The LWF Blog

Fire Safety Engineering for Design – Fire Dynamics – Part 83

June 6, 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 82, LWF looked at fire parameter calculations and design fires, before talking about pre-flashover fires. In part 83, we discuss fire growth rates.

A wide range of fire tests have been carried out to establish the heat release from varying materials when burned. The SFPE Handbook of Fire Protection Engineering contains many relevant examples, such as typical types of warehouse goods and foam-filled furniture.

Fire growth can be characterised in different ways through measurements, such as:

t-squared fires (UK and USA)

t-cubed fires

standard fires, types 1, 2 and 3 (Japan)

growing fires (Australia)

In the USA, a great deal of experimental work was carried out on heat release rates as a function of time. Much of the resulting data is available as a summary in NFPA 92.

There is more data on real fires available in the USA through the National Institute for Standards and Technology and BRE in the UK.

The type of tests carried out are large-scale and with the aim of showing fire growth and decay for groups of objects, and a series of objects. The data shows that fire curves are in fact, closer in shape to spikes with rapid growth and decay.

A heat release peak may be very high but will last only for a limited time and this should be taken into consideration when designing fire safety systems.

The first stages of a fire might be thought of as an incubation period and the fire growth rate will be very much lower than the t-squared fire growth rate. This is the initial period of a fire when it is very small or smouldering and it may continue for an indeterminate length of time. This can largely be ignored for design purposes, however, the fire may be detected in this stage by a nearby automatic heat or smoke detector, or by being discovered by a building occupant.

After the initial incubation period, the heat release rate grows approximately as the square of the time, i.e.:

Qt = at2

Where Qt is the total heat release rate of the fire (kW), a is a constant (kW · s–2) and t is time (s).