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11.3. General Fire Science
Fire loads account for all combustible building contents including furnishings, equipment, as well as combustible construction components. Normally, most of the fire load in a building results from contents that have been introduced after the construction is complete. The fire load is usually expressed in terms of the so-called “wood-equivalent” weight of combustible building contents per unit building floor area (e.g., in psf). The actual weight of combustible contents is adjusted to the wood-equivalent weight based on the estimated potential heat of contents normalized to the potential heat of wood (8000 Btu/lb). Alternatively, the fire load could be expressed in terms of the potential heat of building contents per unit building floor area (e.g., Btu/ft2).
The duration and the maximum temperature of a fire in a building compartment depends on several factors including the amount and configuration of available combustibles, ventilation conditions, properties of the compartment enclosure, weather conditions, etc. In common circumstances, the maximum temperature of a fully developed building fire will rarely exceed 1800°F. The average gas temperature in a fully developed fire is not likely to reach 1500°F. Temperatures of fires that have not developed to post-flashover stage will not exceed 1000°F.
The term “thermal mass” is used sometimes for effective specific heat or heat capacity. Effective specific heat (e.g., in Btu/(lb°F)) is the amount of energy, per unit mass or weight of material, required to raise the temperature of the material by one temperature unit. Similarly, effective heat capacity (e.g., in Btu/((ft3°F)) is the amount of energy, per unit volume of material, required to raise the temperature of the material by one temperature unit. For most construction materials, specific heat and heat capacity values (as well as thermal conductivity values) are temperature dependent, i.e. these values change significantly in the temperature range associated with building fires (50-1800°F), because many materials undergo physicochemical changes at elevated temperatures. These thermal properties are also sensitive to the testing method used; therefore it is very common to find different values of material properties for the same material in different literature sources.
“Heat sink” refers to anything absorbing large amounts of heat through physical and/or chemical processes. Usually, materials containing large amounts of chemically combined water in their structure, e.g., gypsum or concrete, can absorb significant amounts of heat due to the energy consumed in the water evaporation process. Materials with high thermal conductivity and high effective heat capacity will also act as a heat sink. Materials with low thermal conductivity will reflect, and not absorb, heat.
“Flashover” is an abrupt transition from the burning of a small number of items in the room, or a small portion of the room, to full-room involvement in a fire. Room fire flashover is, in essence, a state in which all the combustible contents of the room experience a nearly simultaneous ignition. Most fires never reach flashover, because they are detected and extinguished before this can occur. Some fires self-extinguish due to insufficient air supply, or when the fire is of a low enough energy source that multiple targets are not ignited. Flashover is very unlikely to occur in sprinklered premises. The time to flashover depends on many factors, but most notably the properties of combustibles, size of the room, and the ventilation conditions. In larger rooms, the time to flashover will typically be longer. In well-ventilated, large premises (e.g., open parking garages, large atriums, etc.) flashover is highly unlikely.
11.3.6. Where can one find the thermal conductivity or thermal resistance values of different fire-protective materials?
Information on the properties of fire-protective materials is scattered. One good source is the SFPE Handbook of Fire Protection Engineering by the Society of Fire Protection Engineers. For some materials, such information may not be available at all.