provision achieved within the building due to the provision of a single escape stair.
Any fire engineered solution is subject to acceptance by the approving authority. One difficulty with this approach is that there is no performance requirement given for the code compliant layout so there are no defined acceptance criteria for the performance of an alternative method. In order to demonstrate the effectiveness of a fire engineered solution there are different approaches that can be followed.
A comparative approach attempts to compare the conditions achieved using a code compliant solution and show that the engineered solution can achieve the same or better conditions.
A deterministic approach sets out acceptance criteria based upon physical properties required for life safety such as temperature, toxicity of smoke or visibility and then attempts to show that these criteria are met.
A probabilistic approach attempts to show that the probability of a defined hazard is so low that the risk can be considered to be acceptable.
In all of the above approaches some degree of computational modelling is likely to be required to quantify results.
If the flat layout follows the recommendations of the prescriptive guidance (i.e. BS 5588: Part 1[2] or Approved Document B), the internal planning of flats would be deemed to meet the functional requirements of the Building Regulations. By following BS 5588: Part 1 [2], the most common approaches to the internal layouts are:
• For studio flats, the kitchen should be positioned remotely from the entrance door. The travel distance is limited to 9m (measured from the furthest point to the flat entrance door).
• Provision of an entrance hall is required for flats with one bedroom or more. The travel distance within the entrance hall is 9m (measured from flat entrance door to any habitable room).
A common complaint with many prescriptive internal flat layouts is that they are monotonous and dull due to inflexible standards. Some of the typical features that may have the effect of influencing the internal planning of flats are:
• Inner rooms
• Open plan layouts with extended travel distances
• Duplex flat layouts
Where it is desirable to vary the internal planning of flats this may be possible but often requires a fire engineering approach.
In prescriptive flat layouts, the flat is approached from the common corridor via a protected entrance hall. A common internal layout that does not comply with the recommendations in Approved Document B is a flat with bedrooms as inner rooms. Some of the common fire safety measures used by fire engineers to justify non-compliant flat layouts are the provision of one or more of the following:
• Early warning of a fire
• Automatic suppression systems
The minimum provision for residential flats is a Category LD3 detection system, in accordance with BS 5839: Part 6 [4]. The LD3 system covers the flat entrance hall only. Therefore, it could be argued that the provision of a Category LD1 coverage system (i.e. smoke detectors in bedrooms, living room and a heat detector in close proximity to the cooking space) is in excess of the code compliant system r. For a compliant Category LD3 system, smoke from a fire in the closed kitchen/lounge would have to leak into the protected entrance hall before smoke would be detected and the alarm would then have to be heard by the sleeping occupants through the closed bedroom doors. The benefit of enhanced detection systems is significant due to the reduced time taken to detect a fire. Recent research on life safety of open plan flats in the event of fire undertaken by the National House Building Council (NHBC) Foundation, concluded that although a significant benefit can be seen there is concern that enhanced detection alone cannot satisfy the requirements of the Building Regulations.
It is recognised that the activation of an automatic suppression system has the effect of reducing the toxicity and temperature levels within a flat. In addition, an automatic suppression system is considered as a compensatory measure in Approved Document B. However, research on the effectiveness of residential sprinklers was undertaken by the Building Research Establishment (BRE) [7] and observations of the visibility level indicated that there was no improvement with sprinkler coverage. Hence, approving authorities often raise concerns with the tenable conditions in terms of visibility and request further justifications. Very often the method used for justifying this approach may involve fire modelling.
However, recent research undertaken by NHBC has demonstrated that open plan flat layouts with an automatic suppression system in accordance with BS 9251 and a Category LD1 detection system provides a level of safety equivalent to or better than the minimum recommended provision in the Approved Document B (note: research was based on flat layouts with a maximum size of 12m x 16m and larger layouts would require further analysis).
Other active fire engineering systems such as smoke curtains, mechanical or natural smoke control systems are common practice in large assembly and public buildings e.g. shopping centres, atrium and office buildings. Where used these systems could potentially maintain clear layer heights, limit temperatures and keep escape routes clear.
With respect to internal planning of flats the application of active fire engineering systems are rare but there is scope for this type of system in unconventional dwellings where protected routes are compromised by an open plan layout [8].
Maintenance of Active Systems
The application of active fire safety systems can be very effective however some approving authorities may have concerns with regards to the ongoing maintenance of these systems and some assurance of how this will be achieved is often required.
This bulletin describes various fire safety challenges in residential developments and fire engineering approaches available for tackling the non-compliance items. It is clear that current fire safety guidance in the UK does not always adequately address what many homebuilders want to build i.e. open plan flat layouts with inner rooms. It must however be noted that all building layouts are unique and have different compartment geometries, risk level and fire load. Therefore, each case should be assessed individually; but without adequate guidance for open plan flats it is likely that the design approach will be time consuming and discussion with the approving authority should be undertaken prior to any detailed design of residential buildings where a significant departure from the standards is proposed.
LWF has a broad range of experience in developing fire engineered solutions in residential developments. Fire engineered solutions in residential buildings can be advantageous to your project through potential cost savings and more flexible layouts. Fire engineering can often add value to large or complicated projects as well as aid architects and designers to better fulfil their creative aspirations.
This bulletin has been written by James Lee MEng (Hons) AIFireE
1. The Building Regulations 1991, Fire Safety, Approved Document B, HMSO 2006 edition
2. BS 5588: Part 1, Code of practise for residential buildings, British Standards Institution, 1990
3. PD 7974: Part 7: The application of fire safety engineering principles to the design of buildings – Part 7: Probabilistic risk assessment, British Standard BS 7974: Part 7: 2003, London, BSI, 2003
4. BS 5839: Part 6: 2004 Code of practise for system design, installation and maintenance of fire detection and fire alarm systems in dwellings, British Standards Institution, 2004
5 Fraser-Mitchell J, Williams C, 2009, Open plan flat layouts Assessing life safety in the event of fire, NHBC Foundation
6. BS 9251 Sprinkler systems for residential and domestic occupancies, British Standards Institution, 2005
7. Williams C, Fraser-Mitchell J, Campbell S, Harrison R, 2004, Effectiveness of sprinklers in residential premises, BRE project report 204505
8. CIBSE Guide E: Fire Engineering, Chartered Institution of Building Services Engineers, 2010
9 S. Dabin, “Life of luxury” Risk Management Journal pp. 21-23, December 2010
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