Factors to Consider When Attempting to Determine POINT OF ORIGIN
A comparison of Figures 1.1 and 1.2 will conceptually depict the fire’s progression in an upward and outward path on a vertical surface through the addition of two new bricks on top of the first original brick.
F = Flame—Allows heat transfer through convection.
P = Plume—The area on the side of the flame.
C = Conducted heat
R = Radiated heat
V = “V” pattern—Created by the pyrolysis and ignition of the available fuel.
Finding a fire’s point of origin is similar to piecing together a jigsaw puzzle—the only thing is that when you arrive at the fire scene, you may or may not have all of the pieces.
The only way to discover what pieces are missing is through your ability to correctly, and logically, interpret those pieces that are present. By placing these pieces into their proper perspective, you might then be able to determine the successive events that occurred for the scene to have attained its present state of fracture.
With some exceptions, all fires, no matter how large, start out as a small spark.
The path of fire travel is upward and outward. This path is the direct result of:
- The transfer of heat by convection (because heated air and fire gases are lighter than the surrounding air and will rise).
- The transfer of heat by conduction (if the fuel is composed of one integral unit).
- The transfer of heat by radiation (a form of non-visible energy present in all fires).
- The pyrolitic role the heat plays on the fuel.
- The fire itself providing an ignition source for pyrolyzed vapors at ignition temperature.
The fire increases in size by oxidizing available fuel. Most of this fuel is usually above the fire, and the pattern of charring that it leaves in its wake is that of an inverted cone or a “V”.
This upward and outward path is like building a brick pyramid on its apex. The first “brick” is the initial “spark” of flame. The next two successive bricks ignite as a result of the first brick’s ability to heat the fuel above it, and so on, until there is no more fuel.
The fire’s intensity and rapidity depends on the source and type of fuel, the available oxygen, and an uninhibited chain reaction. The cause of the fire is whatever generated the initial spark. All of the above factors impact upon the investigator’s ability to determine the point of origin.
The first step in determining the point of fire origin is a visual examination of all exterior surfaces on the fire building. You must examine all sides and the roof. Of course, if the structure is attached or semi-attached, this job will be diminished.
Exterior bum patterns
At this point, the only conclusive results of this visual examination will be to determine if and at what location the fire vented the structure and what probable conditions existed both within and outside of the fire building.
Exterior burn patterns above and around vertical openings in a structure will vary depending on the duration and intensity of the fire conditions within the structure, and on fire suppression activities and wind velocity on the outside of the structure.
An equilateral and equiangular “V” pattern on an exterior vertical surface indicates an unobstructed and undeflected venting of the fire at this opening.
When the pattern is much wider than the opening, it is an indicator that a low intensity, heavy volume of flame vented thereat. As the volume of flame is increased, the pattern should get wider and higher on the exterior vertical surface. In this instance, the depth of char is an indicator of the duration of that venting flame.
When the “V” pattern is the same size as the opening, only rising up a limited distance and coupled with a deep char around the upper edges of that opening, it is an indicator of an intense, pushing fire condition within the structure. The depth of char above this opening on the exterior vertical wall is an indicator of the duration of that venting.
A second, and usually justified, conclusion that can be reached through this exterior examination is that the area of origin can be discerned by the greatest amount of visible fire damage.
Some things that could occur to fault the validity of this assumption are:
- Forced or unnatural ventilation created by fire suppression activities.
- Highly flammable fuels stored at that location.
- Prevailing winds creating either a positive or negative pressure thereat.
However, with “normal” fire progression and extension, a general rule is that the area of heaviest exterior fire damage is a good indicator of the interior area of origin.
Your fire scene examination at this point will cover all of the conditions on and around the exterior of the building. Besides fire damage, you should note in your memo pad and exterior sketches such things as broken bushes, broken windows, and other damaged items of evidence in the area.
When you enter the building, begin your examination from the areas of least fire damage to the areas of greatest fire damage.
Usually the areas of least and heaviest fire damage are readily discernable, even to the untrained eye. It is when this is not the case that your ability to interpret char patterns and other conditions enables you to locate the area of heaviest damage by determining the direction of the fire’s travel.
Direction of fire travel
If everything in an area of a structure appears to be destroyed uniformly, a close examination of the remaining structural members and any contents is called for.
The sides of those items that have the deepest char on them will be the sides that were exposed to the fire for the longest period of time. When these items are viewed collectively in relation to their physical location, and in respect to one another, the direction of fire travel can be deduced.
