The LWF Blog

Means of Escape Assessment Series | Part 1 – Occupancy

March 20, 2014 11:17 am

How do I know people in my building can escape safely in case of fire? How many people can I have at any one time in my premises? Where should I locate the fire exits? There are many questions which architects, designers and building managers cannot effectively answer until a fire safety advisor or engineer is involved in the project.

The main objective of the Building Regulations 2010, Approved Document B 2006 edition – Amended 2013 (ADB) – Fire Safety is Life Safety. This blog will take into account measures that exist to ensure that an ideal level of life safety is achieved when designing or refurbishing means of escape in buildings.

The compliant solution

The assessment of means of escape within a building – whether it is an existing or a new build design, depends upon 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. The first point of action is 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. Approximately 120 documents currently in place can be used as design guidance for fire safety. An alternative approach to the “prescriptive way” is fire safety engineering, which will be discussed later in the series.

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.

A fire consultant/engineer will undertake similar calculations to design the 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. as per ADB Appendix C table C1). 

As an alternative to using the values in that table, the floor space factor may be determined by reference to valid data taken from similar premises. 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 the intended future use of the building.

When considering the renovation of existing premises, the maximum occupancy of a building is dependent upon the size of the escape routes. While 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 Section 3 Table 3 recommends use of the following figures:

Up to 60 persons – 1 fire exit

More than 60, less than 600 persons – 2 fire exits

More than 600 persons – 3 fire exits

The positioning 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 upon the number of occupants needing to evacuate, their location is largely dependent upon the travel distances within the building.

Approved document B can be downloaded, free of charge, from the Planning Portal, for reference.

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.

Lawrence Webster Forrest Limited is a fire engineering and fire risk management consultancy, with over twenty-five years experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.