IFSTA 7th ch 4.docx

Physical Science Terminology Physical science is the study of matter and energy and includes chemistry and physics. This theoretical foundation must be translated into a practical knowledge of fire dynamics. To remain safe, you need to be able to identify the fire dynamics present in a given situati...

Physical Science Terminology Physical science is the study of matter and energy and includes chemistry and physics. This theoretical foundation must be translated into a practical knowledge of fire dynamics. To remain safe, you need to be able to identify the fire dynamics present in a given situation and anticipate what the next stages of the fire will be along with how fire fighting operations may impact the fire's behavior. The world around you is made up of matter in the form of physical materials that occupy space and have mass. While matter can undergo many types of physical and chemical changes, this chapter will concentrate on those changes related to fire. A physical change occurs when a substance remains chemically the same but changes in size, shape, or appear-ance. Examples of physical change are water freezing (liquid to solid) and boiling (liquid to gas). A chemical reaction occurs when a substance changes from one type of matter into another, such as two or more substances combining to form compounds. Oxidation is a chemical reaction involving the combination of an oxidizer, such as oxygen in the air, with other materials. Oxidation can be slow, such as the combination of oxygen with iron to form rust, or rapid, as in combustion of methane (natural gas) Energy is the capacity to perform work. Work occurs when a force is applied to an object over a distance or when a substance undergoes a chemical, biological, or physical change. In the case of heat, work means increasing a substance's temperature. Forms of energy are classified as either potential or kinetic.  Potential energy represents the amount of energy that an object can release at some point in the future. Fuels have a certain amount of potential energy before they are ignited, based on their chemical composition. This potential energy available for release in the combustion process is known as the heat of combustion. The rate at which a fuel releases energy over time depends on many variables including:   Chemical composition   Arrangement   Density of the fuel   Availability of oxygen for combustion Kinetic energy is the energy that a moving object possesses. While a fuel such as wood is not "moving" as you might define it, when heat is introduced, the molecules within the fuel begin to vibrate. As the heat (thermal energy) increases, these molecules vibrate more and more rapidly. The fuel's kinetic energy is the result of these vibrations in the molecules. There are many types of energy including:   Chemical   Thermal   Mechanical   Electrical   Light   Nuclear   Sound All energy can change from one type to another. For example, mechanical energy from a machine can convert to thermal energy when friction between moving parts generates heat. In terms of fire behavior, the potential chemical energy of a fuel converts into heat and light during combustion. Energy is measured in joules (J) in the International System of Units (SI). The quantity of heat required to change the temperature of 1 gram of water by 1 degree Celsius is 4.2 joules. In the customary system, the unit of measurement for heat is the British thermal unit (Btu). A British thermal unit is the amount of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit. While not used in scientific and engineering texts, the Btu is still frequently used in the fire service. When comparing joules and Btu, 1055 J = 1 Btu. Chemical and physical changes almost always involve an exchange of energy. A fuel's potential energy releases during combustion and converts to kinetic energy. Reactions that emit energy as they occur are exothermic reac-tions. Fire is an exothermic chemical reaction that releases energy in the form of heat and sometimes light. Reactions that absorb energy as they occur are endothermic reactions. For example, converting water from a liquid to a gas (steam) requires the input of energy resulting in an endothermic reaction. Converting water to steam is a tactic for controlling and extinguishing some types of fires. Fire Triangle and Tetrahedron The fire triangle and fire tetrahedron models are used to explain the elements of fire and how fires can be extinguished. The oldest and simplest model, the fire triangle, shows three elements necessary for combustion to occur: fuel, oxygen, and heat. Remove any one of these elements and the fire will be extinguished. The fire triangle was used prior to the general adaptation of the fire tetrahedron, which includes a chemical chain reaction. An uninhibited chemical chain reaction must also be present for a fire to occur. The fire tetrahedron model includes the chemical chain reaction to explain flaming or gas-phase combustion (fire is an example of gas-phase combustion). Ignition Fuels must be in a gaseous state to burn; therefore, solids and liquids must become gaseous in order for ignition to occur. When heat is transferred to a liquid or solid, the substance's temperature increases and the substance starts to convert to a gaseous state (off gassing). In solids, off-gassing is a chemical change known as pyrolysis; in liquids, it is a physical change called vaporization. Piloted ignition is the most common form of ignition and occurs when a mixture of fuel and oxygen encounter an external heat source with sufficient heat or thermal energy to start the combustion reaction. Autoignition occurs without any external flame or spark to ignite the fuel gases or vapors. The fuel's surface is heated to the point at which the combustion reaction occurs. Once the fuel is ignited, the energy released from combustion transfers to the remaining solid fuel resulting in the production and ignition of additional fuel vapors or gases. This exchange of energy from the burning gases to the fuel results in a sustained combustion reaction. Autoignition temperature (AIT) is the minimum temperature at which a fuel in the air must be heated in order to start self-sustained combustion. The autoignition temperature of a substance is always higher than its piloted ignition temperature. Combustion Fire and combustion are similar conditions. Both words are commonly used to mean the same thing. Combustion, however, is a chemical reaction while flaming combustion is only one possible form of combustion. Combustion can occur without visible flames. There are two modes of combustion: nonflaming and flaming. Nonflaming Combustion Nonflaming combustion occurs more slowly and at a lower temperature, producing a smoldering glow in the material's surface. The burning may be localized on or near the fuel's surface where it is in contact with oxygen. Examples of nonflaming or smoldering combustion include burning charcoal or smoldering wood or fabric. The fire triangle illustrates the elements/conditions required for this mode of combustion. Flaming Combustion Flaming combustion is commonly referred to as fire. It produces a visible flame above the material's surface. Flaming combustion occurs when a gaseous fuel mixes with oxygen in the correct ratio and heats to ignition tem-perature. Flaming combustion requires liquid or solid fuels to be converted to the gas phase through the addition of heat (vaporization or pyrolysis, respectively). When heated, both liquid and solid fuels will emit vapors that mix with oxygen, producing flames above the material's surface if the gases ignite. The fire tetrahedron accurately reflects the conditions required for flaming combustion. Each element of the tetrahedron must be in the proper proportion and in close physical proximity for flaming combustion to осcur. Removing any element of the tetrahedron interrupts the chemical chain reaction and stops flaming combustion. However, the fire may continue to smolder depending on the characteristics of the fuel. Ignition is where the combustion process begins. A heat source pyrolizes a fuel, creating fuel gases. Those gases mix with oxygen and ignite, creating a fire. The fire can be compared to a pump. Fresh oxygen is "pumped in" and mixes with fuel gases. Then as it burns, the fire "pumps out" combustion products that have larger amounts of mass and a higher level of energy than the inlet air. In the case of open burning, the "pump" does not have a well-defined inlet or outlet, as the air is being entrained (drawn in) from all around the burning fuel. The fire also generates heat. As the heat transfer to the gaseous combustion products, they expand and begin to rise and move away from the fire due to buoyancy. In other words, the density of the hot combustion products is less than the surrounding air, and the combustion products float on the dense cool air surrounding the fuel, creating the layers of smoke and fuel gases that fill a compartmen during a fire. Products of Combustion As a fuel burns, its chemical composition changes, which produces new substances. These products of combustion are often simply described as heat and smoke. While the heat from a fire is a danger to anyone directly exposed to it, exposure to toxic gases found in smoke and/or lack of oxygen cause most fire deaths. Smoke is an aerosol comprised of gases, vapor, and solid particulates. Smoke is the product of incomplete combustion. Simply stated, combustion is incomplete when any of the fuel is left after combustion has occurred. Smoke and ash are examples of left over fuel from incomplete combustion. By comparison, under ideal conditions, the entire fuel would undergo a chemical conversion from its current form into an equal amount of new materials. For example, complete combustion of methane in air results in the production of heat, light, water vapor, and carbon dioxide. However, combustion is incomplete in a structure fire, meaning that some of the fuel does not burn, but instead gets entrained with hot gases and rises aloft. This un-burned fuel is smoke, and it has the potential to burn. Most structure fires involve multiple types of fuels (carbon-based fuels [wood, cotton], hydrocarbon fuels [plastics, synthetic fabrics], and other types), and the fires tend to have a limited air supply. When the air supply is limited, the level of incomplete combustion is higher, which produces more smoke. These factors result in complex chemical reactions that generate a wide range of products of combustion including toxic and flammable gases, va-pors, and particulates that comprise smoke. Gases such as carbon monoxide (CO) are generally colorless, while vapor and particulates give smoke its varied colors. Most components of smoke are toxic and dangerous to human life. The materials that make up smoke vary from fuel to fuel, but generally all smoke is toxic. The toxic effects of smoke inhalation are the result of the interrelated effect of all the toxic products present. Keep in mind that the combustion process consumes oxygen from the air, effectively removing it from the en-vironment. As part of the chemical reaction, the consumed oxygen combines with carbon in the smoke to form combustion products like CO or carbon dioxide (CO,). Low oxygen concentrations alone can result in hypoxia or death. The toxic gases in combination with a low oxygen concentration can reduce the time that a victim could survive. Table 4.2 lists some of the more common products of combustion and their toxic effects. Concentrations of the products of combustion and/or low oxygen concentrations can cause asphyxiation (fatal level of oxygen deficiency in the blood). Carbon monoxide (CO) is a toxic and flammable product of the incomplete combustion of organic (carbon-containing) materials. Carbon monoxide is a colorless, odorless gas present at almost every fire. It is released when an organic material burns in an atmosphere with a limited supply of oxygen. CO exposure is frequently identified. CO acts as a chemical asphyxiant. CO poisoning is a sometimes lethal condition in which carbon monoxide molecules attach to hemoglobin, decreasing the blood's ability to carry oxygen. CO combines with hemoglobin about 200 times more effectively than oxygen does. CO does not act on the body, but excludes oxygen from the blood, leading to hypoxia