It would be well to remember and look for some other indicators of the direction of fire travel:
- An ordinary light bulb will begin to soften and distort, flowing towards the direction of the heat source. This forms a conical configuration with the apex pointing towards the heat source and the base away from it.
- Glass will soften between 1,200°F and 1,400°F and will become molten and runny at 1,600°F. Therefore, if you are looking for a point of origin within a room, the panes of glass in the windows may point the way. If you find one pane of glass that appears distorted through softening and a second pane of glass that has been distorted by actually melting and running, the direction of the heat source would be from the former toward the latter, or from the lower temperature indicator towards the higher temperature indicator.
- Wooden structural members that are vertically affixed are also good indicators. Those closest to the heat source would be burned down to a lower level than those that are successively and respectively further away from the heat source. In fact, with “normal” fire progression, if one could construct a descending declined plane across the upper parts of these vertical structural members, the point where the base of the declination leg abuts the floor should identify the area and possibly the point of origin.
- Shiny metal surfaces are discolored by heat, and this discoloration varies with the intensity of the applied heat. The discoloration that results from the highest applied heat is, with the exception of those colors developed at both the high and low ends of the heat spectrum, retained in the metal and is obviously visible. The heat colors developed in metals are the same as the flame colors created at various temperatures and are:
Yellow = 450 °F Brown to purple = 550°F Blue = 600 °F Faint red = 900°F Dark cherry = 1,100°F Full cherry = 1,400°F Salmon = 1,600°F Lemon = 1,800°F White = 2,200 °F Sparkling white = 2,400°F
If shiny metal surfaces exist at points distant from one another, their retained and visible coloration as a result of applied heat can indicate the locations of the least and the most heat, and thus provide a direction towards the heat source.
An old adage is appropriate at this point: A bear can only go into the woods halfway, because as he continues his forward motion, he is coming out on the other side.
This points up the fact that somewhere along our linear travel we will have reached the center of our quest. This will become evident when the indicators that we are examining are in opposition to those that we previously examined and are pointing toward the area just passed as the source of the heat.
By using this method, you can arrive at the area of origin and, once there, continued use of this method should allow you to focus in on the point of origin.
Ceiling damage above the point of origin
When you arrive at the spot that all of the indicators thus far have fixed as the point of origin, your next confirming indicator will be directly above that point. Since this is where the fire began, the fire presumably burned here the longest. If we apply the principle of convection, a valid assumption would be that: The greatest ceiling damage should occur directly above the point of origin because this is where it was the hottest for the longest period of time.
In normal fire propagation, this is true. Therefore, and conversely, the area below the point that has the greatest ceiling damage, either through consumption or collapse, is a good spot to search for the point of origin.
Keep in mind that the point of origin could have occurred at any level, all the way down to the floor. Therefore, when you are searching this area and you arrive at a level that has no burning below it, your point of origin is at this level.
This is the sequential order in which these points of origin indicators should be present in a normal fire occurrence.
WHAT IS NORMAL?
“Normal,” as used in this context, refers to those “ideal” conditions and results that we would expect to obtain if this fire were to occur under strictly controlled laboratory conditions.
Since we do not live in a laboratory, any events that occur can, and will, be effected by a multitude of constantly fluctuating variables that continually alter the results of that event. Therefore, fires that occur in our ambient surroundings would, in 99 out of 100 cases, be considered anything but normal. However, we still refer to them that way in given sets of circumstances.
It must be remembered that our observations of the physical results of a fire upon a fuel can only occur within a limited set of applied variables. Any conclusions that we make must take into account those variables that fit into the overall context of the picture (the whole equals the sum of all of its parts).
This is not a “Catch 22“ situation because a great number of variables can be identified through:
It is only when those indicators present do not fit the results we observe when the known variables are applied to the event that we have to search out the unknown variable. In some instances, this unknown variable, when discovered, may disclose the cause of the fire.
WHAT IS ABNORMAL?
What then do we mean by “abnormal”? In the context that this terminology will be used in relation to fire progression, it means: The results that are being viewed are not consistent with the results viewed in previous incidents that occurred in similar structures and that had a like number of known variables.
If these viewed results differ from the results of our former experiences, we can presume that this is caused by one or more unknown variables. Investigation should disclose what these unknown variables are, and when viewed in relation to the totality of the circumstances, it might help explain the abnormal results and possibly fix the point of origin.
This is not to say that when an abnormal condition exists it will change the cause of the fire (although that is a possibility). However, there does exist within this fire an unknown variable as compared to previous, similar fires.
