The LWF Blog

Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 117

January 30, 2023 12:11 pm

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 116, LWF looked at how the design approach can take into account pre-movement times and what factors should be considered for inclusion. In part 117, we continue discussing the pre-movement time of the first few occupants of a building and its importance, along with how it can be streamlined.

A fire alarm system that incorporates voice instructions can have a significant impact on the pre-movement times of building occupants. The difference between voice instructions and a klaxon is particularly noticeable when it is used in premises which are largely unfamiliar to the occupancy and where the emergency procedures are not known. Visitors to a building may not know the reason for an alarm signal or klaxon, whereas verbal instructions can be immediately understood and acted upon. A reduction in the pre-movement time when using a voice alarm is acknowledged in the design approach used in BS 9999.

The pre-movement time is particularly affected by the standard of fire safety management in the premises. Where fire safety management is not of a high standard, it can lead to extended pre-movement times and so good fire safety management is essential, whether escape provisions are aligned to design codes or fire engineering principles.

In England and Wales, adequate evacuation procedures are required by the Regulatory Reform (Fire Safety) Order 2005 and in Scotland by the Fire (Scotland) Act 2005.

Pre-movement times in establishments where the occupants may be sleeping are longer, despite effective fire safety management and the type of warning used. Due to contributory factors such as sleeping pills or consumption of alcohol, pre-movement times for some individuals may be as much as 30 minutes in hotels, for example.

When considering the distribution of pre-movement times, the initial ‘head’ of the distribution is the first few occupants beginning to move and then follows a log-normal, frequency-time distribution for the remainder of the occupants in an enclosure. The ‘tail’ follows the majority. This type of distribution is especially short in open-plan offices, for example, where the movement of the first few occupants spurs the remainder to action.

A much wider distribution can usually be observed in multiple enclosure buildings and the potential for the fire alarm warning and fire safety management system to have an impact on speed is greater.

In part 118 of LWF’s series on fire engineering we will continue to discuss pre-movement time distribution. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

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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