of the brain and tissues. Death will follow if the process is not reversed. Hydrogen cyanide (HCN), a toxic and flammable substance produced in the combustion of materials containing nitrogen, is also commonly found in smoke, although at lower concentrations than CO. Incomplete combustion of substances that contain nitrogen and carbon produce HCN. The following materials produce HCN:   Natural fibers such as wool, cotton and silk   Resins such as carbon fiber or fiberglass   Synthetic polymers such as nylon or polyester   Synthetic rubber such as neoprene, silicone and latex These materials are found in:   Upholstered furniture   Bedding   Insulation   Carpets   Clothing   Other common building materials and household items HCN is a significant byproduct of the combustion of polyurethane foam used in many household furnishings. HCN is also released during off-gassing as an object is heated. It may also be found in vehicle fires, where new insulation materials give off high amounts of gases and cause fires to last longer. HCN is 35 times more toxic than CO. HCN acts as a chemical asphyxiant but with a different mechanism of action than CO. HCN prevents the body from using oxygen at the cellular level. HCN can be inhaled, ingested, or absorbed into the body, Where it then targets the heart and brain. Inhaled HCN enters the bloodstream and prevents the blood cells from using oxygen properly, killing the cells. The effects of CN depend on the con-centration, length, and type of exposure. Large amounts, high concentrations, and lengthy exposures are more likely to cause severe effects, including permanent heart and brain damage or death. Carbon dioxide (CO,) is a product of complete combustion of organic materials. It is not toxic in the same manner as CO or CN, but it displaces existing oxygen which creates an oxygen deficient atmosphere. CO, also acts as a respiratory stimulant, increasing the respiratory rate. NOTE: Fire gases also contain many other gases than the three highlighted in this section. These additional gases have their own effects and exposure times. The exposure time is based on the combination of gases or the lethal effective dose. Irritants in smoke are substances that cause breathing discomfort and inflammation of the eyes, respiratory tract, and skin. Smoke can contain a wide range of irritating substances depending on the fuels involved. More than 20 irritants in smoke have been identified including hydrogen chloride, formaldehyde, and acrolein. Smoke also contains significant amounts of unburned fuels in the form of solid and liquid particulates and gases. Smoke must be treated with the same respect as any other flammable gas because it may burn or explode. Particulates can interfere with vision and breathing. Pressure Differences Pressure is the force per unit of area applied perpendicular to a surface. For example, atmospheric pressure (1 atmosphere [app. 101 kPa]) at standard temperature (68° F [20° CI) indicates the amount of pressure the atmosphere applies to the surface of the earth. At standard temperature and atmospheric pressure, gases remain calm and move very little. Differences in pressure above or below standard pressure create movement in gases. Gases always move from areas of higher pressure to areas of lower pressure. Even small differences in pressure, such as the 0.1 kPa or less differences created in most compartment fires, create this movement. Heat from a fire increases the pressure of the surrounding gases. This increased pressure will seek to expand and equalize with areas of lower pressure. Heated gases will rise, remain aloft (buoyant) and generally travel up and out. At the same time, cooler, fresh air will generally travel inward toward the fire. This exchange of air creates a convective flow. As the pressure difference between high and low pressure areas increases, the speed with which gases will move from high to low also increases. It is critical for firefighters to understand how small changes to the gas pressure within a structure can dramatically affect fire behavior. Thermal Energy (Heat) A working knowledge of fire dynamics requires an understanding of temperature, energy, and power or heat release rate. Firefighters often use these terms interchangeably because the differences between the terms are not always understood. Difference between Heat Release Rate and Temperature Heat is the thermal kinetic energy needed to release the potential chemical energy in a fuel. As heat begins to vibrate the molecules in a fuel, the fuel begins a physical change from a solid or liquid to a gas. The fuel emits flammable vapors which can ignite and release thermal energy. This new source of thermal energy begins to heat other, uninvolved fuels converting their energy and spreading the fire. Temperature is the measurement of heat. More specifically, temperature is the measurement of the average kinetic energy in the particles of a sample of matter. A block of wood at room temperature has stable molecules and is in no danger of ignition. When thermal energy transfers to the wood, the wood is heated, and the temperature of the wood rises because the molecules have begun to vibrate and move more freely and rapidly. Temperature can be measured using several different scales. The most common are the Celsius scale, used in the International System of Units (SI) (metric system), and the Fahrenheit scale, used in the customary system. The freezing and boiling points of water provide a simple way to compare these two scales. A dangerous misconception is that temperature is an accurate predictor or measurement of heat transfer. It is not. For example, one candle burns at the same temperature as ten candles. However, the heat release rate (kW) of the ten candles is ten times greater than one candle at the same temperature. The increased heat release rate results in an increased heat transfer rate to an object. This energy flow to a unit area (heat flux) is measured in kilowatts per square meter. Translated to an interior fire environment, the temperature in the structure may be within tolerances for personal protective equipment however, the heat flux to the PPE from the fire indicates the real measurement of how long the PPE will protect you. In other words, the temperature tells you it is safe to go in, but the heat transfer rate - not the temperature - tells you how long you can stay in. Sources of Thermal Energy Chemical, electrical, and mechanical energy are common sources of heat that result in the ignition of a fuel. They can all transfer heat, cause the temperature of a substance to increase, and are most frequently the ignition sources of structure fires. Chemical Energy Chemical energy is the most common source of heat in combustion reactions. The potential for oxidation exists when any combustible fuel is in contact with oxygen. The oxidation process almost always results in the production of thermal energy. Self-heating, a form of oxidation, is a chemical reaction that increases the temperature of a material without the addition of external heat. Self-heating can lead to spontaneous ignition which is ignition without the addition of external heat. Oxidation normally produces thermal energy slowly. The energy dissipates almost as fast as it is generated. An external heat source such as sunshine can initiate or accelerate the process. For self-heating to progress to spontaneous ignition, the following factors are required:   The insulation properties of the material immediately surrounding the fuel must be such that the heat cannot dissipate as fast as it is generated.   The rate of heat production must be great enough to raise the temperature of the material to its autoignition temperature.   The available air supply in and around the heated material must be adequate to support combustion. Rags soaked in linseed oil, rolled into a ball, and thrown into a corner have the potential for spontaneous ignition. The natural oxidation of this vegetable oil and the cloth will generate heat if some method of heat transfer such as air movement around the rags does not dissipate the heat. The cloth could eventually increase in temperature enough to cause ignition. The rate of most chemical reactions increases as the temperature of the reacting materials increases. The oxidation reaction that causes heat generation accelerates as the fuel generates and absorbs more heat. When the heat generated exceeds the heat being lost, the material may reach its autoignition temperature and ignite spontaneously. Electrical Energy Electrical energy can generate temperatures high enough to ignite any combustible materials near the heated area. Electrical heating can occur in several ways, including the following:   Resistance heating - Electric current flowing through a conductor produces heat. Some electrical appliances, such as incandescent lamps, ranges, ovens, or portable heaters, are designed to make use of resistance heating. Other electrical equipment is designed to limit resistance heating under normal operating conditions.   Overcurrent or overload - When the current flowing through a conductor exceeds its design limits, the conductor may overheat and present an ignition hazard. Overcurrent or overload is unintended resistance heating.   Arcing - In general, an arc is a high-temperature luminous electric discharge across a gap or through a medium such as charred insulation. Arcs may be generated when there is a gap in a conductor such as a cut or frayed wire or when there is high voltage, static electricity, or lightning.   Sparking - When an electric arc occurs, luminous (glowing) particles can form and splatter away from the point of arcing. Mechanical Energy Friction and compression generate mechanical energy. The movement of two surfaces against each other creates heat of friction that generates heat and/or sparks. Heat is generated when a gas is compressed. Diesel engines use this principle to ignite fuel vapors without spark plugs. This principle is also the reason that SCBA cylinders feel warm to the touch after they are filled. When a compressed gas expands, the gas absorbs heat. This absorption accounts for the way the cylinder cools when a CO, extinguisher is discharged. Heat Transfer The transfer of heat from one point or object to another is part of the study of thermodynamics. Heat transfer from the initial fuel package (burning object) to other fuels in and beyond the area of fire origin affects the growth of any fire and is part of the study of fire dynamics. Heat transfers from warmer objects to cooler objects because heated materials will naturally return to a state of thermal equilibrium in which all areas of an object are a uniform temperature. Objects at the same temperature do not transfer heat. The rate at which heat transfers is related to the temperature differential of the bodies and the thermal conductivity of the materials involved. The greater the temperature differences between the bodies, the greater the transfer rate. A material with higher thermal conductivity will transfer heat more quickly than other materials. Heat transfers from one body to another by three mechanisms: conduction, convection, and radiation. Conduction Conduction is the transfer of heat through and between solids. Conduction occurs when a material is heated as a result of direct contact with a heat source. Conduction results from increased molecular motion and collisions between a substance's molecules, resulting in the transfer of energy through the substance. The more closely packed the molecules of a substance are, the more readily it will conduct heat. For example, if a fire heats a metal pipe on one side of a wall, heat conducted through the pipe can ignite wooden framing components in the wall or nearby combustibles on the other side of the wall. Heat transfer due to conduction is dependent upon three factors: Area being heated   Temperature difference between the heat source and the material being heated   Thermal conductivity of the heated material Thermal conductivity of the hated material of various common materials. For example, copper will conduct heat more than seven times faster than steel. Likewise, steel is nearthy forty times as thermally conductive as concrete. Air is the least able to conduct heat of most substances, so it isa very good insulator. Insulating materials