Thorntons Rule

Thornton`s Rule

I was compelled to pen these comments concerning an article by John D. Wiseman, Jr., “Thornton`s Rule and the Exterior Fog Attack: A Perspective” (July 1996). Based on Wiseman`s conclusions, I guess we can look forward to wearing rubber turnout coats and riding the back step again. Fighting fires using fog streams directed through window openings was common in the 1950s when the “indirect” and “combination” methods of fire attack became popular. But it is not the 1950s and aggressive, interior firefighting is the hallmark of modern fire departments across the nation. To fight any structure fire other than from the interior when conditions are favorable to an interior attack and a thorough search of the building for victims has not been concluded does not merely constitute incompetence; it smacks of utter cowardice.

Despite his many years of service, Wiseman has apparently failed to understand a basic tenet of structure firefighting: Hoselines are not stretched and operated simply to extinguish fire. Hoselines are stretched and operated to save lives–civilian and firefighter–by separating the fire from any trapped occupants; safeguarding the means of egress (hallways and stairs); confining the fire to limit extension; and extinguishing the fire to end production of smoke laden with heated, toxic, and highly flammable combustion gases. The late chief Emanual Fried stated this principle very succinctly some 25 years ago: “Regardless of strategic dictates, the safety of human life should always be the strongest motivating factor in the placement of a hose stream.”1

I have talked at length with Keith Royer. He explained that the Iowa formula is concerned strictly with estimating required fire flow for controlling a fire in the largest room or section of a building or structure. (For an excellent discussion of the Iowa rate-of-flow formula, see “Iowa Rate of Flow Formula for Fire Control,” Fire Engineering, September 1995.) Fire flow needs for complete extinguishment, exposure protection, and fire extension are not taken into account. And occupant life safety was not an issue addressed by the Iowa researchers. For this reason the type of stream to be used (fog, straight, or solid) and the point or points for fire stream application are left up to the individual fire department. Only at this level can decisions about how best to protect the lives of trapped victims and searching firefighters be made. Just as municipal fire departments perform not one but two searches (primary and secondary) at every structure fire to be certain all potential victims have been accounted for, hoselines must be positioned with the intent to save lives. Sticking a nozzle through the window does not usually accomplish this most basic firefighting objective. This leads us to Thornton`s Rule.

What Thornton discovered before World War I was that in any oxygen-regulated fire environment, heats of combustion will be approximately the same for a variety of organic (carbon containing) liquids and gases.2 In the 1970s, Huggett extended Thornton`s research and found that the heat of combustion for a wide range of organic solids is also relatively constant and is a factor of the oxygen available for consumption within the fire compartment.3 This means that regardless of the type of material–be it hydrocarbon- or cellulose-based–heats of combustion do not vary substantially.

Based on the research findings of Thornton and Huggett, scientists have created methods of calculating heats of combustion for various substances based on the amount (or mass) of oxygen consumed. In practical terms, scientists can analyze the combustion products produced by a material burning under strictly controlled conditions in a calorimeter and determine its heat of combustion. Unfortunately, as many scientists and fire protection engineers point out, application of the oxygen consumption method outside the laboratory is not straightforward. Due to problems in testing and measuring techniques (several different types of calorimeters and measurement instruments are in use) and the fact that a laboratory test is being transposed to the highly complex, infinitely variable real world, all results must be viewed very carefully.4

Wiseman ignores another critical point–uncontrolled ventilation. In stochiometric calorimetry, the amount of available oxygen is controllable. Not so at a structure fire, where ventilation parameters are highly variable and may change many times during the course of a fire. And when ventilation is increased, another factor must be considered–the rate at which different materials liberate heat. It is one thing to say that for a given quantity (mass) of available oxygen, two materials–one hydrocarbon-, the other cellulose-based–will have approximately equal heats of combustion. But when provided with increased ventilation, hydrocarbons generally will liberate heat at a rate faster than cellulosic materials. As a consequence, we can expect that rollover and flashover conditions will be achieved in much less time.

Consider this example taken from real-world experience. A ladder company arrives at an early morning apartment fire and begins a primary search. A fire is discovered in the kitchen, but there is no door to close and isolate the fire. Initially, the fire appears small and docile–windows are still intact, heat conditions tolerable. The search team penetrates deeper into the apartment upon radio reports of a missing child. Suddenly, a thermal pane window fails, introducing a fresh supply of oxygen. The oxygen mixes with the highly heated carbon monoxide gas, causing the fire room and hallway to become a mass of orange flames. As the searching firefighters drop to their stomachs and begin reciting the Our Father, the engine company`s quick-acting nozzle team drives the fire back with its powerful solid stream, preventing severe burns to the search team members.

