The LWF Blog
Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 120
February 20, 2023 12:02 pmLWF’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 119, LWF began to examine travel time and to look at how the different types of occupancy can affect travel time. In part 120, we discuss time of travel to an exit during a total or partial building evacuation.
The time taken by a building occupant to travel to an exit is affected by the speed at which they are travelling. In an empty building aside from one occupant, the speed of that person will be the maximum walking speed they are able to achieve. Crowd density, however, can affect travel speed in that when there is a high density of people all trying to move towards an exit, travel speed is reduced.
At a density of 4 persons per m2, the speed of movement can be slowed to the point it makes the occupancy very stressed and uncomfortable. Where occupant density is relatively low, e.g. 2m2 per person, 1.2 m.s-1 can be assumed to be the average walking speed for design purposes.
When descending stairs, the speed is reduced slightly and it can be assumed that the typical free movement speed is 1.1 m.s-1, giving a vertical component of velocity of 0.75 m.s-1.
People with mobility issues too can affect the travel speed possible. Wheelchair users on one level with adequate space to manoeuvre and accessible doors are able to evacuate as quickly as the average person on foot. Persons using walking aids require more time and while this varies widely, it can be assumed reasonably that they are able to achieve around half the speed of the average person on foot, e.g. 0.6 m.s-1.
Once the evacuation process begins and people begin to move towards an exit, the time taken for them to reach the designated place of safety will usually be affected most by the time required to pass through restrictions such as doorways. Doorways are usually designed to accommodate all building occupants in a nominal period of 2.5 minutes. It would be unusual for it to take 2.5 minutes to reach the doorway/exit, the rate of arrival at the door is likely to be greater than the doorway is designed to accommodate and a queue will form. Exceptions may exist where occupancy is low or the doorway is very wide, but generally-speaking, this is expected and implicit in the established fire safety design codes.
In part 121 of LWF’s series on fire engineering we will continue to discuss time of travel to an exit during an evacuation. 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.