slow the conduction of heat from one solid to another. Good insulators are materials that do not conduct heat well because their physical makeup disrupts the point-to-point transfer of heat or thermal energy. The best commercial insulators used in building construction are those made of fine particles or fibers with void spaces between them filled with a gas such as air. Gases do not conduct heat very well because their molecules are relatively far apart. Convection Convection is the transfer of thermal energy by the circulation or movement of a fluid (liquid or gas). In the fire environment, convection usually involves transfer of heat through the movement of hot smoke and fire gases. As with all heat transfer, the heat flows from the hot fire gases to the cooler structural surfaces, building contents, and air. Convection may occur in any direction. Vertical movement is due to the buoyancy of smoke and fire gases. Lateral movement is usually the result of pressure differences (movement from high to low pressure). Heat transfer due to convection is dependent upon three factors:   Area being heated   Temperature difference between the hot fluid or gas and the material being heated   Turbulence and velocity of moving gases Radiation Radiation is the transmission of energy as electromagnetic waves, such as light waves, radio waves, or X-rays, without an intervening medium. Radiant heat can become the dominant mode of heat transfer as the fire grows in size and can have a significant effect on the ignition of objects located some distance from the fire, Radiant heat transfer is also a significant factor in fire development and spread in compartments. Numerous factors influence radiant heat transfer, including:   Nature of the exposed surfaces - Dark-colored materials emit and absorb heat more effectively than light-colored materials; smooth or highly-polished surfaces reflect more radiant heat than rough surfaces.   Distance between the heat source and the exposed surfaces - Increasing distance reduces the effect of radiant heat (Figure 4.19).   Temperature of the heat source - Unlike other methods of heat transfer that depend on the temperature of both the heat source and exposed surface, radiant heat transfer primarily depends on the temperature of the heat source. As the temperature and heat release rate of the heat source increases, the radiant energy also increases. As an electromagnetic wave, radiated heat energy travels in a straight line at the speed of light. The heat of the sun is the best example of radiated heat transfer. The energy travels at the speed of light from the sun through space (a vacuum) until it strikes and warms the surface of the earth. Radiation is a common cause of exposure fires. As a fire grows, it radiates more energy which other objects absorb as heat. In large fires, it is possible for the radiated heat to ignite buildings or other fuel packages a considerable distance away. Radiated heat travels through vacuums and air spaces that would normally disrupt conduction or convection. However, materials that reflect, absorb, or scatter radiated energy will disrupt the heat transmission. While flames have high temperature resulting in significant radiant energy emission, hot smoke or flames in the upper layer can also radiate significant heat. Interaction among the Methods of Heat Transter The methods of heat transfer rarely occur individually during a fire. The fire radiates heat, causes convection of heat through hot fuel gases, and conducts heat through burning materials or metals that are involved in the fire. Convected heat and radiated heat that reaches walls and ceilings heats those surfaces which, in turn, begin to conduct heat to whatever extent possible based upon the material's thermal conductivity. One side of the object is warm and slowly warms through the object until the opposite side is of equal temperature with the heated side. A heated surface will then, in turn, begin to radiate heat which could lead to ignition, combustion, convection, and so on. This cycle continues until interrupted. A good example of this interaction is how your PPE absorbs heat during interior operations. Convected and radiated heat will begin to heat the exterior of your PPE. The longer you are in the heated environment, the more heat that surface will absorb. The PPE has low thermal conductivity, so it will conduct heat slowly. However, eventually the interior surface of the PPE will heat to the same level as the exterior. Wherever the gear is compressed against skin or underclothing, heat will be conducted faster. Where the PPE is not in contact, it will radiate heat to the insulating air layer between your body and the inte rior surface of the gear. This transferred heat can cause heat stress and will eventually cause PPE to fail. The heat absorption and build-up in PPE is a direct result of all of the heat transfer methods acting at the same time. Fuel Fuel is the oxidized or burned material or substance in the combustion process. A fuel may be found in any of three physical states of matter: gas, liquid, or solid. The fuel in a combustion reaction is known as the reducing agent. Fuels may be inorganic or organic. Inorganic fuels, such as hydrogen or magnesium, do not contain carbon. Most common fuels are organic, containing carbon and other elements. Organic fuels can be divided into hydrocarbon-based fuels, such as: Gasoline Fuel oil Plastics Cellulose-based materials (wood and paper) A fuel's chemical content influences both its heat of combustion and heat release rate. The fuel's heat of combustion is the total amount of thermal energy released when a specific amount of that fuel burns. In other words, different materials release more or less heat than others based on their chemical makeup. Many plastics, flammable liquids, and flammable gases contain more potential thermal energy than wood. Synthetic materials are common in modern construction and furnishings. These materials are synthesized from petroleum products, and as a result, they have higher heats of combustion and may generate higher heat release rates than wood on a per-mass basis. Power is the rate at which energy transfers. Another way to describe power is the rate at which energy converts from one form to another. The standard international (SI) unit for power is the watt (W). One watt is 1 joule per second (J/s). In terms of fire behavior, power is the heat release rate during combustion. When a fuel is heated, work is being performed (energy is being transferred). The speed with which this work occurs, heat release rate, is the amount of generated power. Heat release rate is the energy released per unit of time as a fuel burns and is usually expressed in kilowatts (kW) or megawatts (MW). Heat release rate depends on the type, quantity, and orientation of the fuel. Heat release rate directly relates to oxygen consumption because the combustion process requires a continuous supply of oxygen to continue. Typically, the more oxygen is available, the higher the heat release rate. Similarly, the heat release rate decreases if all available oxygen is consumed and not replenished. Gases For flaming combustion to occur, fuels must be in the gaseous state. As previously described, thermal energy is required to change solids and liquids into the gaseous state. Vapor is the common term used to describe the gaseous state of a fuel that would normally exist as a liquid or a solid at standard temperature and pressure. Gaseous fuels such as methane (natural gas), hydrogen, and acetylene, can be the most dangerous of all fuel types because they are already in the physical state required for ignition. When wood burns inefficiently, the combustion products may contain methane, acetylene and other fuel gases. Vapor density describes the density of gases in relation to air. Air has a vapor density of 1. Gases with a vapor density of less than 1, such as methane, will rise while those having a vapor density of greater than 1, such as propane, will sink. Vapor densities are based upon the assumption that the density is measured at standard temperature and pressure. Heated gases expand and become less dense; when cooled they contract and become more dense. Liquids Liquids have mass and volume but no definite shape except for a flat surface or the shape of their container. Unlike gases, liquids will not expand to fill all of a container. When released on the ground, liquids will flow downhill and pool in low areas. Just as gas density is compared to air, liquid density is compared to water. Specific gravity is the ratio of the mass of a given volume of a liquid compared to the mass of an equal volume of water at the same temperature. Water is assigned a specific gravity of 1. Liquids with a specific gravity less than 1, such as gasoline and most other flammable liquids, are lighter than water and will float on its surface. Liquids with a specific gravity greater than 1, such as corn syrup, are heavier than water and will sink. To burn, liquids must vaporize. Vaporization is the transformation of a liquid to vapor or a gaseous state. Unlike solids, liquids retain their state of matter partly due to standard atmospheric pressure. For vaporization to occur, the escaping vapors must be at a greater pressure than atmospheric pressure. The pressure that vapors escaping from a liquid exert is known as vapor pressure. Vapor pressure indicates how easily a substance will evaporate into air. Flammable liquids with a high vapor pressure present a special hazard to firefighters.  The vapor pressure of the substance and the amount of thermal energy applied to it determines the rate of vaporization. For example, a puddle of water eventually evaporates because of slow heat transfer from the sun. When the same amount of water is heated on a stove, however, it vaporizes much more rapidly because there is more thermal energy applied. The volatility or ease with which a liquid gives off vapor influences how easily it can ignite. The size of a liquid's surface area also influences the extent to which the liquid will give off vapor. In many open containers, the surface area of liquid exposed to the atmosphere is limited. Flash point is the minimum temperature at which a liquid gives off sufficient vapors to ignite, but not sustain combustion, in the presence of a piloted ignition source. Fire point is the temperature at which a piloted ignition of sufficient vapors will begin a sustained combustion reaction. Flash point is commonly used to indicate the flammability hazard of liquid fuels. Liquid fuels that vaporize sufficiently to burn at temperatures under 100°F present a significant flammability hazard. Firefighters must know how liquid fuels react with water. Solubility describes the extent to which a substance (in this case a liquid) will mix with water. Solubility may be expressed in qualitative terms (slightly or completely) or as a percentage (20 percent soluble). Materials that are miscible in water will mix in any proportion. Some liquids are lighter than water and do not mix with it, such as hydrocarbon fuels (gasoline, diesel, and fuel oil). Flammable liquids called polar solvents such as alcohols (methanol, ethanol) will mix readily with water. Liquids that are less dense (lighter) than water are more difficult to extinguish using water as the sole extinguishing agent. Because the liquid fuel is less dense and will not mix with water, adding water to the liquid fuel may disperse the burning liquid instead of extinguishing it, which could potentially spread the fire to other areas. Firefighters should use the appropriate foam or chemical agent to extinguish liquid fuels that are not water-soluble. Water-soluble liquids will mix with some water-based extinguishing agents, such as many types of fire fighting foam. The extinguishing agent will mix with the burning liquid and become much less effective at extinguishing the fire. To avoid this mixture, firefighters should use alcohol-resistant fire fighting foams specifically designed for polar solvents. Solids Solids have definite size and shape. Different solids react differently when exposed to heat. Some solids such as wax and metals will change their state and melt, while others such as wood and plastics will not. When solid fuels are heated, they begin to pyrolize (off-gas) and release fuel gases and vapors The solid fuels begin to decompose and emit combustible vapors. If there is enough fuel and heat, the process of pyrolysis generates sufficient flammable vapors to ignite in the presence of sufficient oxygen or another oxidizer. When wood first heats, it begins to pyrolize and decompose into its volatile components and carbon. These vapors are usually white in color. Pyrolysis of wood begins at temperatures below 400°F, which is lower than the temperature required for ignition of the released vapors. Home construction still uses wood based prod. ucts for the framing, subfloor and roof decking. However the insulation, exterior and interior finish materials, and the contents are likely to be or contain synthetic materials, like polyvinyl chloride, polyethylene, polystyrene, polypropylene, and polyurethane. Today, flexible polyurethane foam is one of the most common materials used in upholstered furniture. Solid fuels have a definite shape and size which significantly affects how easily they ignite. The primary consideration is the surface area of the fuel in proportion to its mass, called the surface-to-mass ratio. One of the best examples is that of a large tree:   To produce lumber, the tree must be felled and cut into a log. The surface area of this log is low compared to its mass; therefore, the surface-to-mass ratio is low.   