Fire Growth and Flashover: The Importance of Rapid Response to Residential Fires

The Stages of Flashover Defined:



The Smoldering Phase

The first stage of any fire is the smoldering stage.  When heat is applied to a combustible material, the heat oxidizes the material’s surface into combustible gases.  The oxidation process is exothermic, meaning that the oxidation process itself produces heat.  The heat from oxidation raises the temperature of surrounding materials, which increases the rate of oxidation and begins a chemical chain reaction of heat release and burning.  A fire can progress from the smoldering phase immediately or slowly, depending upon the fuel, nearby combustibles, and the availability of oxygen in the surrounding air.

 

 

The Free Burning Phase

The second stage of fire growth is the “free” or “open burning” stage.  When the temperature of a fire gets high enough, visible flames can be seen.  The visible burning at this stage is still limited to the immediate area of origin.  The combustible process continues to release more heat, which heats nearby objects to their ignition temperature, and they begin burning.  In a wildland fire the surrounding growth will ignite and the flames will spread, quickly if wind and dry growth are present.  A structure fire is different, because the gaseous products of combustion, most of which are flammable and lighter than air, rise and are contained in the upper levels of the structure.  When this occurs, the structure fire is at a critical point: either the fire has insufficient oxygen available to burn and it progresses back to the smoldering stage, or it has sufficient oxygen available to move on to the next stage.


“Fire Growth in a Confined Space”[1]

 

 

When an object in a room starts to burn, for some time after ignition, it burns in much the same way as it would in the open.  After a short period of time, however, confinement begins to influence fire development.  The smoke produced by the burning object rises to form a hot gas layer below the ceiling; this layer heats the ceiling and upper walls of the room.  Thermal radiation from the hot layer, ceiling, and upper walls begins to heat all objects in the lower part of the room and may augment both the rate of burning of the original object and the rate of flame spread over its surface.

 

 

At this point, the fire may go out if, for example, the first object burns completely before others start, or if sufficient oxygen cannot get into the room to keep the object burning.  Sometimes, however, the heating of the other combustibles in the room continues to the point where they reach their ignition temperatures more or less simultaneously.  If this occurs, flames suddenly sweep across the room, involving most combustibles in the fire.  This transition from the burning of one or two objects to full room involvement is referred to as “flashover.” [2]

 

Flashover

The third stage of fire growth is called flashover.  It is the most significant moment of any structure fire.  As combustible gases are produced by the two previous stages they are not wholly consumed.  They rise and form a superheated gas layer at the ceiling.  As the volume of this gas layer increases, it begins to bank down to the floor, heating all combustible objects regardless of their proximity to the burning object.  In a typical structure fire, the gas layer at the ceiling can quickly reach temperatures of 1,500 degrees Fahrenheit.  If there is enough existing oxygen, usually near floor level, flashover occurs and everything in the room breaks out into open flame at once.  The instantaneous eruption into flame generates a tremendous amount of heat, smoke, and pressure with enough force to push beyond the room of origin through doors and windows.  Usually at the time of flashover, windows in the room will break, allowing for the entry of fresh air.  The introduction of fresh air serves to further fuel the growth of the fire, increase the temperature of the fire, and aid in the spread of the fire beyond the room of origin.  The combustion process then speeds up because it has an even greater amount of heat to move to unburned objects.

 

 

Flashover is a critical stage of fire growth for two reasons.  First, no unprotected living thing in a room where flashover occurs will survive and the chance of saving lives drops dramatically.  Second, flashover creates a huge jump in the rate of combustion, and a significantly greater amount of water is needed to reduce the burning material below its ignition temperature.  A post-flashover fire burns hotter and moves faster, requires more resources for fire attack, and compounds the problems of search and rescue, exposure protection, and containment.[3]

 

 


“The Fire Propagation Curve”

 

The ability of adequate fire suppression forces to greatly influence the outcome of a structural fire is undeniable and predictable.  Data generated by the National Fire Protection Association provides empirical proof that rapid and aggressive interior attack can substantially reduce the human and property loss associated with structural fires.  At each stage of a fire’s extension beyond the room of origin, the rate of civilian deaths, injuries, and property damage grows exponentially.

