Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
As designers of buildings, we must acknowledge that any fire safety design can fail, i.e. a fire could occur. The risk of fire will always be present due to the very nature of its component parts, fuel, oxygen and heat. There are very few instances when these three are never combined.
With this acknowledgement made, we must then design our buildings on the basis that a fire will occur and occupants within it will need to escape, to a place of relative safety and ultimately to ‘fresh air’.
The term ‘a place of relative safety’ is usually within a building, that is deemed to be safe, these places are termed safe usually because of their physical distance from a fire and/or the fire resisting construction separating the designated area from the fire. Typical places of relative safety include, protected corridors and protected stairs (separated from accommodation by fire resisting construction). However, other strategies can also be implemented into a buildings design to facilitate an evacuation. For example, within hospitals/care homes, where occupants may not be ambulant/ have mobility issues, fire compartments are created to enable occupants to move horizontally from an area affected by fire, into an unaffected area without having to negotiate the stairs.
The design philosophy adopted will be significantly influenced by the occupancy of the building, this is evident from the example above. Similarly, the size and height of the building will have an impact of the strategy implemented. Tall buildings often house significant numbers of people and as such, to evacuate all persons simultaneously (one out, all out) is often unfeasible, also the stair widths that would be required to do so could make the design space/cost prohibitive. For this reason, an alternative evacuation philosophy, known as phased or staged evacuation is adopted whereby those persons most at risk are evacuated initially, with remaining floors remaining in situ or notified by a pre-alarm condition. In this instance, the floor of fire origin is evacuated, along with other higher risk zones, due to their proximity to the fire/occupants that may have a more difficult escape route, for example the floor above, floor below, plant areas (roofs etc) and basements.
There are many other factors that need to be considered when designing the means of escape. One of the simplest design objectives to be considered is the ability of a person to be able to turn their back on a fire and make their way towards an exit. On this basis, guidance documents limits ‘dead end’ conditions – the distance occupants can travel in a single direction. Additionally, design guidance places limitations on the distance that can be travelled prior to reaching a place of relative safety/fresh air, ensuring that persons within the compartment of fire origin are not within that compartment for an excessive time.
A common theme arises with all aspects of means of escape design, time. All factors influencing the escape design revolve around time, trying to ensure that as the fire develops (over time) the occupants are progressing with their escape, ensuring that no occupants are overcome by smoke or flames. To achieve this also, it must be ensured that there is not excessive queuing at exits. For example, should 1,000 people be required to pass through a typical single door width, it is clear that this would take considerable time. For this reason, the number of occupants within a building needs to be considered as part of the means of escape design. Once more, design guidance stipulates escape door widths set against occupancy numbers, following this guidance should ensure occupants can escape in adequate time. Similar guidance is provided for all parts of the escape routes, for example, stairs and final exits.
In more recent times, fire engineering has been used to apply a scientific and building specific approach to means of escape design. An ‘ASET vs RSET’ (available vs required safe evacuation time) model has been adopted, whereby the required time to evacuate is set on a timeline against the time available prior to conditions becoming untenable for occupants. Evacuation and fire modelling is often undertaken, along with looking at all influencing factors, for example, ceiling heights, likely fire conditions (growth rate, fire load etc), fire protection systems amongst others. This aspect of means of escape design is extremely technical and should only be undertaken by a qualified person.
It is clear from this bulletin that means of escape design is complex with numerous variables. Should the design be incorrect, this may result in Enforcing Authorities placing restrictive conditions on the buildings use, or occupants not escaping in a satisfactory manner.
We very much look forward to hearing from you and helping you with your means of escape and fire safety designs.
If you would like to know more – or would like to arrange an appointment with one of our senior fire safety advisers – simply call Peter Gyere on 020 8668 8663.
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