The log is then cut into planks. This reduces the mass of the individual planks compared to the log. The resulting surface area increases, thus increasing the surface-to-mass ratio.   The chips and sawdust produced as the planks are cut into boards have an even higher surface-to-mass ratio.   If the boards are milled or sanded, the shavings or sawdust have the highest surface-to-mass ratio of any of the examples. As this ratio increases, the fuel particles become more finely divided, like shavings or sawdust. Therefore, the particles' ability to ignite increases tremendously. As the surface area increases, more of the material is exposed to the heat and generates combustible pyrolysis products more quickly. The proximity and orientation of a solid fuel relative to the source of heat also affects the way the fuel burns. For example, if you ignite one corner of a sheet of ⅛-inch plywood paneling that is lying horizontally (flat), the fire will consume the fuel at a relatively slow rate. The same type of paneling in a vertical position (standing on edge) burns much more rapidly because the heated vapors rise over more surface area and transfer more heat to the paneling. Oxygen Oxygen in the air is the primary oxidizing agent in most fires. Normally, air consists of about 21 percent oxygen. The energy release in fire is directly proportional to the amount of oxygen available for combustion. When a fire ignites in an open area where air is plentiful, the fire will release energy based on the given surface area. In con-trast, when a fire ignites within a compartment with limited air supply the fire can only react with oxygen from the compartment's air and any additional oxygen supplied through openings. Thus, in most compartment fires, the energy released is proportional to the limited amount of oxygen available, not the amount of fuel available to burn.  At normal ambient temperatures (68°F), materials can ignite and burn at oxygen concentrations as low as 15 percent. When oxygen concentration is limited, the flaming combustion will diminish, causing combustion to continue in the nonflaming mode. Nonflaming or smoldering combustion can continue at extremely low oxygen concentrations even when the surrounding environment's temperature is relatively low. However, at high ambient temperatures, flaming combustion may continue at considerably lower oxygen concentrations. When the oxygen concentration is higher than normal, materials exhibit different burning characteristics. Materials that burn at normal oxygen levels will burn more intensely and may ignite more readily in oxygen-enriched atmospheres. Some petroleum-based materials will autoignite in oxygen enriched atmospheres. Many materials that do not burn at normal oxygen levels will burn in oxygen-enriched atmospheres. One such material is Nomex® fire-resistant fabric, which is used in many types of protective clothing. At normal oxygen levels, Nomex® does not burn. When placed in an oxygen-enriched atmosphere of approximately 31 percent oxygen, Nomex® ignites and burns vigorously. Fires in oxygen-enriched atmospheres are more difficult to extinguish and present a potential safety hazard. Firefighters may find these conditions in hospitals and other healthcare facilities, some industrial occupancies, and even private homes where occupants use breathing equipment containing pure oxygen. For combustion to occur after a fuel converts into a gaseous state, the fuel must be mixed with air (an oxidizer) in the proper ratio. The range of concentrations of the fuel vapor and air is called the flammable (explosive) range. The fuel's flammable range is reported using the percent by volume of gas or vapor in air for the lower explosive (flammable) limit (LEL) and for the upper explosive (flammable) limit (UEL). The LEL is the minimum concentration of fuel vapor and air that supports combustion. Concentrations below the LEL are said to be too lean to burn. The UEL is the concentration above which combustion cannot take place. Concentrations above the VEL are said to be too rich to burn. Within the flammable range, there is an ideal concentration at which there is exactly the correct amount of fuel and oxygen required for combustion. Self-Sustained Chemical Reaction The self-sustained chemical reaction involved in flaming combustion is complex. As flaming combustion occurs, the molecules of a fuel gas and oxygen (0) break apart to form free radicals (electrically charged, highly reactive parts of molecules). Free radicals combine with oxygen or with the elements released from the fuel gas to form new substances (molecules) and even more free radicals. The process also increases the speed of the oxidation. The combustion of a simple fuel such as methane and oxygen provides a good example. Complete oxidation of methane releases the elements needed to create carbon dioxide and water as well as release energy in the form of heat and light. The elements released when methane molecules break down (carbon and hydrogen) recombine with oxygen in the air to form CO, and H, (carbon dioxide and water) At various points in the combustion of methane, this process results in production of carbon monoxide and formaldehyde, which are both flammable and toxic. When more chemically complex fuels burn, their combustion creates different types of free radicals and intermediate combustion products, many of which are also flammable and toxic. Flaming combustion is one example of a chemical chain reaction. Sufficient heat will cause fuel and oxygen to form free radicals and initiate the self sustained chemical reaction. The fire will continue to burn until it consumes the fuel or oxygen or an extinguishing agent, applied in sufficient quantity, interferes with the ongoing reaction. Chemical flame inhibition occurs when an extinguishing agent, such as dry chemical or Halon-replacement agent, interferes with this chemical reaction, forms a stable product, and terminates the combustion reaction. Compartment Fire Development Typically, when we think about a fire, we tend to limit our perspective to the burning fuel itself. However, in the sections that follow, we will see that the compartment surrounding that burning fuel has a significant impact on the available ventilation, access to additional fuel, and heat losses or gains. Compartment fire development depends upon whether the fire is fuel-limited or ventilation-limited. When sufficient oxygen is available for flaming combustion, the fire is said to be fuel-limited. Under fuel-limited condi-tions, the fuel's characteristics such as heat release rate and configuration control fire development. As long as the fire can reach more ignitable fuel, it will continue to burn. Conversely, ventilation-limited fires have access to all of the fuel needed to maintain combustion. However, the fire does not have access to enough oxygen to continue to burn or to spread to all available fuels. All compartment fires begin in the incipient stage as fuel-limited fires. Once the fire reaches the growth stage, the fire will either remain fuel-limited, if there is enough oxygen to support continued growth, or the fire will consume all available oxygen and become ventilation-limited. A fuel-limited fire will usually progress through the stages of fire development in order. Ventilation-limited fires tend to enter an early state of decay at the end of the growth stage because there is no longer enough available oxygen for the fire to become fully developed. This section will define the stages of fire development and then describe the progression of a fire in a compartment. The examples in the information boxes describe fire behavior in a room with one exterior window, an exterior doorway, and typical modern furnishings found in a residential living room. Stages of Fire Development Fires develop through four stages: incipient, growth, fully developed, and decay. These stages can occur with any fire; however, there are three key factors that control how the fire develops: the fuel properties, the ventilation available, and heat conservation. Depending on these factors, the fire development stages exhibit different characteristics or may occur in a different sequence. The four stages of fire development can be generally defined as follows:   Incipient Stage — The incipient stage starts with ignition when the three elements of the fire triangle come together and the combustion process begins. At this point, the fire is small and confined to a small portion of the fuel first ignited.   Growth Stage - As the fire transitions from incipient to growth stage, more of the initial fuel package becomes involved and the production of heat and smoke increases. If there are other fuels close to the initial fuel package, radiant heat from the fire may begin to pyrolize nearby fuels which could spread the fire to new fuel packages. The fire may continue to grow to become fully developed or may enter an early state of decay depending upon available oxygen.   Fully Developed Stage - The fully developed stage occurs when all combustible materials in the compartment are burning at their peak heat release rate based on the available oxygen. The fire is consuming the maximum amount of oxygen that it can. If the fire is limited to one fuel package, the fully developed stage occurs when the entire fuel package is on fire and the fire has reached its peak heat release rate.   Decay Stage — As the fire consumes the available fuel or oxygen and the heat release rate begins to decline, the fire enters the decay stage. Fuel-limited fires may self-extinguish in this phase or reduce to smoldering fires. Ventilation-limited fires may also self-extinguish. However, if oxygen becomes available during the decay stage before complete extinguishment, these fires are likely to reenter the growth stage and rapidly become fully developed. Open burning or a free burn condition provides the most basic fire growth curve. Open burning is representative of a fuel-limited fire, such as a campfire, a pile of wood pallets, or a sofa in a large, open, empty warehouse. This fire is considered fuel controlled because a single item burning either outside or in a large, well-ventilated space means there is sufficient oxygen available to burn the fuel until it can no longer sustain combustion. As heat and fire gases are produced, they move away from the fuel and disperse throughout the environment remote from the burning fuel. The only limit or control on the heat release rate of a fire burning out in the open is the fuel itself. Incipient Stage Once ignition occurs and the combustion process begins, development in the incipient stage depends largely upon the characteristics and configuration of the fuel involved (fuel-limited fire). Air in the compartment provides adequate oxygen to continue fire development. The following describe what occurs when a compartment fire enters the incipient stage:   Radiant heat warms the adjacent fuel and continues the process of pyrolysis. A thin plume of hot gases and flame rises from the fire and mixes with the cooler air in the compartment.   