The LWF Blog

Fire Safety Engineering for Design – Fire Dynamics – Part 81

May 23, 2022 12:01 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 80, LWF looked at pre-flashover and flashover fire behaviour. In part 81, we continue to discuss the development of compartment fires and flashover, before beginning to address the calculation of fire parameters.

In very large or high space compartments with minimal fuel loads, flashover is much less likely to occur than in smaller areas or where fuel loads are high. Some examples of large compartments where flashover is unlikely are airport concourses, shopping centre atria and indoor sports stadia.

The risk of fire spread within large or tall compartments to other areas by radiative heat transfer is possible and should be considered in terms of the placement of combustibles within such spaces at the design stage. Guidance may be found in BS 9999 Annex B.

While flashover is unlikely to occur in many types of large compartment, it is possible and more likely where a high fire load is present, such as in certain types of warehousing. Compartments with this type of fire risk usually require the installation of appropriate sprinkler systems, which work to ensure the smoke does not reach the required temperature for flashover.

A fuel-bed controlled fire has excess air available and so the combustion rate, temperature reached and fire growth are limited by the amount of fuel available to burn.

Fire Parameter Calculations

A design fire is generally characterised by the heat release rate variation with time

The design fire has certain assumptions. In its initial stages of fire growth, there is sufficient ventilation and the rate of burning will be indicated by the type, amount and placement of the fuel source. The design fire is assumed to be initially confined to a single object or group of objects acting as fuel.

As it develops, the fire may spread to adjacent fuel sources and once flames reach the ceiling, flashover may occur resulting in the whole room or compartment becoming involved in a fully-developed fire.

Post-flashover, smoke production is increased greatly and to the point where smoke control becomes impracticable. However, where there is a post-flashover fire in a small room, a smoke-control system in an adjacent large-volume space may be a possible fire safety solution, to help when smoke emerges from apertures in the compartment of fire origin.

In part 82 of LWF’s series on fire engineering, we will continue to discuss fire parameter calculations and design fires. 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 for over 25 years 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.