The fire sevice did not abandon booster lines and 112-inch hose based on exterior application of water fog. The advent of 134-inch and two-inch hose was inevitable given the dangers of rollover and flashover faced by firefighters advancing hoselines inside highly heated structures.

Wiseman also reminds us that, according to the Iowa formula, if the correct flow is provided and the water is applied at the proper point(s), the nozzle need only be opened for 30 seconds or less to achieve control. Royer and Nelson stated that by keeping the nozzle open much longer than necessary to “black out” the fire would overcool the fire area and negate subsequent ventilation efforts. Unfortunately, this does not reflect the reality of most interior firefighting operations. Closing a nozzle immediately after knockdown requires discipline and a degree of experience many of today`s nozzlemen simply do not have. In addition, even after knockdown, the nozzle must sometimes be kept open to cool a burning room to permit search and overhaul operations. Apparently, Wiseman has yet to experience the tremendous heat that radiates from lath and plaster walls or concrete ceilings once a postflashover fire has been controlled.

Wiseman is entitled to his opinion, but he was educated in a much different school of firefighting than I was. If we adopt this approach, it is back to the 1950s. Exterior fire attack will become the rule, and victims will be an afterthought. We can return to the use of 34-inch booster line and store our sophisticated turnout clothing and SCBA away in the closet. I`m sure the public we are sworn to protect will understand, as will their attorneys.

1. Fried, Emanual. Fireground Tactics. 1972. Chicago: H.M. Ginn Corp., 11.

2. Babrauskas, V., and S.J. Grayson, eds. Heat Release in Fires. 1992. London and New York: Elsevier Applied Science, 31.

3. Ibid.

4. SPFE Handbook of Fire Protection, Second Edition. 1995. National Fire Protection Association, Quincy, Mass.; Society of Fire Protection Engineers, Boston, Mass., 3-25.

Andrew A. Fredericks


City of New York (NY) Fire Department

Rescue rope testing should be scientific

I feel obligated to respond to Chuck Dean`s “Rescue Rope: Lifeline vs. the Bottom Line” (July 1996).

Dean seems to feel that any form of testing is better than no testing at all. I vehemently disagree. While it is true that scientific testing is costly and time-consuming, its validity can hardly be argued. Real “backyard” testing leaves one with nothing but questions regarding the intepretation of the results. We cannot compare results of one kind with the other. Finally, if scientific testing is not necessary, why do we rely on and, in fact, demand it as the basis of equipment testing and rating by manufacturers and other certifying agencies?

I have not seen any documentation on Dean`s test regarding the location of the break in the ropes he tested, but I am willing to wager that they almost all broke at the first bend in the figure-eight knot (an area we already know is the weakest point). You see, Dean has not tested the rope strength; he tested the knot strength.

So where has Dean led us? We are asked to look at these “backyard” results, compare them with scientific results, and base our decisions on when to retire rescue ropes with this incomplete knowledge. Not a good scenario. The only positive to these results is that we are told the rope strength was far less than it probably is, which offers a good margin of safety. I do not advocate using rope until you see through it, but I feel this testing is misleading.

The answer is to compare the results from scientific tests with those of testing done in the exact same manner. Only then can we make clear, concise decisions based on evidence and not on hearsay.

Ron James

Cleveland (OH) Division of Fire

Don`t limit choices or tactics

for fire scenario

In “Fog Attack for Ship Fires” (July 1996), Commander John P. Farley uses only two methods of attacking a given fire scenario. One had positive results (short bursts of fog to the overhead, followed by straight-stream application to the fuel). The other had negative results (a line with a straight stream application was advanced directly to the seat of the fire), in that the firefighters received burns. Of course, they received burns advancing to the seat of the fire in near-flashover conditions. I know of no one who would advocate this type of attack. Because the second method did not work, the author concludes the first method should have been used. But what about other options such as hitting the overhead with a straight stream and allowing the water drops to cool the fuel?

The choices of the size of the line, type of nozzle, nozzle pattern, position of the line, and application rate are made after considering many different variables. The choices should always take into consideration our goal of protecting life, reducing injuries, and keeping property damage to a minimum. We shouldn`t focus on only one factor such as using the least amount of water possible or using one attack method.

Ted Goldfarb

Deputy Chief, Division 8

City of New York (NY) Fire Department

Flashover survival strategy questioned

“Flashover Survival Strategy” (August 1996) is very interesting and worthy of careful study. However, the analysis is blurred by the failure to include the one significant fact about flashover that has been determined by numerous experiments–that is, that flashover occurs at an average ceiling temperature of 1,000°F. Thus, the analysis could be clarified by using the 1,000°F temperature to separate the four types of flashover discussed.

