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
LWF PROFESSIONAL BULLETIN
Means of Escape Assessment – The Basics
How do I know people in my building can escape safely? How many people can I have at any one time in my premises? Where should I locate my fire exits? So many questions architects, designers and building managers cannot answer until a fire safety advisor / engineer is involved. The main objective of the Building Regulations, Approved Document B – Amended 2002 (ADB) – Fire Safety is Life Safety. This bulletin therefore concentrates on the various parameters that must be taken into account to ensure such a level of safety is achieved when designing or refurbishing means of escape in buildings.
The assessment of means of escape within a building; whether it is existing or new build design, depends on the characteristics of that building. The approach taken for such an assessment will vary if the building is an office, a shop, a cinema, a tube station, etc. As a common practice, the first point of action will be to follow the design guidance within ADB. Taking this approach is using what is called a compliant solution, i.e. following strictly what the design standards recommend. Such design standards are not only limited to the Building Regulations and ADB. An approximate total of 120 documents, currently in place, can be used as a design guidance for fire safety. An alternative approach to the “prescriptive way” is fire safety engineering, which will be developed later in this bulletin.
Number of occupants
As a first point of concern, the designer must determine the maximum number of occupants who will be present in the premises at any one time. In the same way architects carry out occupancy calculations in order to determine numerous requirements such as the number of toilets necessary in a building, the fire consultant/engineer undertakes similar calculations to design the building’s means of escape. ADB allows for two methods to be used when undertaking such exercise. The first method is based on floor space factor calculations; i.e. using a value of 6m2/person to determine how many people are likely to occupy an office building (7m2/person for a library, etc. – See ADB table 1). As an alternative to using the values in that table, the floor space factor may be determined by reference to actual data taken from similar premises (albeit reference able material). Where appropriate, the data should reflect the average occupant density at a peak trading time of year. The maximum number of people likely to occupy a building can therefore be determined, as a second method, based on what the future owner is intending to use the building for.
Outside design objectives and when concentrating more on existing premises, the maximum occupants’ capacity of a building is dependant upon the size of the escape routes. When the first method determines the number of occupants in order to define the escape routes, the second method works the other way around.
Number and location of escape routes
Once the maximum number of people in the building under consideration has been determined, it is possible to identify the minimum number of escape routes necessary to ensure all occupants will evacuate safely. As a rule, ADB Table 4 recommends use of the following figures:
Maximum No. of people Minimum No. of exits
More than 600 3
Location of escape routes and final exit doors must be such that these are clearly visible and accessible by all. As the number of escape routes is mainly dependant on the number of occupants to evacuate, their location is largely dependant on the travel distances within the building.
The travel distance is defined as the actual distance to be travelled by a person from any point within a floor area to the nearest storey exit, having regard to the internal layout of walls, partitions and fittings. The travel distances vary with the type of premises under consideration and can be found in ADB Table 3. All buildings are different and present different degrees of fire risk based on their type. As such, travel distances allowed in offices buildings (15m in one direction / 45m in more than one direction) are higher that those allowed in enclosed plant rooms (9m in one direction / 35m in more than one direction).
It is common within existing premises that were designed prior to the application of current design standards, and also in the design of new buildings, that travel distances are in excess of that recommended by the Building Regulations – ADB. These non-compliances must therefore be justified and compensated for using a fire engineering approach.
Width of escape routes
The width of escape routes and final exit doors is an additional element in the design of means of escape. The escape width is the driven factor that determines the evacuation flow of people through a door, corridor and stair. Approved Document B Tables 5, 6, 7 & 8 provide guidance on the escape widths to be used based on the number of occupants to evacuate.
Simultaneous and phased evacuations
Most buildings around the
As a different type of strategy, a phased evacuation may be adopted. In phased evacuation the first people to be evacuated are all those of reduced mobility and those of the storeys directly affected by the fire, usually the floor of fire origin and the floor above. This is commonly the case in high rise and residential buildings. Healthcare premises have a similar type of strategy whereby the phased evacuation is based on a compartment by compartment basis, with ‘Evacuation’ and ‘Alert’ zones. When adopting a phased evacuation strategy, the maximum number of people to be evacuated at any one time is limited compared to simultaneous evacuations. As explained in previous paragraphs this results in a reduction in the means of escape necessary, albeit the escape routes are afforded a significantly greater level of protection.
The Fire Safety Engineering Approach
Where design standards provide recommendations based on building types, i.e. all offices buildings together, shops, restaurants, cinemas, etc. The fire safety engineering approach considers every building (and not type of building) individually. It may therefore be acceptable to justify an extended travel distance in the design of a shop 1 using an approach A, but it may totally be unacceptable to do it in the design of a shop 2 using a similar approach.
The evacuation process can be broadly broken into three parts that take place one after the other: Recognition time ® Response time ® Travel time.
The Recognition Time is the interval between the time at which a warning of a fire is given and the first response to the warning.
The Response Time is the interval between the time at which the first response occurs and the time at which the first move is made towards an exit.
The Travel Time is the time needed, once movement towards an exit has begun, for all occupants of a specified part of a building to reach a place of safety.
The various elements of the overall building’s fire strategy are linked together and impact on one another. The escape width and travel distance will have a major impact on the travel time during an evacuation. Similarly, the fire detection and alarm system will have an impact on the recognition time. And again, an important staff presence and management will impact on the response time.
As with the rest of fire safety precautions usually installed in a building (e.g. fire alarm system, fire suppression system, etc.), the means of escape design may be proven adequate for life safety, even though it is not compliant with recommended design standards, on the basis that compensation features are provided. Hence, it may be acceptable to have an extended travel distance in a building if a sprinkler system is installed in that building*.
Note *: Justifications other than a sprinkler system may be used.
In order to assist in the design of means of escape, a number of computer models have been developed. These allow the designer to establish a building geometry, specify the escape route, final exit doors and their respective widths. Once these parameters are in place, the software requires the input of occupants: numbers, locations, direction of escape, etc. Most softwares also take into consideration whether occupants are adults, children, elderly or people with disabilities, by allowing the designer to specify the walking speed of each occupant. Once the setup is in place, the simulation is run and provides various outputs such as the total building evacuation time, a specific room evacuation time, the number of people to evacuate through a specific fire exit door, etc. Further to analysis of the results, alterations of the parameters are made (Reduction of travel distance by moving an exit door, increase in the width of an exit route, etc.) and the simulation is run again. Computer models are tools that require a high level of expertise in order to design or review means of escape design. These constitute fire engineering tools that are more and more commonly used in the justification of means of escape designs.
This bulletin provides an overview of the basics when designing means of escape in a building. The structure of this document was broken down into several paragraph for description and presentation purposes. It must however be noted that all the elements discussed should NOT be considered separately, but as a whole as shown in the diagram presented below.
For example, a building occupied by 60 people only requires one escape route; however, the provision of additional escape routes may be necessary if travel distances within that building are excessive or have an insufficient width.
The design of means of escape becomes difficult within complex buildings that can contain different types of accommodation and occupants. It is therefore necessary to consider merging flows of people at different locations and from various directions. Within such schemes, the use of a specialist fire safety engineer is recommended.
1. The Building Regulations – Approved Document B 2000 (Amended 2002)
2. BS 7974: Part 6: 2004 – Application of fire safety engineering principles to fire safety design of buildings. Human factors: Life safety strategies – Occupant evacuation, behaviours and condition.
The LWF Bulletin is designed to give general information on fire safety risk management. Readers should take specific advice when dealing with particular situations. LWF accept no responsibility for action taken as a result of information contained in the document. The information in this document is correct at the time of going to publication.
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