 

 


“The Relationship between Fire Extension and Fire Loss” [4]


Rate Per 1,000 Fires

 

Fire Extension in Residential Structures:

Civilian

Deaths

Civilian

Injuries

Average Property

Damage

Confined to Room of Origin

2.07

24.30

$1,505.00

Confined to Floor of Origin

18.60

80.44

$12,134.00

Beyond Floor of Origin

27.23

55.37

$21,343.00

 

 

 

The Importance of a Rapid Response in Initiating Safe and Effective Fire Suppression and Rescue Operations:

Any delay in the initiation of fire suppression and rescue operations translates directly into a proportional increase in expected property, life, and economic losses (reference “The Relationship between Fire Extension and Fire Loss,” above).  It warrants emphasizing that if a structure has no automatic suppression or detection system, a more advanced fire may exist by the time the fire department is notified of the emergency and is able to respond.  Fires of an extended duration weaken structural members, compromising the structural integrity of a building and forcing operations to shift from an offensive to defensive mode.[5]  This mode will continue until enough resources can be amassed to then change to an aggressive, offensive attack.

 

 

 

Fire Growth and the Importance of a Rapid Response To a Fire in a High-Rise Structure:

 

As noted in a comprehensive study of adequate staffing and resources conducted by the Dallas Fire Department, one of the primary differences between a high-rise fire and those in other structures is the scale of the operation.[6]  Whereas a residential structure could be two stories and thirty feet in height and occupy 2,000 square feet, a high-rise building may be multiple stories, hundreds of feet high, and cover several thousand square feet.  Significantly affecting fire potential is the fire load, including office furniture, files and papers.  Many, if not most, floors can be expected to have a significant load of computer and electronic equipment, adding to the fire load.

 

 

 

Several additional factors complicate fire suppression and rescue operations at the scene of a high-rise fire.  Firefighters can be faced with an increased danger if the windows at the fire floor have vented, resulting in a “blow-torch” effect, and multiple victims of fire can be expected to become trapped or unaccounted for.  Effective fire suppression and rescue operations under such conditions hinge upon the availability and reliability of building elevators.  The Dallas Study illuminates the major issues associated with elevators in a high-rise fire as follows:

 

 

 

  1. There are a limited number of elevator cars and the cars have limited capacity.  Therefore, multiple trips must be made.  To control elevator car movement, a firefighter must be assigned to operate the car manually.  Elevator systems were never designed to operate in fire environments.  The products of combustion, heat, and water can disrupt the elevator programming and cause the cars to move erratically.  Inevitably, delays occur while waiting for, traveling in, loading, and unloading cars.[7]

     

  2. Due to elevator unreliability, firefighters are often required to use the stairs.  As previously mentioned, it is difficult to deliver firefighters and equipment to the upper floors due to falling glass and debris, a lack of water, difficulty in ventilating the structure, and heavy smoke in the stairwells in which firefighters are attempting to ascend while panicked occupants are attempting to descend.

     

  3. A high-rise fire also presents logistical difficulties similar to those experienced in commercial structures.  For example, when a firefighter depletes an air cylinder at the scene of a residential structure fire, it can be easily replaced at the incident scene, requiring little more than a return to the incident staging area where the cylinder can be easily and rapidly replaced.  Conversely, in a high-rise structure it is impractical to return to the street level from an upper floor of the building to obtain tools and equipment, such as air cylinders for self-contained breathing apparatus (SCBA) and fire hose.  Provision of sufficient personnel must be made to deliver these and other items to the locations in the building where they are needed.

     

  4. A final distinction between a residential fire and that in a high-rise building is the time frame of the operation.  As compared to a residential structure, “the relative inaccessibility of the high-rise building, the elevated location of the fire, the dependency on elevators, the larger size and number of potential fire areas, the greater exposure of occupants, the larger quantities of water required for control of the fire, and the more hostile fire environment all contribute to a more prolonged operation which cannot be attacked with the same speed.”[8]  Factors such as these require a greater number of firefighters to initiate safe and effective fire suppression and rescue operations.



[1] Image courtesy of University of California at Davis Fire Department

[2] J.R. Mehaffey, Ph.D., Flammability of Building Materials and Fire Growth, Institute for Research in Construction (1987)

[3] The University of California at Davis Fire Department website; site visited April 2, 2009.<

[4] Source: National Fire Protection Association

[5] According to the NFPA, “it’s important to realize that every 250 GPM stream applied to the building can add up to one ton per minute to the load the weakened structure is carrying.”

