The new code of practice for fire safety in the design, management and use of buildings; BS9999 is published by BSI and came into effect on 6 October 2008. The British Standard BS9999:2008 supersedes most of the BS5588 series (Part 0 and Part 5 to Part 12) and DD9999:2005 which will be withdrawn on 6 April 2009. This bulletin concentrates on the effect that the new standard has on means of escape design and the various parameters that must be taken into account to ensure a satisfactory level of safety is achieved when designing or refurbishing means of escape in buildings.
Introduction to the new Code of Practice for Fire Safety in the Design, Management, and Use of Buildings
BS9999:2008 provides guidance that gives a more transparent and flexible approach to fire safety design through use of a structured approach to risk-based design. Much of the guidance in the British Standard is based on fire safety engineering principles and research although it is not intended to be a guide to fire safety engineering.
BS9999:2008 is designed as a co-ordinated package covering the four main areas that influence fire safety measures, namely:
• The provision of means of escape;
• The provision of access and facilities for fire-fighting;
• The structural protection of escape facilities and the structural stability of the building in the event of a fire;
• Fire safety management levels.
Designing Means of Escape
The guidance is based on the principal of the Available Safety Egress Timeline. The time to escape to a place of relative safety should be less than the allowable travel time which is based on the risk profile. Four major stages of occupant response should be taken into account in determining the provision of means of escape:
• The time to detect a fire and sound an alarm;
• The pre-movement time which consists of the recognition and the response time;
• The travel time, including queuing, to a place of relative safety; • The movement time within a place of relative safety.
A minimum package of fire protection measures covering management level, automatic fire detection & alarm, lighting & signage etc must be provided to allow the occupants to escape from the building in the event of a fire. Indeed, 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 categorised as risk profiles, which provide a basis against which the risk to occupants can be assessed. A risk profile should be established for each building in order to determine the appropriate means of escape provisions.
There are a number of general principles used to determine whether the means of escape are acceptable. However circumstances can vary and the means of escape chosen for a building should reflect the particular needs of that building. A graphic representation showing the basic process for designing means of escape is given in Figure 1.
Figure 1: Designing means of escape
Horizontal Means of Escape
The provision of means of escape from any point in a storey to the nearest storey exit of a protected area on the floor in question defines the horizontal means of escape.
Number of occupants
First the engineer must determine the maximum number of persons who are present in the premises at any one time. A realistic estimate of the maximum occupancy of a room, storey, building or part of a building is either:
• The maximum number of persons it is designed to hold or,
• The number calculated by dividing the area of a room or storey (m2) by the appropriate floor space factor (m2/person), such as those given in Table 10 of BS9999.
Table 10 sample as given in BS9999
Number of escape routes
Once the maximum occupancy in the premises has been determined, it is possible to identify the minimum number of escape routes necessary to allow all occupants to evacuate safely. Table 11 of BS9999 gives some figures for the minimum number of escape routes and exits from any room, tier or storey.
Table 11 as given in BS9999
However the number of escape routes and exits that will be needed is mainly dependant on the risk profile, the number of occupants and the limits on travel distance to the nearest storey exit.
The travel distances given in Table 12 of BS9999 have been determined according to the risk profile and are based on the time available to reach a place of relative safety.
Travel distance should not exceed the value given in the Table 12 of BS9999 for the appropriate risk profile; if additional fire protection measures are provided the travel distance may be increased subject to certain limitations.
Table 12 as given in BS9999
Width of doors, corridors and escape routes
The width of the doors given in Table 13 of BS9999 depends on the risk profile. The total door width should be not less than the aggregate of the exit widths given in Table 13 and not less than 800mm.
The width of a corridor or escape route should not be less than the calculated width of any door leading on to it, or 1200mm.
Table 13 as given in BS9999
Vertical Means of Escape
Vertical escape involves the transition from horizontal escape from a building to a place of ultimate safety. Therefore the design for vertical means of escape should meet the recommendations for horizontal escape for each storey exit in a building.
Design of escape stairs
Generally, every internal escape stair should be a protected stair i.e. it should be constructed within a fire-resisting enclosure.
Every protected stairway should discharge directly to a final exit, or by way of a protected exit passageway to a final exit.
Under certain conditions such as having a single escape stair, a stair serving any storey more than 18m high or in a building designed for phased evacuation, the stair should be designed with a protected lobby or corridor or a pressure differential system.
