The LWF Blog

Fire Safety Engineering for Design – Exit Widths for Evacuation – Part 123

March 13, 2023 11:44 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 122, LWF looked at what can be done when the route to the exit for evacuation purposes is unavoidably complex. In part 123, we begin to discuss exit widths as the final part of exit routes.

The width of an exit for evacuation purposes is important because it affects how many people can pass through the exit in a fire situation in a given amount of time. Traditional fire safety codes base the width requirement on how many building occupants there are predicted to be.

The notional time given for evacuation is 2.5 minutes in most guidance and requirement documents and this will apply to the majority of situations. The figure does not take into account any potential delays that may occur before a person responds to the fire alarm sounder, however. The notional time may be extended upwards to around 8 minutes for the design of open-air sports stadiums, for instance.

While the ‘standard’ notional time of 2.5 minutes has proven largely suitable, there is no scientific basis for the period, it was simply thought to be appropriate.

Where the period for smoke-filling of an area is prolonged, e.g. due to a very high ceiling height or where smoke ventilation is provided, the exit widths may be based on longer evacuation times. British standards give a notional exit time of 5 minutes for shopping centres. This extension is justified by the smoke control and sprinkler systems normally required in large shopping centres.

Due to the likelihood of delays before people recognise the fire alarm and begin to evacuate, and potentially further delays as they follow a less than optimal route to the final exit, the notional exit time may prove to be much less than the actual time taken to evacuate a space.

A fire engineered design is likely to calculate exit widths based on various factors, although the basic tenets of provision of additional fire protection and smoke control measures will extend the period building occupants can safely remain in the building while evacuating.

In part 124 of LWF’s series on fire engineering we will begin to look at exit flows and how this can be calculated. 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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