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Fire Safety for Facilities Management Personnel – Means of Escape – Part 121

October 25, 2021 12:34 pm

Lawrence Webster Forrest (LWF) is a specialist fire engineering and fire risk management consultancy whose aim is to give information on best practice in fire safety for facilities management personnel through this blog series. In part 120, LWF looked at means of escape in a fire engineered plan. In part 121, we continue discussing means of escape.

As a part of a safe and practical evacuation plan for a building, there must be sufficient exits and stairways for the anticipated occupancy, and those provided must be of sufficient width to avoid a bottleneck.

There is an established and simple method which can be used to calculate the required number and width of exits. The average shoulder width of an adult is assumed to be 0.53 metres. An exit of this width would therefore only allow people to leave ‘single-file’. It is also assumed that people passing through an exit in single-file would do so at a rate of 40 persons per minute.

In practice, an exit with a width of 0.53 metres would be impractically narrow and a minimum exit width common to many codes for existing buildings is 0.75 metres. However, depending on if your property is subject to Part M building regulations, there may also be minimum sizes to allow for mobility access.

Any exit, doorway or stairway which is larger than a single exit width of 0.75 metres, but smaller than a double width exit will still only be able to be classed as a single-file exit for evacuation calculations. A double-width exit will, of course, double the potential discharge capacity as people can pass through two at a time.  If this is based on the average shoulder width of 0.53 metres, then an exit with a width of 1.06 metres should, theoretically, be able to discharge 80 persons per minute.

With the required evacuation time defined, it is then possible to calculate the number of building occupants that may be served by any exit. An evacuation time of 2.5 minutes is traditionally required, based on studies of major fires.

Approved Document B (PDF) gives information on calculating exit and escape route capacities.

The guides supporting the Regulatory Reform (Fire Safety) Order 2005 have a more traditional approach for places of assembly. This is based on a unit of exit width of 525 mm and increases in exit width above two units are made in increments of 75 mm, which cater for 15 people.

The guides also lay out three differing evacuation times – 2 minutes, 2.5 minutes and 3 minutes depending on if they pertain to high, medium or low risk category buildings respectively. The risk category relates to the predicted rate of fire spread, or the likely time between detection and alert.

In part 122 of this series, LWF will continue to discuss means of escape, starting with stairways. In the meantime, if you have any queries about your own facilities or wish to discuss this blog series, please contact LWF on freephone 0800 410 1130.

Lawrence Webster Forrest is a fire engineering consultancy based in Surrey with over 25 years’ experience, which provides a wide range of consultancy services to professionals involved in the design, development and construction and operation of buildings.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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