The hot gases in the plume rise until they encounter the ceiling and then begin to spread horizontally. This flow of fire gases is called the ceiling jet.   Hot gases in contact with the surfaces of the compartment and its contents transfer heat to other materials. In this early stage of fire development, the fire has not yet influenced the environment within the compartment to a significant extent. The temperature, while increasing, is only slightly above ambient in areas that the fire, plume, and ceiling jet directly affect. During the incipient stage, occupants can safely escape from the compartment, and a portable extinguisher or small hoseline can safely extinguish the fire. The transition from incipient to growth stage can occur quickly (in some cases in seconds, depending on the type and configuration of fuel involved A visual indicator that a fire is leaving the incipient stage is flame height. When flames reaction fuel in mondie y, radiated heat begins to transfer more heat than convection. The fire will then enter the growth stage. Growth Stage Within the growth stage, a variety of fire behaviors can occur, depending upon the number of ventilation sources. The fire may consume all of its available oxygen and enter a ventilation-limited state of decay or ventilation may provide enough oxygen for rapid fire development and/or growth to full development. Rapid fire development usually occurs during the growth stage. Understanding fire dynamics is largely an understanding of everything that can happen during the growth stage. As the fire transitions from incipient to growth stage, it begins to influence more of the compartment's environment and has grown large enough for the compartment configuration and amount of ventilation to influence it. The first effect is the amount of air that is entrained into the fire. Unconfined fires draw air from all sides and the entrainment (drawing in) of air cools the plume of hot gases, reducing flame length and vertical extension. In a compartment fire, the location of the fuel package in relation to the compartment walls affects the amount of air that is entrained and thus the amount of cooling that takes place. The following tenets describe entrainment based on the positioning of fuel packages. Fires in fuel packages in the middle of the room can entrain air from all sides.   Fires in fuel packages near walls can only entrain air from three sides.   Fires in fuel packages in corners can only entrain air from two sides. Therefore, when the fuel package is not in the middle of the room, the combustion zone (the area where sufficient air is available to feed the fire) expands vertically and a higher plume results. A higher plume increases the temperatures in the developing hot gas layer at ceiling level and increases the speed of fire development. In addition, heated surfaces around the fire radiate heat back toward the burning fuel which further increases the speed of fire development. A fire is said to be in the growth stage until the fire's heat release rate has reached its peak, either because of a lack of fuel or a lack of oxygen. In other words, when a fire cannot grow without the introduction of a new fuel source or a new oxygen source, it has left the growth stage and become fully developed. Two common routes to full development are as follows:   Fires that consume all available oxygen and transition to a state of ventilation-limited decay.   Fires that have enough oxygen and move through the growth phase and possibly into rapid fire development. Thermal Layering Once the ceiling jet reaches the walls of the fire compartment, the hot gas layer begins to develop. Thermal layer. ing is the tendency of gases to form into layers according to temperature, gas density, and pressure. Provided that there is no mechanical mixing from a fan or a hose stream, the hottest gases will form the highest layer, while the cooler gases will form the lower layers (Figure 4.34). In addition to the effects of heat transfer through radiation and convection described earlier, radiation from the hot gas layer also acts to heat the interior surfaces of the compartment and its contents. Changes in ventilation and flow path can significantly alter thermal layering. The flow path is defined as the space between the air intake and the exhaust outlet. Multiple openings (intakes and exhausts) create multiple flow paths. The products of combustion from the fire begin to affect the environment within the compartment. As the fire continues to grow, the hot gas layer within the fire compartment gains mass and energy. As the mass and energy of the hot gas layer increases, so does the pressure. Higher pressure causes the hot gas layer to spread downward within the compartment and laterally through any openings such as doors or windows. If there are no openings for lateral movement, the higher pressure gases have no lateral path to follow to an area of lower pressure. As a result, the hot gases will begin to fill the compartment starting at the ceiling and filling down. Isolated or intermittent flames may move through the hot gas layer. Combustion of these hot gases indicates that portions of the hot gas layer are within their flammable range, and that there is sufficient heat to cause ignition. As these hot gases circulate to the outer edges of the plume or the lower edges of the hot gas layer, they find sufficient oxygen to ignite. This phenomenon frequently occurs before more substantial involvement of flammable products of combustion in the hot gas layer. The appearance of isolated flames is sometimes an immediate indicator of flashover. The interface between the hot gas layers and cooler layer of air is commonly referred to as the neutral plane because the net pressure is zero, or neutral, where the layers meet. The neutral plane exists at openings where hot gases exit and cooler air enters the compartment. At these openings, hot gases at higher than ambient pressure exit through the top of the opening above the neutral plane. Lower pressure air from outside the compartment entrains into the opening below the neutral plane.  Transition to Ventilation-Limited Decay Most residential fires that develop beyond the incipient stage become ventilation-limited. Even when doors and windows are open, insufficient air entrainment may prohibit the fire from developing based on the available fuel. When windows are intact and doors are closed, the fire may move into a ventilation-limited state of decay even more quickly. While a closed compartment reduces the heat release rate, fuel may continue to pyrolize, creating fuel-rich smoke. As the interface height of the hot gas layer descends toward the floor, the greater volume of smoke begins to interrupt the entrainment of fresh air and oxygen to the seat of the fire and into the plume. This interruption causes the fire within the compartment to burn less efficiently. As the efficiency of combustion decreases (incomplete combustion), the heat release rate decreases and the amount of unburned fuel within the hot gas layer increases. The fire is now in a state of ventilation-limited decay because:   There is not enough oxygen to maintain combustion.   The heat release rate has decreased to the point that fuel gases will not ignite. Although the heat release rate decreases when a fire is ventilation-limited, the temperature in the room may remain high. Because there is not enough oxygen to maintain combustion, the fire has a lower heat release rate, but that does not mean that the environment is tenable. The compartment fills with fuel-rich gases that only need more oxygen to ignite because of the higher temperatures in the compartment. Even if temperatures decrease, pyrolysis can continue. Under these conditions, a large volume of flammable products of combustion can accumulate within the compartment. These gases are fuel that can ignite, given a new source of oxygen. If no other source of oxygen exists, the compartment will fill with black smoke and slowly cooling fuel gases. The compartment will show no visible flames. The characteristics of the fuel and fuel load in today's typical fires will cause fires to quickly become ventilation-limited. In order for a ventilation-limited fire to grow, it needs a new supply of oxygen. Ventilation introduces outside air to the fire as this new source of oxygen. If windows or doors fail, the sudden introduction of fresh air creates a rapid increase in the heat release rate and growth of the fire. This rapid increase can also occur when firefighters open a door or window to enter the compartment for extinguishment, which creates a new flow path. The pressure outside the compartment is lower than the pressure inside the compartment. Because of these pressure differences, any ventilation to the outside - opening an interior or exterior door, or breaking or opening a window - provides a flow path along which the hot gases can now move from the high pressure area inside to the low pressure area outside. Rapid Fire Development Rapid fire development refers to the rapid transition from the growth stage or early decay stage to a ventilation-limited, fully developed stage. Among these events are flashover and backdraft. Rapid fire development has been responsible for numerous firefighter deaths and injuries. To protect yourself and your crew, you must be able to:   Recognize the indicators of rapid fire development   Know the conditions created by each of these situations   Determine the best action to take before they occur In this section, rapid fire development conditions are described along with their indicators. Flashover. Rapid transition from the growth stage to the fully developed stage is known as flashover. When flashover occurs, the combustible materials and fuel gases in the compartment ignite almost simultaneously; the result is full-room fire involvement. Flashover typically occurs during the fire's growth stage, but may occur during the fully developed stage as the result of a change in ventilation. Flashover conditions are defined in various ways; however, during flashover, the environment of the room changes from a two-layer condition (hot on top, cooler on the bottom) to a single, well mixed hot gas condition from floor to ceiling. The environment is untenable, even for fully protected firefighters. As flashover occurs, the gas temperatures in the room reach 1,100 °F or higher. A significant indicator of flashover is rollover. Rollover de scribes a condition where the unburned fire gases that have accumulated at the top of a compartment ignite and flames propagate through the hot gas layer or across the ceiling. Rollover may occur during the growth stage as the hor gas layer forms at the ceiling of the compartment. Flames may appear in the layer when the combustible gases reach their ignition temperature. While the flames add to the to. tal heat generated in the compartment, this condition is not flashover. Rollover will generally precede flashover, but it may not always result in flashover. Rollover contributes to flashover conditions because the burning gases at the upper levels of the room generate tremendous amounts of radiant heat which transfers to other fuels in the room. The new fuels begin pyrolysis and release the additional gases necessary for flashover. The transition period between preflashover fre conditions (growth stage/ventilation-limited decay) to postflashover (fully developed stage) can occur rapidly. Radiation from the compartment's upper layer heats the compartment's contents until they reach their ignition temperature simultaneously. When the upper layer ignites, the amount of radiation increases to levels which rapidly ignite contents in the room, even if they are remote from the fire. During flashover, the volume of burning gases can increase from approximately ¼ to ½ of the room's upper volume to fill the room's entire volume and extend out of any openings from the room. When flashover occurs, burning gases push out of compartment openings (such as a door to another room) at a substantial velocity. There are four common elements of flashover:   Transition in fire development - Flashover represents stage. a transition from the growth stage to the fully developed   Rapidity - Although it is not an instantaneous