The Behaviour of Fire in Buildings

About this course.

Fire is a chemical reaction: the rapid oxidation of a fuel that releases heat and light. For that reaction to start and continue, three ingredients must be present together — fuel, oxygen and heat — supported by a self-sustaining chemical chain reaction. Take away any one of them and the fire cannot begin or cannot continue. This is the idea behind the familiar "fire triangle" of fuel, oxygen and heat, and the "fire tetrahedron" that adds the chain reaction as a fourth side. Understanding these elements is the foundation for everything else in fire safety, because every method of preventing or extinguishing fire works by removing or interrupting one of them.

Fuel is any material that can burn. On a construction site and in a finished building this includes timber and engineered wood, paper and packaging, plastics and foams, fabrics and furnishings, insulation, flammable liquids such as solvents and fuels, and gases such as LPG. Most solid materials do not burn directly; instead, heat causes them to break down and give off flammable vapours in a process called pyrolysis, and it is these vapours that actually ignite and flame. This is why finely divided or vapour-rich materials catch far more readily than dense solid ones, and why the amount and type of combustible material in a space — the "fuel load" — has such a strong influence on how a fire will behave.