In fact, the discussion of the Type 4 flashover appears to be a discussion of the hot, smoldering fire in which temperatures range from 1,200°F to 1,800°F. In such a fire, the oxygen level drops below 15 percent; this fire has a blowout or backdraft potential. If enough oxygen is introduced into such a fire, an explosion will occur. I think that it is a mistake to call such an explosion a “flashover.”

The analysis could be further clarified by including some basic facts about glass behavior. The authors state that “Energy efficient windows (multipane, airtight, insulated) … will keep in heat and flammable gases much longer than conventional single panes.”

Of course, the truth of this statement depends on the period of time meant by the phrase “much longer.”

A different perspective is presented by Dr. Robert W. Fitzgerald in his article “Structural Integrity During Fire” in the NFPA Handbook (17th edition), pages 6-71. With reference to glass used as glazing for windows or doors, he states, “It quickly breaks because of the temperature differences between surfaces. Double glazing does not provide much improvement.” So, apparently, double-pane windows do not hold much longer than single-pane windows.

John A. Campbell in his article “Confinement of Fire in Buildings” in the NFPA Handbook (15th edition), pages 5-95, adds that …”Glass panes normally start breaking when heated to 550°F to 600°F, although some breaking may occur at lower temperatures.”

This is a critical fact about glass behavior. If a fire has broken a window, and especially if fire is burning out a window, then immediately you know what the temperature level is inside that room. This fact should be included as one of the key survival tactics.

The authors also state that the fire environment has changed with the addition of plastics that generate high heat and dense smoke when they burn. However, for confined fires the rate of heat release is constant, or nearly constant, for almost all organic solid compounds. The rate of heat released is 13.1 mJ/kg for molecular oxygen consumed irrespective of the type of fuel burning. This is Thornton`s Rule, which you can find in Appendix A of the NFPA Handbook (17th edition).

I have carefully studied the survival tactics suggested by the authors. I am impressed with their statement that feeling heat through the turnout gear in use today cannot be relied on as a warning since you cannot detect dangerous levels of heat. Furthermore, the authors state that rapid, almost immediate escape is the key survival tactic, perhaps the only survival tactic. However, there is no guarantee that having observed any warning sign guarantees enough time to escape. In fact, citing several instances in which firefighters were trapped within a few feet of an exit offers little assurance that this survival tactic will work.

The only real assurance offered by the authors is that they know of no instance in which a firefighter was injured or killed by flashover while operating a hoseline. That`s the sort of guarantee I like–100 percent. So why not simplify this whole analysis and conclude: Always take a charged line with you?

That makes sense as a surefire survival tactic. By the way, I would take a fog nozzle with me. Since its my life at stake, that`s my choice.

Finally, I would like to pass along a piece of advice I received from Keith Royer, former director of the Fire Service Institute at Iowa State University. He has a simple tactic for handling a flashover. He says if you stand up and can`t stand the heat, get out. That should take care of the problem.

John D. Wiseman, Jr.

Murfreesboro, Tennessee

Collapsed springs still valuable in fire investigation

Ronald K. Marley`s “The Facts about Coiled Metal Springs in Fire Investigation” (August 1996) is a well-written work that shows the result of painstaking research. It will be interesting to see how metallurgists critique the facts as presented. It is hoped they are unanimous in their approval.

My concern is the closing sentence: “The only thing they [collapsed springs] prove is that heating occurred in the area of the springs for the period of time it took to cause permanent spring collapse.” This leaves the fire-cause investigator where he started: Which came first, the chicken or the egg?

A suggestion is offered. In investigating fires involving upholstered furniture for about 40 years, I believe springs have much to tell us. A lot depends on the stage at which the fire was discovered and the method of extinguishment. Rest assured that if the structure burns to the ground, all springs will be “flat.”

The true value lies in the gutted room or seriously damaged residence where destruction is not total. An area of a room may be singled out if a chair is reduced to metal parts while a matching sofa still has its wood frame. You will find a remarkable difference in the springs even though both the chair and sofa are “destroyed.” The floor beneath, wall behind, and ceiling above tell a consistent story that shows a “core” of destruction centered at a single piece of furniture.

In this case, you can be reasonably certain that a fire smoldered in the chair for an hour or longer before progressing to flame. This slow destruction was preparing the room for a flashover when a window failed or other source of combustion air was introduced. This is the classic “smoker” fire. Steel springs are completely annealed.

When this type of fire is discovered earlier, you can find varying degrees of spring damage. Look for the telltale “core” where the smoldering cigarette first ignited upholstery, usually in a crease at the base of an armrest.