event, flashover happens rapidly, often in a matter of seconds, to spread fire completely throughout the compartment. Compartment - There must be an enclosed space such as a single room or enclosure. then shows rapid fire Pyrolysis of all exposed fuel surfaces - Fire gases from all of the combustible surfaces in the enclosed space ignite, combustion. provided that there is sufficient oxygen to support flaming Two interrelated factors determine whether a fire within a compartment will progress to flashover. First, there must be sufficient fuel and the heat release rate must be sufficient for flashover conditions to develop. For example, ignition of discarded paper in a small metal wastebasket may not have sufficient heat to develop flashover conditions in a large room lined with gypsum drywall. On the other hand, ignition of a sofa with polyurethane foam cushions placed in the same room will likely result in fashtrer prodide-The second factor is ventilation. Regardless of the type, quantity, or configuration of fuel, heat release depends on oxygen. A developing fire must have sufficient oxygen to reach flashover, an amount that a sealed room mas not provide. The available air supply limits the heat release. If there is insuficent natural ventilation, the fire may enter the growth stage but not reach the heat release rate or gaseous fuel production to transition through flashover to a fully involved fire. Survival rates for firefighters are extremely low in a flashover. At the floor level, a heat flux of approximately 20 kW/m is also typical of rollover conditions at the start of the flashover. Once flames begin to affect a surface, the heat flux could range from 60 to 200 kW/m'. For frame of reference on heat flux, consider that NIST testing conducted in 2013 (Purtoti, 2013) has shown that SCBA face pieces begin to fail after 5 minutes of exposure to a heat flux of 15 kW/m'. You must be aware of the following flashover indicators to protect yourself: Building indicators - Interior configuration, fuel load, thermal properties, and ventilation Smoke indicators - Rapidly increasing volume, turbulence, darkening color, optical density, and lowering of the hot gas layer and/or neutral plane Heat indicators - Rapidly increasing temperature in the compartment, pyrolysis of contents or fuel packages located some distance away from the fire, or hot surfaces Flame indicators - Isolated flames or rollover in the hot gas layers or near the ceiling Levels of the neutral plane observed from the exterior of the structure are also good indicators of fire behavior within the structure as follows: High neutral plane - May indicate that the fire is in the early stages of development. Remember that high ceilings can hide a fire that has reached a later development stage. A high neutral plane can also indicate a fire above your level. Mid-level neutral plane - Could indicate that the compartment has not yet ventilated or that flashover is approaching. Very low-level neutral plane - May indicate that the fire is reaching backdraft conditions. This occurrence could also mean that a fire is below you (basement fire or lower story). When a fire is in ventlation limited decay, the introduction of new oxygen can trigger flashover quickly. able a roweh. over quickly. Flashen a fre is in ventilation limted decay then ventilation are available for fire shoulda lowever, in an over mayled situation, it may be difficult to identify what stage a fire is in, so firefighters should assume that fash. over may occur at any time that the conditions are right. Flashover may not occur in every compartment fire, such as in large-area compartments of compartments with high ceilings. Fire development may take an alternative path in a compartment that quickly becomes ventilation. limited, before the thermal energy can build within the compartment. The fire may not progress to flashover but instead become ventilation-limited, limiting heat release rate and causing the fire to enter the decay stage while continuing the process of pyrolysis and increasing the fuel content of the smoke. Backdraft.  A ventilation-limited compartment fire can produce a large volume of flammable smoke and other gases due to incomplete combustion. While the heat release rate from a ventilation-limited fire decreases, elevated temperatures may still be present within the compartment. An increase in ventilation such as opening a door or window can result in an explosively rapid combustion of the flammable gases, called a backdraft. Backdraft occurs in a space containing a high concentration of heated flammable gases that lack sufficient oxygen for flaming combustion. When potential backdraft conditions exist in a compartment, the introduction of a new source of oxygen will return the fire to a fully involved state rapidly (often explosively). A backdraft can occur with the creation of a horizontal or vertical opening. All that is required is the mixing of hot, fuel-rich smoke with air. Backdraft conditions can develop within a room, a void space, or an entire building. Anytime a compartment or space contains hot combustion products, firefighters must consider potential for backdraft before creating any openings into the compartment. Backdraft indicators include: Building indicators - Interior configuration, fuel load, thermal properties, amount of trapped fuel gases, and ventilation Smoke indicators - Pulsing smoke movement around small openings in the building; smoke-stained windows Air flow indicators - High velocity air intake Heat indicators - High heat, crackling or breaking sounds Flame indicators - Little or no visible flame The effects of a backdraft can vary considerably depending on a number of factors, including: Volume of smoke Degree of confinement Temperature of the environment Pressure Speed with which fuel and air mix Do not assume that a backdraft will always occur immediately after an opening is made into the building or involved compartment. You must watch the smoke for indicators of potential rapid fire development including the air currents changing direction, or smoke rushing in or out. To some degree, the violence of a backdraft depends upon the extent to which the fuel/air mixture is confined in the compartment. The more confined, the more violent the backdraft will be. Fully Developed Stage The fully developed stage occurs when the heat release rate of the fire has reached its peak, because of a lack of either fuel or oxygen. There are two main types of fully developed fires: ventilation-limited and fuel-limited fires. The factor limiting the peak heat release rate is used to identify which type of fully developed fire exists. Firefighters often misinterpret the term "fully developed" to mean that the fire can no longer grow. A more accurate description would be that the fire has grown as much as it can. New sources of fuel introduced after full development will allow fuel-limited fires to grow. Likewise, new sources of oxygen introduced after full development will allow ventilation-limited fires to grow. Fuel-Limited Conditions The available fuel limits the peak heat release in a fuel-limited, fully developed fire. The most effective method of increasing the heat release rate is to provide more fuel. A campfire located in a fire ring is a good example of fuel-limited conditions. The fire reaches its peak when all the fuel becomes involved. The fire ring separates the burning fuel from other potential fuel resulting in a fuel-limited, fully developerate. Adding additional fuel or frewood would increase the energy release of the fire to a new peak heat release rate. Technically speaking, most compartment fires, even those that are ventilated and have untenable interior environments, are ventilation limited. Adding ventilation points to a compartment fire that is already ventilated will add oxygen that will allow the fire to grow. Fuel-limited full development usually occurs when fires are not contained within compartments such as wildland fires, vehicular fires, or fires burning in collapsed structures. Ventilation-Limited Conditions In contrast, a fully developed, ventilation-limited fire lacks the oxygen available to grow because the number and size of openings in the compartment limit the entrainment of air. The fire reaches a peak when it consumes all the available oxygen from the air intake, typically with incomplete combustion. Additional fuel is available and gaseous fuel is leaving the compartment in the smoke; however, the fire cannot release any more energy. Allowing additional air into the compartment via an additional opening or enlarging the existing opening will provide more oxygen, resulting in a higher peak heat release rate. Ventilation-limited, fully developed fires present a hazardous situation to firefighters. The potential for a window failure to provide fresh oxygen and increase the peak heat release rate can endanger both firefighters and potential victims. To reduce the risk of the unpredictable window failure, firefighters must transition the fire from ventilation-limited to fuel-limited. With the high heat of combustion found in modern furnishings, the only mechanism to transition the fire is to extinguish some of the burning fuel. It is not possible to make enough openings in a compartment to transition a fire from ventilation-limited to fuel-limited conditions. Decay Stage A fire is said to be in the decay stage when it runs out of either available fuel or available oxygen. Either fuel or oxygen is an integral part of the fire triangle introduced earlier in the chapter. Without all three components of the triangle, the fire will decay and extinguish. In fuel-limited fires, the decay stage is usually the fire's final stage, leading to the fire's self-extinguishment when it runs out of available fuel. Ventilation-limited fires can also self-extinguish due to lack of oxygen. Both of these situations can result in the termination of the combustion reaction. However, just like throwing another log on top of a smoldering campfire, introducing new oxygen to a ventilation-limited fire can cause it to reenter the growth stage. Fuel-Limited Decay After a fuel-limited fire reaches the fully developed stage the fire will decay as the fuel is consumed. As the fire consumes the available fuel and the heat release rate begins to decline, the fire enters the decay stage. The heat release rate will decrease, but the temperature of surrounding objects may remain high for some time due to absorbed heat. Compartment fires rarely enter a state of fuel-limited decay unless the compartment burns all the way to the ground. If the compartment fails and the fire opens to the atmosphere, then the amount of fuel available would limit the fire's ability to grow. Ventilation-Limited Decay When a fire enters a ventilation-limited state of decay, this stage is not necessarily the last stage of the fire's devel-opment. As stated earlier, the fire awaits a new supply of oxygen to return to the growth stage. This statement is true even if compartment ventilation has already occurred. To ensure that the decay stage of a ventilation-limited fire is the fire's final stage, a controlled transition from ventilation-limited to fuel-limited must take place. To provide this control, firefighters must cool the hot fire gases before any further ventilation occurs or immediately following any forcible entry. This tactic will lessen the likelihood of the gases igniting when supplied fresh oxygen. If the compartment has no ventilation openings, the heat release rate will eventually decrease to the point that the heat in the compartment naturally transfers through the compartment itself to the outside. This process takes time and is rarely a viable fire fighting strategy because firefighters must ensure that no ventilation occurs until the compartment transfers all of the heat. Structure Fire Development Structures are essentially composed of individual compartments connected by hallways, stairways, or openings such as doorways. If a fire starts within one of the compartments, how it grows or decays is based on the model growth curves presented in previous sections. However, in a structure, the fire has the potential to involve more than one compartment or could spread beyond the contents of a compartment and involve the structural members of the building itself. Fighting a fire