Where stairs are not lobby protected it is necessary to discount a whole stair when considering the means of escape capacity for the building.
The final exit route from a stairway should be at least as wide as the stair leading to it. Where an exit route from a stairway also forms the escape route from the ground and/or basement floors, the width of the exit route may need to be increased accordingly and is be calculated using the merging flow formula given in BS9999.
Formula for merging flow at final exit:
W = [(N/2.5) + (60S)] /80
W: width of final exit in metres
N: number of people served by ground floor
S: stair width in metres
Number of escape stairs
The number of escape stairs is controlled by:
• The horizontal escape routes design,
• The acceptability of a single stair,
• Mixed occupancy building,
• The provision of adequate width for escape assuming that a stair may not be available due to fire or smoke.
Width of escape stairs
The width of the stair depends on how the building is designed in terms of evacuation. It can be either simultaneous evacuation or phased evacuation.
The width of escape stairs is based on the occupant flow rate considering a simultaneous evacuation i.e. the escape stairs should have the capacity to allow all floors to be evacuated at the same time and should:
• Not be less than the width of any exit affording access to them;
• Not be reduced at any point on the way to a final exit;
• Not be less than the dimensions given in Table 14 of BS9999 for the appropriate occupancy characteristic.
Table 14 as given in BS9999
For the simultaneous design, the escape stairs should be wide enough to accommodate all floors being evacuated simultaneously. The width therefore depends on the number of people using the stair on each storey. The width of the stair should not be less than the greater value of the dimensions given in Table 14 or Table 15 of BS9999 for the appropriate risk profile.
Table 15 sample as given in BS9999
It can be advantageous to design stairs in high buildings on the basis of phased evacuation. It allows for narrower stairs than would be the case for simultaneous evacuation. The aggregate width of the stairs should not be less than the dimensions given in Table 14 or Table 15 of BS9999 for the appropriate risk profile and the maximum capacity on any two floors.
If an escape stair forms part of the only route from the upper storeys of a building it should not be continued down to serve any basement storey.
If there is more than one escape stair from the upper storeys, only one of the stairs serving the upper storeys need be terminated at ground level. Others stairs may connect with the basement if there is a ventilated protected lobby, or a ventilated protected corridor between the stairs and accommodation at each basement level.
Additional Fire Protection Measures
Every building should incorporate the minimum level of fire protection recommended. However if additional fire protection measures are provided, such as enhanced automatic detection and warning systems or high ceilings it is permissible to increase the travel distance and reduce the door and the stair widths. These travel distances, door and stair widths are subject to specified maxima.
Automatic sprinklers indirectly by changing the risk profile
The provision of automatic sprinklers affects the risk profile. Such provision restricts fire growth, prevents fire spread and limits heat and smoke generation. This means that if sprinkler systems are installed in a building or part of a building, the fire growth rate can be reduced within the sprinklered room or building. Therefore an A3 building can be considered as an A2 building where sprinklers are provided.
Automatic detection and warning systems
The provision of automatic smoke detection can be of significant benefit in terms of providing early warning for the occupants. The response of occupants is generally enhanced by the installation of a fire warning system that provides information about a fire incident. Where a clear benefit resulting from the addition of detection and alarm systems is demonstrated, a 15% increase in allowable travel distance and 15% reduction in door width, corridor width and stair can be applied.
Effect of ceiling heights
For rooms with high ceilings, it is permissible to increase the travel distance and decrease the door, corridor and stair width as they have a greater capacity to hold smoke and delay the time taken to fill with smoke to a level that affects escape.
Table 16 sample as given in BS9999
Where additional fire precautions are provided, such as enhanced AFD or high ceilings, variations in travel distance and door and stair widths are allowed however a clear benefit must be shown: e.g. going from L2 to L1 may not show clear benefit.
This bulletin provides an overview of the process when designing means of escape in a building under the new Code of Practice for Fire Safety BS9999. It must be noted that all the elements discussed previously should be considered as a whole. Indeed fire safety provisions can be seriously compromised by a lack of management of fire safety, inadequate facilities for fire-fighting or a lack of appropriate related measures in the construction of the building.
It is not acceptable therefore to ‘cherry pick’ from the standard those parts which are considered to benefit the design.
Considerable benefit can be given to a design through the use of the BS9999 standard where extended travel distances are required.
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