The LWF Blog

Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 109

November 28, 2022 12:04 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 108, LWF discussed mechanised walkways and other measures which may be used for means of escape design. In part 109, we look at the difference between the previously discussed and prescriptive measures for fire safety and fire safety engineering design approaches.

There is much guidance available on prescriptive solutions for fire safety provision and while the best fire safety solution may be found within those documents, there are alternatives. Fire safety engineering design allows for the architect and team to approach building design without the constraints implicit in prescriptive guidance. Fire engineering works to provide the optimum solution given the building design in question and is, therefore, not a one-size-fits-all solution, but a made-to-measure one.

Prescriptive guidance talks about travel distances and exit widths but does not touch on the time taken to escape, which is surely a significant factor to those evacuating during a fire.

Fire engineering design is based on the time available before conditions become untenable. The available time must be greater than the required time to escape. This can be shown as:


Available Safe Egress Time (ASET) and Required Safe Egress Time (RSET). The time between ignition of the fire and the onset of untenable conditions (ASET) must be greater than the time following ignition, when all occupants are notified of the fire and able to leave the building or fire affected space and reach a place of safety (RSET).

A fire engineer may use a relatively simple equation to work out the RSET of a given building, the resulting amount of time will include a margin of safety.

The remainder of the RSET is made up of detection time, alarm time and evacuation time. Evacuation time is made up of pre-movement time and travel time. And pre-movement time is made up of recognition time and response time. It is important that all elements are taken into consideration and any contributory factors that might influence any part are quantified or mitigated.

A simple example is that when evacuating, people whose children are on site, in a crèche for example, are unlikely to follow instructions to evacuate the building. This is a contributory factor that should be taken into account and mitigated where possible. For example, if the crèche is situated in an area with direct access to the outside, parents can be shown where the children will exit if the fire alarm sounds and therefore, where they should meet them outside the building.

In part 110 of LWF’s series on fire engineering we will consider tenability limits for design. 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