in a structure, as opposed to a stand-alone single compartment, is challenging because firefighters will need to size up the building, find the fire, and then find a way to attack the fire. Flow Path In a structure fire, the method by which the fire receives the needed oxygen to sustain the combustion reaction occurs through one or many flow paths. The flow path is composed of two regions: the ambient air flow in and the hot exhaust flow out. The flow is always unidirectional due to pressure differences where the ambient air flows toward the seat of the fire and reacts with the fuel. The products of combustion flow away from the fire toward the low pressure outlet. In a structure fire, the floor plan and openings within the structure determine the available flow path. For ex-ample, hot gases from a fire in a bedroom will travel out of the doorway and into the hallway if the door is open. If other doors in the structure are also open, the adjoining rooms also become possible parts of the flow path. The pressure in these other rooms is lower than the pressure in the fire room; therefore, hot fire gases and smoke will travel toward those areas unless the direction of flow is altered, for example, through tactical ventilation or door control. Air in those rooms will entrain toward the fire as the structure fills with fuel gases and the fire grows and spreads. A flow path's effectiveness to transport ambient air to the seat of the fire is based on the following: Size of the ventilation opening Length of the path traveled Number of obstructions Elevation differences between the base of the fire and the opening When firefighters advance a hoseline or ventilate windows to make entry into a building, they establish new flow paths between the fire compartment and exterior vents of the building. These new flow paths may allow air and thus oxygen to reach the fire, increasing the heat release rate. In addition, hot, fuel-rich fire gases may flow toward a vent opened to the building's exterior because the hot gases are at a higher pressure than the lower pressure air outside the vent. When the hot gases mix with the outside oxygen, they may be hot enough to autoignite. Since any ventilation creates new flow paths for oxygen and hot gases, firefighters must use tactics that control the oxygen available to the fire and the fire's generated heat to prevent unwanted fire spread. When hot gases follow the flow path from areas of high to low pressure, they convect heat to a larger portion of the structure. They also carry the products of combustion into new areas of the structure. Since these gases are also fuel, fire can propagate through them, out of the fire room. As a result, firefighters should know the location of the flow path for these gases in the structure and coordinate their ventilation and interior activities accordingly. Firefighters working in the exhaust portion of the flow path will feel the increase in temperature as the velocity and/or turbulence increases, causing increased convective heat transfer. Convective heat transfer is a similar phenomenon to wind chill, except energy is transferred from a hot fluid (gas) to a solid surface (your PPE) rather than from a hot surface (your skin) to a cooler fluid (air). If ventilation is not well coordinated, this heat transfer - such as that associated with flashover or backdraft - can be unsurvivable even when wearing PPE. If you must perform operations in the flow path, recognize that these operations are risky and potentially life threatening. The time that firefighters are operating in the flow path should be strictly limited. They should not be in the area any longer than necessary. A structure fire that extends beyond the room of origin may have two compartments involved, each in different stages of development. The room of origin may be in a fully developed, ventilation-limited stage while the adjacent compartment may be in the growth stage and nearing flashover. Understanding the model growth curves and what conditions to expect based on fire dynamics will aid firefighters in finding or establishing a tenable environment for their interior operations. Tactics employed for fire suppression, ventilation, and search and rescue will directly relate to the fire dynamics occurring on a given incident. Ventilation and Wind Considerations Beginning an attack on a ventilation-limited structure fire with ventilation alone will progressively increase the fire's heat release rate and spread as additional vents are made. Once the fire has filled the structure's compartments with hot, unburned, gaseous fuel, using ventilation as the only tactic will not enable you to get ahead of the fire and limit fire growth and spread.  Unplanned Ventilation  Unplanned ventilation is when a structural member fails - usually because of exposure to heat and introduces a new source of oxygen for the fire. This new oxygen source can result from failure of: Window  Roof Doorway Wall The source of new oxygen does not have to originate from outside the building. When floors fail above basement fires, the interior air in the structure becomes a new oxygen source. Unplanned ventilation is often the result of: Occupant action Fire effects on the building (such as window glazing) Actions other than planned, systematic, and coordinated tactical ventilation Unplanned ventilation, by definition, is unexpected. When it occurs, situational awareness is essential to ensure your safety and that of other crew members. Wind Conditions The wind can increase the pressure inside the structure, drive smoke and flames into unburned portions of the structure and onto advancing firefighters, and/or upset tactical ventilation efforts. You must be aware of the wind direction and velocity and use it to your advantage to assist in tactical ventilation. Wind conditions can also create differences in pressure that can cause windows to fail. The exterior pressure on the upwind side of a structure will be higher than the pressure on the downwind side of the structure. As a result, the ambient air on the outside of the structure is constantly trying to move through the structure along the path from high to low pressure. If heat exposure weakens windows in this path, wind pressure could cause them to fail which introduces a new flow path for oxygen and hot fire gases. Smoke Explosions A smoke explosion occurs when a mixture of unburned fuel gases and oxygen comes in contact with an ignition source. When smoke travels away from the fire it can accumulate in other areas and mix with air. When the fuel and oxygen are within the flammable range and contact an ignition source, the result will be explosive, rapid combustion. Smoke explosions are violent because they involve premixed fuel and oxygen. Effects of Fire Fighting Operations Limiting or interrupting one or more of the essential elements in the combustion process depicted in the fire tetrahedron controls and extinguishes fire. Firefighters can influence fire dynamics in a number of ways: Temperature reduction - Using water or a foam agent to cool fire gases and hot surfaces for the purposes of extinguishment. Fuel removal - Eliminating sources of fuel in the path of the fire's spread that could provide a new source of fuel; typically a tactic in wildland fires or liquid and gas fires. Oxygen exclusion/flow path control - Using door control and tactical ventilation techniques to control the amount of air available to the fire. Chemical flame inhibition - Using extinguishing agents other than water and foam, such as some dry chemi-cals, halogenated agents (Halons), and Halon-replacement "clean" agents, to inhibit or interrupt the combustion reaction and stop flame production. NOTE: Tactics for these methods and further descriptions of their benefits are provided in Chapter 14, Fire Control. Reaction of Building Construction to Fire As buildings burn, the fire creates a variety of dangerous conditions. You must be aware of these conditions to remain safe during an emergency incident. An already serious situation can worsen if firefighters fail to recognize the potential of the situation and take the wrong actions. Two primary types of dangerous building conditions are: Conditions that contribute to the spread and intensity of the fire Conditions that make the building susceptible to collapse These two conditions are related; conditions that contribute to the spread and intensity of the fire will increase the likelihood of structural collapse. The following sections describe some of these conditions. Construction Type and Elapsed Time of Structural Integrity Most building codes rate the various construction types according to how long each construction type maintains its structural integrity over a certain period of time. Table 4.13, p. 166 shows some examples of the expected fire resistance of the five types of construction. How long a building will maintain structural integrity is not an exact science. Observations made at a fire scene must be used to reevaluate estimations based upon building construction type. When you respond to a fire, there are many unknown factors that are not reflected in fire resistance estimates, such as the following: The duration of the fire up to the time of arrival The building's contents Ways) the building contents affect the heat release rate The heat release rate and intensity of the fire Renovations to the interior that may have compromised fire resistance Fuel Load of Structural Members and Contents The total quantity of combustible contents of a building, space, or fire area is referred to as the fuel load (some documents may use the term fire load). All combustible materials in the building's construction comprise the fuel load, such as: Wood framing Floors Ceilings Furnishings Combustible materials within the building The more materials that are combustible, the more fuel is available to pyrolize and burn. Your knowledge of building construction and occupancy types will be essential to determining fuel loads. At a scene, you will only be able to estimate the fuel load based upon your knowledge and expe-rience. For example, a concrete block structure with a steel roof assembly containing stored steel pipe will have a much smaller fuel load than a wood-frame structure used for storing flammable liquids. In buildings where the construction materials are flammable, the materials themselves add to the structure's fuel load. For example, in wood-frame buildings, the structure itself is a source of fuel. The orientation of the fuels as well as their surface-to-mass ratio will also influence the rate and intensity of fire spread. The contents of a structure are often the most readily available fuel source, significantly influencing fire development in a compartment fire. When contents release a large amount of heat rapidly, both the intensity of the fire and speed of development will increase. For example, synthetic furnishings, such as polyurethane foam, begin to pyrolize rapidly under fire conditions even when the contents are located some distance from the fire's origin. The chemical makeup of the foam and its high surface-to-mass ratio speed the process of fire development (Figure 4.51). The proximity and continuity of contents and structural fuels also influence fire development. Fuels located in the upper level of adjacent compartments will pyrolize more quickly because of heat radiating from the hot gas layer. Continuous fuels such as combustible interior finishes will rapidly spread the fire through compartments. Similarly, the fire's location within the building will influence fire development. Fires originating on upper levels generally extend downward much more slowly following the fuel path or as a result of structural collapse. When the fire originates in a low level of the building, such as in the basement or on the first floor, convected heat currents will cause vertical extension through: Atriums Stairways Vertical shafts Concealed spaces Additionally, if the structural elements of the building become involved in the fire, not only does the structure itself provide a new source of fuel, but the fire may be burning in hidden cavities throughout the building. These hidden spaces make finding and extinguishing the fire more difficult and increase the potential risk of building collapse. In commercial, industrial, and storage