The LWF Blog
Fire Safety for Facilities Management Personnel – Means of Escape – Part 122November 1, 2021 1:52 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 121, LWF continued discussing means of escape in a fire engineered plan in terms of how to measure how many people can pass through a doorway in an evacuation situation. In part 122, we continue discussing means of escape, starting with stairways.
The capacity of a stairway in an evacuation situation is based on the width of the aperture, in the same way as the width of a doorway, but also gives an allowance for the number of people who can be accommodated within the (protected) stairway itself. In situations where the stairway only serves one floor, the discharge capacity will be considered the same as a doorway in a corridor or an outside wall. If it serves more than one floor, the capacity will be greater and comprises the original capacity (one floor) plus the standing capacity between the original floor and the additional floor.
Working out the capacities mathematically could be complex, but stairway capacities are given in most relevant codes based on the width of the staircase in mm’s and the number of floors served.
Simultaneous evacuation is not always used, particularly in high rise buildings. If certain fire safety conditions are in place, such as protected corridors or lobbies leading to stairways, and fire alarm and public address systems are in use, it may be acceptable to evacuate specific floors, based on risk. This is subject to sufficient fire resistance being in place between floors. Where phased evacuation is a part of the fire safety design for a building, it may be possible to limit the width of stairways as appropriate for the evacuation of two storeys at a time.
BS 9999:2017 gives the relevant stairway capacities for phased evacuation.
In the past, it was traditional to build redundancy into the means of escape provision for a building by calculating exit and stairway provision assuming one exit or stairway may be inaccessible. It is still the case when calculating storey exit widths, however, it is no longer necessary to discount one stairway, provided each stairway is approached through a protected lobby on all floors (other than the top floor), or provided the stairway is protected by pressurization against smoke entry.
In part 123 of this series, LWF will begin to look at floor space factors when calculating occupant capacity for means of escape. 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.