HIGHEST HEAT GENERATED AT POINT OF ORIGIN THEORY
Thus far during your examination for the point of origin, the focus has been from the area of least damage to the area of greatest damage. The theory behind this being that the greatest damage would occur in the area where the fire started and had been burning the longest. Concurrent with that theory is that this is the area where the most heat will be liberated.
Under normal circumstances, this assumption would be valid. Therefore, do we always stop our examination and designate the area of heaviest damage as that area in which our point of origin will be found? No!
While it may turn out that the above statement is in fact the case, we must examine all the possible variables that might alter this as the designated location and cause a continuation of our examination.
EXCEPTIONS TO HIGHEST HEAT GENERATED AT POINT OF ORIGIN THEORY
Storage of highly volatile materials
If, while conducting an examination/investigation, you discover that highly volatile materials were stored in the area where the heaviest charring occurred, you must try, through the application of all the known variables, to discern if this is a case of secondary ignition or is, in fact, the true point of origin.
Forced or unnatural ventilation
Any opening that allows a stream of air (oxygen) to enter a fire scene while the fire is in progress will only enhance the rate of fuel combustion. This stream of air will both push the flames ahead of it and draw the flames behind it, causing the fire and resulting damage to intensify.
These openings can be the result of:
- Fire suppression activities
- Structural design
- Criminal activity.
If a fire were to originate at the base of an open window (structural design), the physical movement of air into the structure would accelerate due to the venturi effect of the convected fire gases and heated air rising in front of and across the window’s opening. This draws the fire upward and across the ceiling with a great velocity, allowing the fire to bank down on the other side of the room. It also provides, along the course of its travel, an insulating layer of cool air under the mushrooming condition.
This process will continue until a state of equilibrium is reached through normal fire progression processes. Therefore, during the fire’s incipient stage and up until equilibrium is attained, if a low ignition fuel source is present on the other side of the room, it will combust without any evidence of extension from the original fire. If the fire is extinguished at this point, you would be left with a false indication of a second and separate point of origin. Adding to this false point of origin is the fact that equilibrium is first reached between the hot convected gases and the cool fresh entrained air at the ceiling and wall junction opposite the open window and the true point of origin. The pyrolyzation and ignition of these surfaces at this juncture causes greater damage to occur at the ceiling here than at the ceiling above the true point of origin. This condition is known as thermal inversion.
The application of hose streams, especially those of high caliber, will in and of themselves create unnatural, forced ventilation conditions.
High winds are self-explanatory. These are prevailing conditions of weather that could alter the fire’s direction, intensity, or depth of charring, and provide a false indicator of the area of origin.
Flashover is the result of the feedback of radiant heat. This condition occurs in an area where the heat cannot be otherwise dissipated, like in an enclosed room. As the ignition temperature of the materials at the upper levels are reached, they auto-ignite. Since the combustible materials within a room have the same general ignition temperature, they will ignite simultaneously, giving the appearance of a wide area of origin. The charring will usually be present only on the exposed upper surfaces of combustible materials.
Materials with a low ignition temperature
If certain materials within an area have a lower ignition temperature than other materials around them, they will ignite and burn freely, raising the surrounding materials up to their ignition temperature. Eventually, all the combustibles will join in this free burning. This added burning time for low ignition temperature materials allows these materials and these immediate surroundings to have a deeper char and give a false indication that there are several points of origin.
Falling burning debris
The above described results could also be achieved through the action of burning debris falling and striking or landing upon combustible materials and causing them to ignite. This could produce false indicators of multiple points of origin that are separate and distinct from one another.
When a metal electrical conduit, such as Bx cable, carries an overload of current for an extended period of time, it heats up, sometimes to incandescence. Where this conduit passes around, behind, through, or nearby any combustible material, including structural members, it may cause them to char and ignite. If these materials are allowed to free burn, the resulting char pattern might give the false impression of separate and distinct points of origin.
When a conducting medium, other than a live electrical conduit (steam pipe, steel girder, etc.), traverses through the area of a body of fire, it too will become heated, sometimes to incandescence. If you find what you consider a point of origin at a remote distance from the main body of fire but abutting this conducting medium, before you reach any conclusion consider:
- The intensity and/or volume of the fire.
- The relative conductivity of the medium.
- The distance necessary for the heat to travel through the medium.
- The ignition temperature of the material at the remote suspected point of origin.
This will help you determine if this is in fact a point of origin or if it’s possibly a secondary ignition through conduction.
