New Developments in Reading Fire

During the first 14 years of my career as a firefighter, I attended numerous incidents where the fire behaved unexpectedly. The textbooks at the time were somewhat vague in relation to fire behavior, so I asked senior colleagues if they could explain what the different smoke colors and patterns meant. Unfortunately, no one could give an answer that I found to be clear and consistent.

So I started looking to authorities in other countries to get the answers. To my surprise, I found a similar lack of knowledge, with one exception: the Swedish Fire Service. In the late 1970s, Krister Giselsson and Mats Rosander (Swedish firefighters and fire engineers) realized that the proliferation of plastics in modern furnishings, combined with energy-efficient construction, was creating a new paradigm. Their research led to new explanations that challenged the “traditional” understanding of compartment fire behavior and firefighting tactics.

Thus began for me a journey to understand fire behavior and, more importantly, to translate the knowledge into tactics that firefighters all over the world could use. In this article, I’ll provide an update on where we are in understanding fire behavior, especially unusual or unexpected behavior.

Flashover & Backdraft: Signs & Symptoms

By late 1999, I had spent nearly three years developing a nationally recognized fire behavior training program. At this stage, flashover and backdraft indicators were described in terms of “signs and symptoms.”

Signs and symptoms of flashover

• Indicators of a ventilated fire
• Hot surfaces
• Flames at ceiling level
• Lowering of the neutral plane
• Increased rate of pyrolysis
• Increased turbulence of the neutral plane
• Crews forced low by high temperatures
• Painful radiant heat

Signs and symptoms of backdraft

• Smoke pulsating through small gaps in openings
• Fire with limited or no ventilation
• Thick black, yellow and/or cold smoke
• Blue flames
• Hot doors and windows
• Soot-blackened windows
• Lack of a visible flame
• Air being drawn in (whistling noise)
• Type and form of material involved?
• Length of time the fire has been burning

These signs and symptoms had been validated in the field and were useful for teaching the difference between flashover and backdraft. However, I felt they were incomplete. Some of the case studies I examined revealed situations in which very few of these indicators were present prior to firefighters being caught in sudden and unexpected fire development.

Then, in 2000, while fighting a fire in an old picture theatre that had been converted to a restaurant, I was involved in a near-miss incident where smoke had found its way from a basement fire through the ground level and had accumulated in the large concealed ceiling space. On recognizing a small track of fire progressing toward this space, I ordered the crew to withdraw, but before we could completely exit, accumulated smoke violently ignited, bringing down the suspended ceiling. Buffer zoning actions (application of a light film of water to the walls and linings) conducted on entry helped to delay fire spread, and we were able withdraw to safety. But the incident confirmed for me that there was more work to be done to truly be able to understand and predict fire behavior on the modern fireground.

The Effect of Modern Fuel Loads

The major flaw in the existing textbooks when it came to describing fire behavior: They were based on legacy fuel loads–prior to the widespread adoption of plastics in household furnishings.

Synthetic materials produced from petrochemicals have two to three times the energy value of natural products such as wood, cotton and paper. Not only are they higher in calorific value, but they have a more complex molecular structure that requires a very good supply of air, as well as exposure to high temperatures, to burn relatively freely. As a result, the concentration of unburnt fuel in the smoke has been increasing in proportion to our increasing use of synthetic materials in almost every part of our homes, workplaces and means of transport. Today, it’s very likely that high concentrations of unburnt fuel in the smoke will drift a significant distance from the room of origin, forming a flammable mixture that lies in wait for an ignition source.

Traditional teaching highlights the danger associated with thick, hot, dark smoke. What is missing is a recognition that when smoke moves through a structure, it mixes with air, which thins out the smoke, decreasing the temperature and thickness. Although this gives the impression that the conditions are less dangerous, in reality, the partial pre-mixing can help to dilute rich smoke into the flammable range. If an ignition source is introduced to accumulated smoke that happens to be close to an ideal mixture, the result can be a very rapid and powerful (even explosive) event.

Fire Gas Ignition

There are a wide range of terms for the event that occurs during the above scenario: smoke gas explosion, fire gas explosion, smoke explosion, etc. Sometimes these events have even been incorrectly labelled a flashover or backdraft. The intensity of the event depends on the amount of unburnt fuel and the degree of pre-mixing at the time of ignition. This can vary enormously, and for this reason I was never comfortable with any term that included the word “explosion.”

Instead, I use the term “fire gas ignition”: an ignition of accumulated fire gases and combustion products, existing in or transported into a flammable state. If the percentage of unburnt fuel is low, the ignition could behave somewhat like a flashover. If the concentration is high, then the event will be more like a backdraft. If it occurs with a mixture that is close to “ideal,” then the result can be explosive.

So, what are the signs and symptoms of fire gas ignition?

• Smoke flows though openings or gaps.
• Smoke accumulates in voids and spaces that are not involved in fire.
• Smoke can cool and thin due to mixing with cool air.
• It may be difficult to detect accumulated smoke/fuel in hidden or unexpected voids.
• There may be a lack of heat indicators.
• Lack of indicators can lead to a false sense of security.

SAHF: A Fireground Model

After identifying why fire gas ignition occurred, the challenge became developing a simple and reliable fireground model. I found it was difficult to apply “signs and symptoms” to the highly stressful and rapidly changing environment of the fireground–it was useful in the classroom, but trying to apply it at a fire was like trying to remember the contents of three shopping lists. I saw the need to develop a logical and easy-to-remember process that could help firefighters rapidly build a profile of the fire conditions and gain insight into what changes were likely to occur.

