# The LWF Blog

## Fire Safety Engineering for Design – Fire Growth – Part 84

March 31, 2023 9:45 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 83, LWF discussed fire growth rates. In part 84, we will continue to discuss fire growth.

A t-squared fire growth rate is defined by the time taken for the heat output to reach 1055 kW (approximately 1 MW). It is known as the characteristic growth time. Fires are sometimes classified as ‘slow’, ‘medium’, ‘fast’ and ‘ultra-fast’ depending on the characteristic growth time.

For example, the fastest burning upholstered furniture such as sofas, and plastic goods stacked to a height of over 4 metres give ‘ultra-fast’ growth rates. Lower piles of plastic goods and other upholstered furniture give ‘fast’ growth rates. Tightly rolled paper gives a ‘slow’ growth rate.

Testing has shown that fire growth rates depend not only on what material is used for fuel but also how it is arranged. The simplest demonstration of this is the components to make a fire in a grate may not ignite and burn if not arranged properly, but will burn much faster when arranged in the preferred manner.

A high rack warehouse may exhibit growth rates modelled on a t-cubed fire, as Qt = 0.045 t3

This indicates a rapid fire growth, where the incipient stage is significant and the curve is valid up to 10 MW for a 10 metre high rack. It should be noted that there is no data recorded for fires greater than 10 MW.

Unit heat release rates can be estimated per unit floor area or per unit fuel area for various different commodities as provided in the SFPE Handbook and NFPA 92.

A steady-state fire which is not sprinklered occurs when the fire has spread from item to item until all available fuel is burning. The heat output is able to reach a steady value which declines as the fuel decays.

A steady-state fire when sprinklered is a more contentious calculation. Some exposit that no further items of fuel ignite once the sprinklers are operational and therefore the value of mass flow in the plume is calculated accordingly. After operation, it may be assumed that the sprinklers cool much of the smoke layer to a temperature less than the operating temperature of the sprinklers.

For calculation purposes, it may be assumed that an average smoke layer temperature of 100 °C will be attained with conventional sprinklers while in operation.

Part 83 of this series can be read here