The LWF Blog

Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 118

February 6, 2023 11:33 am

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 117, LWF discussed the pre-movement time of the first few occupants of a building and its importance, along with how it can be streamlined. In part 118, we continue to discuss pre-movement time distributions.

While wide and broad pre-movement time distributions often indicate a large building with multiple enclosures, a short and narrow one may be achieved in buildings where the occupants are awake and familiar with their surroundings, such as in an open-plan office.  Where there is only one enclosure for building occupants, and they are alert in familiar surroundings, pre-movement time should be less than one minute. Where there are multiple occupancies and enclosures and people are sleeping, the pre-movement time distribution can be anything up to 30 minutes.

While evacuation time can be broken down into pre-movement time and travel time for the purposes of examining where the time is spent and can be saved, it would not be appropriate to simply add total pre-movement time and travel times, as the result would overestimate the total evacuation time.

Where a building or space has a high occupant density, the period before the first individuals begin to move toward an exit that is the important pre-movement factor. After this time, the travel time comes into play.

E.g. In a space with high occupant density, the time before the first person makes moves to begin to evacuate is the most important in terms of pre-movement time. After that, the first person is already in the travel time proportion of the distribution while the second and third people are just beginning to evacuate, and so on in an overlapping fashion.

In an occupancy which is low density, total evacuation time is equivalent to the sum of the total pre-movement time and the travel time.

In an environment where fire safety and evacuation is well-managed, some guide values of pre-movement times are as follows:

Dwellings                            5 minutes
Hotel rooms                      20 minutes
Halls of residence             20 minutes
School, Lecture hall         1 minute
Office                                  1 minute
Bank                                    2 minutes
Shop/Dept Store               3 minutes

In part 119 of LWF’s series on fire engineering we will begin to examine travel time. 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.

Share this post