The LWF Blog

Fire Safety Engineering for Design – Fire Dynamics – Part 82

May 30, 2022 10:52 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 81, LWF discussed the development of compartment fires and flashover, before beginning to address the calculation of fire parameters. In part 82, we continue to look at fire parameter calculations and design fires, before talking about pre-flashover fires.

The rate of heat release of a fire, commonly referred to as the ‘fire size’, is the most reliable indicator of the way a fire and its products behave. When planning a fire engineering design for a building, it is important to define a series of design fires at the outset which should define the potential worst fire situations that may occur in the building. To ensure that the design fires chosen are suitable for the building in question, theoretical and experimental information is available to designers to assist with selection.

Pre-flashover fires

A design fire may be categorised for design purposes as follows: a growing fire; a fire having a fixed size for a finite time; or a steady state fire.

The size of the design fire will depend on the type and arrangement of fuel.

A fixed size or steady state fire will grow to the limit, which is reached before encountering a restriction to further growth. This would usually be one of the following:

• Fire control methods, such as an automatic or manual fire suppression
• Distance to next fuel source is too great (sufficient space separation)
• In the case of hydrocarbon pool fires, the leakage vs the burning rate or, if bunded, the extent of the bund.

A fixed design fire size which would be applicable to all situations is not feasible, it would also be inappropriate for use, particularly when designing for means of escape or estimating the activation time of automatic fire detectors.

A design should be based on a growing fire, using the design fire calculations including t-squared growth rates and with a maximum heat release rate. It should be noted that it is not possible to calculate the incubation period of the fire and therefore this period should be ignored when using this approach. The result of leaving out that aspect means the design calculation is inherently conservative in estimations.

In part 83 of LWF’s series on fire engineering, we will continue to look at pre-flashover 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.