That’s how SAHF (pronounced “safe”) was born–a simple acronym that describes fire behavior indicators in order of importance.

Smoke

• Height of neutral plane
• Color and thickness (optical density)
• Volume and location
• Buoyancy and pressure

Air Track

• Air track profiles (inlets, exhausts or combinations)
• Velocity and direction
• Flow–turbulent or smooth
• Pulsations
• Whistling sounds

Heat

• Blackening of windows and no flame showing
• Cracking or crazing of glass
• Discoloration of glass
• Blistering or discoloration of paintwork
• Sudden heat build-up

Flame

• Color
• Volume
• Location

Note: Flame is placed last because there can be a tendency to “see nothing else” when flame is showing. Visible flame at least identifies one part of the structure that is lost and defines the point of attack that will prevent further loss of life and property. Smoke, on the other hand, has the potential to suddenly become flame when the conditions are right. The movement of air into the involved compartment, combined with heat indicators, can assist in determining how close we are to the fire origin, sudden changes and likely direction of fire spread.

Remember the Building

In 2005, I was delivering training at the National Fire Academy in Malaysia with my colleague Chief Ed Hartin. Ed and I had worked closely together as co-authors of a book called 3D Firefighting and we had numerous discussions on the finer points of reading fire. He convinced me that it was critical to include B for “building” in my acronym: B-SAHF (“be safe”).

Although I thought it was a good idea, it was only later that I realized how vital it is. My practical experience was based on fighting fires in the city of Brisbane, which has a humid, sub-tropical climate. Our construction typically focuses on making the most of mild winters and staying cool in the hot summer months. It’s common for a fire to reveal its location as the heat is transferred through single-glazed windows and lightly insulated walls and doors. It’s not uncommon for the fire to self-ventilate when the windows fail or when the fire burns through the ceiling and roof.

My discussions with Ed highlighted the impact of different construction methods on what indicators were likely to be visible. Construction found in temperate climates usually features heavy insulation with double- or triple-glazed windows. This not only impacts what indicators are likely to be present, but it can have a significant influence on how the fire will develop.

Further, the global trend toward energy-efficient construction and the increasing use of composite engineered building materials is making it more difficult to predict the rate of fire development and time to collapse. The skill of reading the fire provides us with an advantage, but our knowledge is only complete if we know all of the key factors, and this isn’t possible within minutes of arriving at a fire. For this reason, we should continually reassess the situation in the context of the particular building construction and temper our actions with an understanding that sometimes the indicators can be difficult to see.

Currently, I’m attempting to describe the relevance of the building construction in the context of likely air flow and the type of fire development that is likely to occur. The type of building construction will have an enormous impact on how the fire develops and how long the structure will be stable. The use or occupancy of the building may give some indication of the likely fire load, type and location. Some initial findings include:

• Flashover will occur in most buildings if sufficient air is available. Compartments with limited natural air flow are less likely to flash over before the available air is consumed. Heavy brick or cement-rendered walls will absorb a lot of energy, which could delay flashover.
• Backdraft is more likely in energy- efficient buildings with good insulation and sealed windows (often double- or triple-glazed). The developing fire may consume the available oxygen before it’s able to flashover, or it may rapidly decline due to insufficient oxygen and the increasing volume of combustion products. The superior insulation and double glazing will tend to hold the heat energy in the compartment and some of the heat indicators will be less obvious.
• Fire gas ignition is most likely to occur in structures featuring voids, ducts, shafts, balloon frame construction, large open plans, high ceilings, and false or suspended ceilings. These features allow smoke to be transported and accumulate in areas adjacent to the compartment of origin, or some distance from it. Modifications can create unexpected openings or voids. Poor or damaged smoke/fire stopping can be found in original or modified buildings. The unburnt fuel in the smoke is often partially mixed with fresh air and can accumulate to flammable concentrations.

Why Read Fire?
The most obvious outcome of reading the fire is to be able to identify the location of the fire and predict where it will go with or without our intervention. The key factor to determine: whether the fire is fuel-controlled or ventilation-controlled. If a high percentage of the structure’s volume is filled with smoke and the indicators point to a ventilation-controlled fire, there’s a very high potential for rapid and aggressive fire spread when ventilation (unplanned or even planned) occurs. The tactical ventilation plan must anticipate this and include actions that will minimize this potential and deal with it in a controlled manner. This could include actions such as deliberately keeping sections of the structure under-ventilated (anti-ventilation), delaying ventilation until hoselines are in place to cut off the potential fire progression, or cooling the gases before opening up the building.

Fuel-controlled fires are less likely to react suddenly after opening up the structure. However, it’s still important to have a tactical ventilation plan to remove, confine or dilute the accumulated fuel. Door entry techniques, safe zoning/buffer zoning (application of a light film of water to the walls and linings), positive pressure ventilation, and gas cooling are examples of tactics that may prove helpful in fuel-controlled fires.

The Next Step
The chart on p. 40—41 attempts to summarize the key points in relation to B-SAHF. It remains a work in progress that will continue to mature through collaboration with motivated, knowledgeable and experienced international firefighters and fire scientists. We cannot do this alone or in isolation. We must build bridges between firefighters and fire scientists. There is an emerging “new breed” of fire scientist that does not view firefighting as the “lower end” of fire protection; people like Stefan Svensson (Sweden), Steve Kerber (UL) and Dan Madrokowski (NIST) are shining examples of people who continue to apply a scientific approach to improving a very practical calling. Their work will be critical to advancing our knowledge of fire behavior and reducing fireground injuries and deaths–but it also takes a commitment from each and every firefighter to study this new science, and adjust tactics as needed.