facilities with large fuel loads, the fire can overwhelm the capabilities of a fire suppression system and make it difficult for firefighters to gain access during fire suppression operations (Figure 4.52). Performing and updating preincident surveys is the most effective means of establishing awareness of these hazards. Assuming that there is available oxygen, the higher the fuel load, the more likely the fire will behave in the following ways: If structural members are part of the fuel load, structural integrity of the building will deteriorate faster. The longer the fire burns, the more fire spread accelerates. The fire may have a higher heat release rate. The structure may self-ventilate, introducing even more oxygen to the fuel-limited fire and accelerating fire development and involvement of combustible structural members. If fires are ventilation-limited, higher fuel loads indicate a greater amount of unburned fuel that could reignite with the introduction of a new oxygen source. There may also be a greater amount of unburned fuel gases in the air because fuel packages pyrolized but did not begin combustion before the building became oxygen-limited. Such buildings are subject to backdrafts and flashovers if firefighters do not coordinate ventilation. Furnishings and Finishes In addition to structural members, combustible interior finishes and furnishings, can be a significant factor that influences fire spread and are a major factor in the loss of lives in fires. The interior finishes include the window, wall, and floor coverings such as drapes, wallpaper, and carpet. Furnishings may include: Tables Sofas Desks Beds Other items found in occupancies Combustible Exterior Wall Coverings Flammable material that contributes to the structure's fuel load often covers exterior walls. Exterior wall coverings may add carbon fuels (wooden siding) or petroleum fuels (vinyl siding) to the fuel load. The wall coverings may be installed atop exterior insulation which, in turn, is another fuel source. When exterior coverings become exposed to heat and catch fire, they can spread the fire to other areas of the structure or to adjacent exposures such as vegetation or neighboring buildings. Combustible Roof Materials The combustibility of a roof's surface is a basic concern to the fire safety of an entire community. Some of the earliest fire regulations imposed in North America related to combustible roof coverings because they were blamed for several conflagrations caused by flaming embers flying from roof to roof. Wood shakes, even when treated with fire retardant, can significantly contribute to fire spread. This is a problem in wildland/urban interface fires where wood shake roofs have contributed to large fires. Firefighters must use exposure protection tactics to protect combustible roofs on structures adjacent to a fire building or wildland fire. Fire-resistant metal roof decking may be covered with combustible layers of foam insulation and felt paper covered with asphalt waterproofing. A fire below the metal deck can melt and ignite combustible materials, causing a second fire above the roof. Building Compartmentation The arrangement of compartments in a building directly affects fire development, severity, possible duration, and intensity. Building compartmentation is the layout of the various open spaces in a structure and includes: Number of stories above or below ground Floor plan Openings between floors Continuous voids or concealed spaces Barriers to fire spread Each of these elements may contribute to either fire spread or containment. For instance, an open floor plan space may contain furnishings that provide fuel sources on all sides of a point of ignition. Conversely, a compart-mentalized configuration may have fire rated barriers, such as walls, ceilings, and doors, separating fuel sources and limiting fire development to an individual compartment. Any open space with no complete fire barrier dividing it is considered a compartment. Two rooms that a doorway connects are considered two compartments only if the door between them is closed. When the door is open, a fire in either room can access the oxygen from the adjoining room. The rooms will affect one another more slowly than if the intervening wall weren't there at all, but a closed door will slow the effect of a fire in one room on the adjoining room even further. Firefighters should use doors to their advantage during interior operations, closing doors whenever appropriate to control available oxygen sources. Given enough available fuel, fire will follow oxygen through a building along any available flow path. Firefighters can take advantage of compartmentation to control the flow path to create more predictable fire behavior during operations. Effects of Building Construction Features Building features such as lightweight materials or open floor plans have direct effects on how fire will spread in the structure. If you do not take building construction features into account, then fire fighting activities may worsen an emergency - possibly catastrophically - rather than help to extinguish the fire. Remember, a structure fire is the place where fire dynamics and building construction interact. The sections that follow highlight some of the construction features that firefighters should consider when fighting structure fires. Modern vs. Legacy Construction Over the past 50 years, building construction in North America has changed drastically. In single-family resi. de theast 50 years, bure to cage of houses increased over 150 percent between 1973 and 2008, At the same time lot sizes have shrunk, reducing firefighter access and increasing potential exposure risks (Kerber, "Analysis..." 2012). Residential interior layouts and construction materials have also changed. Older structures (prior to approxi. mately 1990) had the following features: Smaller compartments More compartments within the same square footage as modern homes Windows that could be opened for ventilation Air pockets in empty wall cavities; this construction technique used the air as insulation Modern single-family structures may feature: Open floor plans High ceilings Atriums Lightweight manufactured structural components Sealed windows Wall cavities Synthetic insulation Construction materials and interior finishes consisting of synthetic materials and light composite wood components add to the fuel load of the structure and contribute to the creation of toxic gases during a fire. Because of energy-efficient designs, the structures also tend to contain fires for a longer period of time, thus creating fuel-rich, ventilation-limited environments. These problems are magnified in large-area residential structures. Commercial, institutional, educational, and multifamily residential structures also rely on energy conservation measures that increase the intensity of a fire and make the use of tactical ventilation difficult. Open plan commercial structures, such as warehouse-style stores, have high fuel loads in the contents and no physical barriers to prevent the spread of fire and smoke in the space. The use of plastics and other synthetic materials has also dramatically increased the fuel load in all types of occupancies. These synt, teach hira produce large quantities of toxic and combustible gases. Fires in these fuel opes an escalate rapidly, reach high temperatures, and consume the structure's available oxygen quickly. Knowledge of the building involved is a great asset when firefighters make decisions concerning tactical vent-lation. This information can come from preincident plans, inspection reports, or observations of similar types of structures. Building characteristics to consider include the following: Occupancy classification Construction type Square footage and compartmentation Ceiling height Number of stories above and below ground level Number and size of exterior windows, doors, and other wall openings Number and location of staircases, elevator shafts, dumbwaiters, ducts, and roof openings External exposures Extent to which a building connects to adjoining structures Type and design of roof construction Type and location of fire protection systems Contents Heating, ventilation, and air conditioning (HVAC) system Compartment Volume and Ceiling height A fire in a large compartment will normally develop more slowly than one in a small compartment. Slower fire development is due to the greater volume of air and the increased distance radiated heat must travel from the fire to the contents that must be heated. However, a large volume of air will support the development of a larger fire before the lack of ventilation becomes the limiting factor. A high ceiling can also make determining the extent of fire development more difficult. In structures with high ceilings, a large volume of hot smoke and fire gases can accumulate at the ceiling level, while conditions at floor level remain relatively unchanged. Firefighters may mistake floor-level conditions for the actual state of fire development. If the large hot gas layer ignites, the situation becomes immediately hazardous. Large, open spaces in buildings contribute to the spread of fire throughout. Such spaces may be found in: Warehouses Large atriums Churches Large-area mercantile buildings Theaters Large spaces may also exist between roofs and ceilings and under rain roofs. In these concealed spaces, fire can travel undetected, feeding on combustible, exposed wood rafters. When smoke appears through openings in the roof or around the eaves, the exact point of origin may be deceiving. Thermal Properties of the Building The thermal properties of the building can contribute to rapid fire development. The thermal properties can also make exting roperties of the building reignition possible. Thermal properties of a building include: Insulation - Contains heat within the building which causes a localized inerease in the temperature and fire growth and may introduce an additional fuel source Heat reflectivity - Increases fire spread through the transfer of radiant heat from wall surfaces to adjacent Retention - Maintains temperature by slowly absorbing and releasing large amounts of heat Failure of Lightweight Trusses and Joists The increased use of engineered or lightweight construction and trussed support systems pose a danger to firefighters. Unprotected engineered steel and wooden trusses can fail after 5 to 10 minutes of exposure to fire (Kerber, et al, "Improving. 2012). These trusses can fail from exposure to heat alone without flame contact. For steel trusses, 1,000°F (538°C) is the critical temperature of steel - the temperature at which steel begins to weaken (SFPE 2016). Metal gusset plates in wooden trusses can fail quickly when exposed to heat. Although protective fire-retardant treatments can enhance the fire resistance of both steel and wooden trusses, most trusses lack this protection. The traditional wood-joist roof uses solid wood joists that tend to lose their strength gradually when exposed to fire. This loss of strength causes a roof to become soft or "spongy" before failure, especially with a wood plank roof deck. Although a soft or sagging roof is an obvious indication of structural failure, it should not be considered the only sign of imminent collapse. More modern homes may use engineered joists that burn more quickly and fail before the fire affects the roof decking, so the plywood or OSB used for roof sheathing may not show any signs of sagging during a fire. When the trusses fail first, entire pieces of the decking may fall into the fire. Until they fall, there maybe no indication that a frut ghettes langer of falling through from "sounding" the roof or even standing on the root. Observations about the fire's location, its behavior and activity, and the location of ornera easing on the key to establishing the safety of the roof. Visual observations of the roof's exterior may not be enough. Exterior, interior and roof crews should communicate their observations to one another throughout the incident to monitoring the safety of the roof. An arched or curved outline often indicates a bowstring truss roof. Before 1960, the bowstring truss roof design was one of the most common design types for large commercial and industrial structures. The bowstring truss roof was commonly used in facilities wherever large open floor