The LWF Blog

Fire Safety Engineering for Design – Fire Engineering Design Approaches – Part 111

December 12, 2022 11:17 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 110, LWF considered the tenability limits for design. In part 111, we continue to look at the tenability limits for smoke, before discussing the tenability limits for heat from a fire.

In buildings where there is a clearly defined escape route, a visibility of 10 m is considered acceptable. People will generally continue forwards if they can see that far ahead of them. 10 meters is equivalent to an optical density of 0.1 dB.m-1.

Where integrally lit escape signs are in use (back-illuminated) the visibility distance can be roughly doubled.  However, in public buildings and spaces which are especially large, it may prove more difficult for the building occupants (who may be members of the public not familiar with the building) to identify and follow escape routes. Larger visibility distances may be advisable in such circumstances.

Tenability limits for heat from a fire

For a short period of time, building occupants can withstand breathing air up to 120 °C. This temperature is based upon the air being dry. When the air may be humid, either from the use of sprinkler systems or firefighting the survivable temperature drops to approximately half, therefore a temperature of 60 °C should be adopted for fire safety design purposes.

Breathability of air is not the only aspect to be considered when looking at the tenability limits of heat from a fire. Excess heat can cause severe burns and pain to building occupants. Exposure for any significant amount of time to radiant heat flux of 2.5 kW.m-2 or more can be damaging and painful and therefore this figure is the maximum for short-term exposure.

Thermal radiation at a temperature of 200 °C will emit a radiant heat flux of approximately 2.5 kW.m-2 meaning that people inside the building cannot be expected to escape below a smoke layer at a temperature in excess of this.

In part 112 of LWF’s series on fire engineering we will begin to look at pre-movement time, i.e. what people do between the fire alarm signal sounding and beginning to evacuate. 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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