[6] McManis Associates and John T. O’Hagan & Associates, Dallas Fire Department Staffing Level Study, (June 1984), V-1.

[7] McManis Associates et al., V-1.

[8] McManis Associates et al., V-2.

Fire Growth and the Importance of a Rapid Response To a Fire in a Commercial (High Hazard) Structure

 

Fires in industrial and commercial areas pose unique and significant risks to firefighters operating on the fire ground, and are some of the most difficult fires to control.  Modern warehouses and storage occupancies are especially subject to rapidly developing fires of great intensity because complex configurations of storage are conducive to rapid fire spread, presenting numerous obstacles to fire suppression efforts.  Additionally, windows with iron shutters- or buildings with no windows at all- hamper a fire department’s efforts to gain access to the building.  If passageways are impassable, the fire can be reached only by streams operating through windows, and the opening of shutters may be a time-consuming operation.[1], [2]

 

The logistics of commercial fire fighting operations must not be underestimated.  Even under ideal conditions, successfully fighting a fire requires large numbers of personnel and supplies.  Physical demands on firefighters due to the building’s sheer size requires regular rotation of personnel out of the fire area for rest and rehabilitation.[3]

 

Other required supplies include air cylinders.  The Mansfield Fire Department's self-contained breathing apparatus (SCBA) have only a 30-minute rating and probably last only half that long during strenuous fire-fighting operations.  Firefighters who must walk 300 feet into the building to the actual fire area may only be able to spend 5 to 7 minutes fighting the fire before they must replenish their air supply.  Hence, pre-incident plans should contain provisions for assembling a large pool of trained personnel to assist in fire-fighting operations.[4]

 

It is in the “Organizing for Fire and Rescue Services” section of the Fire Protection Handbook that identifies initial attack response capabilities for low, medium, and high hazard occupancies[5].

Low-Hazard Occupancies

Defined as:

One-, two-, or three-family dwellings and scattered small businesses and industrial occupancies

 

Initial attack response capability:

At least two pumpers, 1 ladder truck (or combination apparatus with equivalent capabilities, 1 chief officer, and other specialized apparatus as may be needed or available; not fewer than 12 firefighters and 1 chief officer, plus a safety officer and a rapid intervention team.

Medium-Hazard Occupancies

Defined as:

Apartments, offices, mercantile and industrial occupancies not normally requiring extensive rescue or fire fighting forces

 

Initial attack response capability:

At least three pumpers, 1 ladder truck (or combination apparatus with equivalent capabilities), 1 chief officer, and other specialized apparatus as may be needed or available; not fewer than 16 firefighters and 1 chief officer, plus a safety officer and a rapid intervention team.

High-Hazard Occupancies

Defined as:

Schools, hospitals, nursing homes, explosives plants, refineries, high-rise buildings, and other high life hazard or large fire potential occupancies

 

Initial attack response capability:

At least 4 pumpers, 2 ladder trucks (or combination apparatus with equivalent capabilities), 2 chief officers, and other specialized apparatus as may be needed to cope with the combustible involved; not fewer than 24 firefighters and 2 chief officers.

 

Extra staffing of units first due to high-hazard occupancies is advised.  One or more safety officers and a rapid intervention team(s) are also necessary.

The Handbook also states, “If properties with considerable life hazard are involved (schools, hospitals, nursing homes, etc.) additional resources should be considered for initial alarms.  Especially large numbers of personnel are needed for search and rescue operation in these properties, with several firefighters needed to ‘sweep and search’ each floor.”


[1] Fire Chief’s Handbook, 4th ed., “Advanced Fire Fighting,” (Saddle Brook, N.J., 1987)498.

[2] National Fire Protection Association, Warehouse Operations, Fire Protection Handbook, 18th ed. (Quincy, MA: NFPA, 1997) § 9-110.

[3] Gary O. Tokle, Fire Protection Handbook: 19th Edition, ed. Arthur E. Cote, P.E. (Quincy, MA: NFPA, 2003), §13, Ch. 17, p. 191.

[4] Ibid.

[5] Gary O. Tokle, Fire Protection Handbook: 19th Edition, ed. Arthur E. Cote, P.E. (Quincy, MA: NFPA, 2003), §7, Ch. 2, p. 36. (Emphasis added).

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