Reading the Fire: Building Factors

First of the critical building features is construction method. Building construction influences both fire behavior and structural stability under fire conditions. Combustible construction such as wood-frame, ordinary (masonry and wood), and heavy...

Non-structural elements such as interior finish can also have a significant effect on fire development. On November 28, 1942; a fire in Boston's Coconut Grove Nightclub claimed the lives of 492 people (Benzaquin, 1959). The extreme number of fatalities in this incident resulted from rapid fire spread due to combustible interior finish and inadequate exits. Similarly, on February 20, 2003; 96 people died in the Station Nightclub fire in West Warwick, Rhode Island (NIST, 2005). In this incident, combustible acoustical foam ignited by pyrotechnics and combustible interior finish along with lack of an automatic sprinkler system contributed to the speed of fire growth and development. Figure 5 illustrates temperatures five feet above the floor in the Station Nightclub 90 seconds after ignition.

When examining non-structural elements as fuel, it is important to not only look up and look around. Look down at the floor. While hot gases rise and convection will generally result in vertical and lateral heat transfer, radiant heat from the fire and the hot gas layer also transfers heat energy to the floor. Like other hydrocarbon based synthetic materials, carpet and carpet padding may have a much higher heat of combustion than wood and can be a significant source of fuel (Division of the State Fire Marshal, 2002; FBU, 1996).

As discussed in factors influencing fire development, insulation and energy efficiency of the structure will have an influence on fire behavior. However, like many other building factors, thermal characteristics of the structure may not be readily visible during firefighting operations. Insulation is designed to reduce heat transfer through the building shell or other structural elements. Insulation is normally intended to retain building heat when ambient temperatures are cold and slow heating when ambient temperatures are hot. Insulation performs the same way under fire conditions. A well-insulated compartment will retain more heat, increasing the speed of fire development (all other things being equal). Reduced building leakage and use of multi-pane glazing in windows also reduces leakage and potential increases in ventilation, speeding the transition to ventilation controlled conditions and increasing the probability of developing backdraft conditions.

Building Contents

A large percentage of compartment fires simply involve contents and non-structural elements such as interior finish of the ceiling, walls, and floor. Key characteristics influencing fuels' burning characteristics include state (solid, liquid, or gas), chemical composition, and distribution. Building contents can include gas, liquid and solid fuels. However, most ordinary contents are in the solid form.

The chemical composition of fuel influences its heat of combustion (the amount of heat released by a given mass of fuel) and the heat release rate (the speed with which that heat is released). Oxidation of a specific amount of fuel (i.e., kilogram) releases a given amount of heat energy (i.e., kilojoules). Kilojoules/kilogram (kJ/kg) is a standard unit of measure for heat of combustion. A fuel's heat of combustion is dependent on its chemical content. The heat of combustion for hydrocarbon fuels such as plastics, gasoline, propane and methane can be considerably higher than that of cellulose fuels such as wood (DiNenno et al., 2002) as illustrated in Figure 6.

The heat of combustion for wood and paper varies with the specific type of material that is burning. The heat of combustion for plastic materials also varies with the specific material involved. However, as noted in Figure 6, some types of plastic have a heat of combustion approaching that of hydrocarbon fuels such as gasoline, propane, and methane. These fuels release considerably more heat energy than ordinary combustibles such as wood and paper.

While the total heat energy released when fuel burns is important, the rate at which it is released is also significant. Heat release rate (HRR) is the energy release per unit of time and is usually expressed in kilowatts (kW). A kW is 1000 joules per second (J/s). Heat release rate is dependent on the type, quantity, and orientation of the fuel as well as the characteristics of the enclosure (if the fire occurs inside a compartment).

The minimum size fire (expressed in terms of heat release rate) that will cause a flashover in a given room is dependent on compartment size and ventilation. HRR varies over time, increasing as more fuel becomes involved and the temperature in the compartment increases (higher temperature increases the speed of the combustion reaction). HRR decreases as fuel is consumed and temperature in the compartment decreases.