Vehicle seats can be helpful even though most springs are the flat corrugated/accordion type. They can show a remarkable difference in their postfire resilience from one side of the seat to the other. Corresponding backrest spring damage can assist. There are also smaller seat positioning springs beneath that can show a varying or corresponding loss of spring tension. These remains can indicate a localized, smoldering fire origin. Damage to these various springs is usually consistent across the seat where the cab is gutted and the fire originates at the dash or elsewhere.

A generally even burn pattern suggests a surface burning fire. If there is no obvious accidental heat source, it is wise to approach this type of fire with suspicion.

Spring steel (and other metals) must be considered a part of the overall burn pattern. If everything does not meld into your analysis, there is work to be done, or the cause must remain “undetermined.”

Robert E. Lowe

Lowe Fire Investigations

San Juan Capistrano, California

Electrocution is always fatal

Without a doubt, Fire Engineering is the premier publication of its type. You are to be congratulated on the stands you have taken regarding the NFPA, firefighter attitudes toward safety, “being our own worst enemy,” and many other extremely worthwhile articles you have published.

Given that welcome change, please help correct what has become more than just a passing misnomer within the past few years.

The article “Jumper on Power Line,” (What We Learned, September 1996) twice describes the victim as “electrocuted.” It then says he survived. This is an impossibility. While the victim`s burns were certainly serious, the fact that he survived indicates he was not electrocuted. Electrocution is always fatal. Webster`s Collegiate Dictionary, 10th edition, defines “electrocute” as “to kill by electric shock.” The description of the victim`s injuries was otherwise accurate. Inclusion of the “e word,” however, should only occur when electrical injuries are fatal.

Again, thank you for your publication.

Jack Ramsey

Health, Safety, and Training Officer

Skyland (NC) Volunteer Fire Department

Four firefighters on fireground not absolute OSHA requirement

I read John Bentivoglio`s series on the volunteer fire department`s relationship with OSHA with great interest but would like to comment on “Legal Limbo, part 4: The Status of Volunteer Fire Departments Under the Federal OSHA Law” (Volunteers Corner, September 1996).

Under the subhead “Operations” is discussed the OSHA IDLH Memorandum from James W. Stanley, dated May 1, 1995, which stated that “at least four firefighters must be assembled and on the fireground before interior structural firefighting operations can be initiated.” It appears Bentivoglio has not read the short news item “OSHA fireground staffing requirements clarified” (News in Brief, February 1996). In it, OSHA`s Thomas Seymour seems to backpedal from the above statement, indicating that OSHA would enforce only some of the provisions of NFPA 1500. In both cases, neither Bentivoglio nor OSHA has addressed the exception to this rule spelled out by NFPA 1500`s Tentative Interim Agreement, 1992, Section 6-4.1.1, which states: “Exception: If, upon arrival at the scene, members find an imminent life-threatening situation where immediate action may prevent the loss of life or serious injury, such action shall be permitted with less than four persons on the scene, when conducted in accordance with the provisions of Section 6-2.”

Although I support the practice of having four personnel on the scene prior to interior attack from a safety perspective, I feel Bentivoglio has done a disservice in implying that this is an absolute OSHA requirement.

Richard Miller

Cedar Crest, New Mexico

John Bentivoglio responds: I compliment Miller for addressing a glaring flaw in OSHA`s May 1, 1995 IDLH Memorandum, although he might be a bit more cautious before accusing someone of doing a “disservice” to Fire Engineering readers.

The IDLH Memorandum states that the agency will enforce the four-firefighters-on-the-fireground rule in NFPA 1500 as a “consensus industry standard” under OSHA`s general duty clause. However, in the lengthy discussion of what is (and is not) required under NFPA 1500, the IDLH Memorandum does not address the exception in Section 6-4.1.1 for “imminent, life-threatening situations.” Will OSHA recognize this exception, since it is an element of the NFPA 1500 TIA (1992), or will it enforce the IDLH Memorandum as written (with no exception)? Miller implies that fire departments need not comply with the literal terms of the IDLH Memorandum and can rely on the “imminent life threat” exception. But OSHA has created a situation where the fireground staffing requirements for fire departments are at best unclear. Rather than forcing fire departments to rely on oral representations by a single OSHA official, OSHA should issue a written clarification of the fireground staffing requirements under NFPA 1500 and the general duty clause.

Unfortunately, OSHA recently made it even more perilous to rely on the exception contained in Section 6-4.1.1. In August 1996, OSHA issued a directive stating that it will require state OSHA agencies to enforce the four-on-the-fireground requirement in NFPA 1500. The directive does not address the exception for life-threatening situations. Until OSHA issues a formal clarification as to whether it will recognize the “imminent life threat” exception, I stand by my earlier view that departments are well-advised to comply with the four-on-the-fireground rule. n

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