The LWF Blog

Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 116

January 23, 2023 12:34 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 115, LWF considered the second part of pre-movement time – response time, before beginning to discuss the design approach. In part 116, we look at how the design approach can take into account pre-movement times and what factors should be considered for inclusion.

Pre-movement times can vary considerably depending upon the different individuals or groups of people involved in evacuating a building. Significant variation can also be found based on the type of building. For example, in a cinema, once the alarm is raised and the lights are up, the time taken on pre-movement is minimal and the audience will all begin to move towards an exit at about the same time. In a hotel, where guests are in individual rooms, the variation can be much greater – partly because there is no ‘herd reaction’ and partly because some guests may be sleeping or showering and will take longer to leave the room than someone ready to go out.

When there is a great variation in pre-movement times, the distribution is likely to have a ‘head’ of early and a ‘tail’ for late starters, with the majority falling between the two. In the immediate area of the fire, those occupants are likely to evacuate the building before people in other areas realise there is a need to take action.

The pre-movement time of the first few people evacuating is particularly important because the evacuation process does not begin until this occurs. As we have established, the pre-movement time will be reliant on occupancy type, the building in which the fire starts, the fire alarm system and how the emergency management procedures are implemented.

If we consider an example of a well-managed office space, the occupants will be awake and already familiar with their surroundings. They will be trained in the fire emergency procedures and so the pre-movement time of the first occupants to respond should be very short – usually less than 20 seconds.

Another example might be in a shop or assembly building where the occupants will also be awake, but largely unfamiliar with their surroundings (apart from the entrance they used on the way in). The pre-movement time can be very short, largely depending upon if the staff are well-trained in taking action to direct customers towards the exits.

In part 117 of LWF’s series on fire engineering we continue looking at how the pre-movement time of the first few occupants of a building is important and can be streamlined. 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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