Laterally radiated heat
There will be times when you find a suspected separate point of origin in a room adjacent to the room containing the seat of the fire. When this occurs and it cannot be attributed to any of the aforementioned means of extension, you must examine for laterally radiated heat.
Since radiated heat is waves of energy, it can travel through solid walls and open spaces. If the amount of energy absorbed by the combustible material within the wall does not raise that material to ignition temperature because of a heat loss through conduction, then there will be an imperceptible change to the wall on the side away from the body of fire.
If low ignition temperature materials are inside that adjacent room, then when they absorb enough of those radiated energy waves, they will auto-ignite when they reach their ignition temperatures. When this phenomenon occurs, it is an extension of the original fire via secondary ignition and not a separate point of origin. As above, a careful examination of all of the facts is necessary before reaching a conclusion.
Spontaneous combustion is the result of spontaneous heating when all the right conditions are met. Spontaneous heating and combustion occurs in three ways:
- Chemicals reacting with one another. This is the truest form of spontaneous ignition, especially when it involves a hypergolic reaction like the one which occurs when you combine the metal sodium and water.
- Fermentation or microbial thermogenesis. This is the breakdown of vegetable or organic matter with the evolution of heat.
- Oxidation. For this to come to our attention, it is usually a continuation of the fermentation process or in conjunction with pyrophoric carbon.
If you believe this occurred at what you have designated as the point of origin, you must carefully examine the source of fuel. If the proper chemicals are present, test their reaction with one another at a safe remote location. If the fuel is organic material, then it should be consumed from the interior to the exterior. If the fuel is from a petroleum base, then while it might be the point of origin, it will not be because of spontaneous ignition.
This phenomenon is caused by the application of heat, usually over a long period of time, upon a thick piece of wood. The heat will pyrolyze the wood and reduce its normal ignition temperature of about 450°F-500°F down to as low as 200°F, creating a substance that also has a tendency to auto-ignite because of its great affinity for oxygen.
Before you consider this location as the point of origin and not the secondary ignition of a low ignition temperature fuel, your investigation must determine that all the proper conditions were present.
The conditions that create the backdraft (the rapid introduction of oxygen and subsequent ignition of superheated combustible gases) can ignite materials in the upper portions of the structure, especially during the vertical ventilation process, if those materials have been heated to their ignition temperature by these gases. The resultant char patterns would give false indicators of separate points of origin.
Elimination of these as points of origin is usually accomplished via investigation, and interviews with the last person at these locations prior to the fire and with those firefighters who could observe the upper portions of the structure.
Also, if the structure is vented horizontally, prior to any vertical ventilation, there is a possibility of backdraft or smoke explosion if available oxygen had been depleted and the fire has been smoldering for some time.
If a backdraft does occur under these conditions, it has a great propensity for distributing flaming debris over its entire route of travel, thereby causing false indicators of multiple points of origin for the same reasons that falling burning debris does.
Certain burn patterns lead fire investigators to point to flammable liquids as the origin and cause of the fire. Significant indicators in the burn pattern that allow for this interpretation are:
- Low burning
- Flooring material charred
- Flooring material charred in a “puddle configuration” over a wide area.
For a complete discussion on recognizing and identifying flammable liquid burn patterns, see page 24 of the June 1984 issue of FIRE ENGINEERING.
Since flammable liquid is a low ignition temperature fuel like pyroforic carbon, we must investigate before we declare this as a point of origin and not as a point of secondary ignition.
Part of the investigation will be to determine if this liquid is indigenous to the premises. Assuming that all of the other variables also point to this area as that in which the fire originated, we may then proceed. If the liquid is not indigenous, then it was introduced and is a point of origin. If it is indigenous, then its distribution pattern will be the pivotal factor in declaring it a point of origin.
Just as a brief summary, here is a list of the elements to consider when investigating the point of fire origin:
Searching for the point of origin at a fire scene is like piecing together a giant jigsaw puzzle;
Therefore, we must examine each piece in context with the whole puzzle;
Indicators are the visible pieces of the puzzle;
Correlate and collate those indicators to fit known variables;
Key pieces might be the unknown variables that we may infer from known facts;
Extraneous pieces must be eliminated to avoid confusion;
Value and importance of each indicator might vary and be subject to change as your examination proceeds;
Each fact must be listed in its proper order of sequence to create a chain of evidence;
Record all pertinent factors to enable you to articulate the chronology of events;
Success in finding the point of origin, therefore, will depend upon your ability to:
- Correctly identify and interpret the indicators and fit them to the known facts.
- Develop a logical linear sequential pattern that can bring you to no other point but the one you have designated.