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Assessing Fire Hazard & The Life of a Fire


It often appears that building regulations and the officials that enforce them are just another obstacle in the process of building our dream home. Understanding that these regulations are in our mutual best interest is sometimes difficult. Yet, we all agree that a safe, secure residence is our ultimate goal. When it comes to fire protection and life safety, the regulations are based on volumes of research and investigative findings with the goal being to provide an appropriate response to the assessed fire hazard.

What is the assessed fire hazard?
The potential for ignition: What types of activities can be expected to be performed within the structure? What types of materials will be present that may constitute fuel? If any of that fuel happens to ignite, to what other surfaces can it spread?

The potential for smoke: When the fuel is burning, how much smoke is produced? Where will it go? Can it be vented away from the occupants? Will the occupants have time to escape the deadly heat, gases, and suffocating fumes associated with smoke?

The potential for damage: Can the fire travel down the hall or go upstairs? Is the fire inside or outside? Can it go through a window? Is there a risk to/from neighboring structures? Is there any method readily available to extinguish the fire? How long will it take for firefighters to arrive and become effective?

In response to the potential hazards, fire and code experts have created classifications and requirements for new construction. Design professionals are well versed in these requirements and integrate them into their designs as standard practice. They can be grouped as follows:

  • Occupancy groups (e.g., residential, institutional, industrial, educational, etc.)
  • Types of construction (e.g., non-combustible types, combustible types, and intermediate types such as heavy timber) defining the ability of the structure to endure the damaging effects of flame and heat
  • Minimum requirements for fire resistive construction to contain fire to the room in which the fire is ignited
  • Detection and alarm systems that alert the occupants of the building to smoke, heat, or flame
  • Suppression methods (adding materials to cool, smother, dilute, or chemically react with the fuel source or using turbulence to emulsify the fuel to disrupt combustion) that use extinguishing agents (e.g., water, carbon dioxide, dry chemicals, foaming agents, halogenated agents)
  • Means of egress (the means to escape from a burning building, facilitated by access to exits so as to minimize occupant panic), especially properly sized windows in sleeping rooms.


The Life of the Fire
The decomposition of combustible materials through the action of heat is the definition of pyrolysis. It begins when a heat source raises the temperature of the material to its ignition point.

  • Phase I: Over a period of minutes, hours, or days, the early stage of pyrolysis progresses into a smoldering stage where smoke develops, a flame may appear, and ambient temperatures increase.
  • Phase II: Upon developing a flame, ambient temperatures rapidly (within 5 minutes!) rise to 1000oF.
  • Phase III: In the next 5 minutes, the confinement of hot, expanding, combustible gases culminates in an explosion known as flashover.
  • Phase IV: “Fully developed” after only ten minutes of fire, this stage can last for hours.
  • Phase V: Depending on the extent of fuel and the efforts to suppress the fire, the fire is finally extinguished. At this point, the material enters the cool-down stage that may last for hours or days.

The life of a fire through these phases is described in a model known as the Standard Time-Temperature Curve. The importance of this model is to understand how quickly the occupants must evacuate the building (within 5 minutes), how long the structure can withstand extreme temperatures (over 1200oF), and how long the suppression/extinguishing effort is required to cool materials below the ignition point.
Firefighters and scientists recognize that fire is a living, breathing enemy. Feeding on combustible materials in an environment of free oxygen or oxygen released by chemical reaction, the ignition source gives birth to flame. The combustible material is not the only fuel; the chemical reaction caused by the mixture of fuel and oxygen generates hot, toxic, combustible gases. To eliminate the potential for fire, one must remove the source of the fuel, oxygen, or ignition. To extinguish fire, the oxygen to fuel ratio must be diluted or suppressed.

Another factor in the growth of fire is called the fuel array. The quantity of material is described as the fuel load, but the array describes the size of the individual particle and its storage arrangement. Typically, the smaller the particle, the easier it is to ignite. Spontaneous combustion becomes a real possibility when storage of smaller particles is concentrated in a container that is improperly ventilated, or even allowed to reach critical pressure. Which is more combustible, a pile of sawdust and wood chips or a stack of lumber or a log?