Fire Development in Modern Buildings

When was the last time you had a fire behavior class?

If you were honest it has probably been several years and most likely half of the class slept or paid very little attention because most firefighters like hands on drills were they can tear up stuff.

Those that have done fire behavior training recently what materials did you use? Because there has been major changes to fire development in the modern fire environment and most text books have not caught up.

Hopefully this post will help bring a lot of new material from UL , NIST and many other places together in a post that firefighters can use to train their next shift.

Fire Dynamics

Fire Dynamics is the study of how chemistry, fire science, material science and the mechanical engineering disciplines of fluid mechanics and heat transfer interact to influence fire behavior. In other words, Fire Dynamics is the study of how fires start, spread and develop. But what exactly is a fire?

Defining Fire

Fire can be described in many ways – here are a few:

  • NFPA 921: “A rapid oxidation process, which is a chemical reaction resulting in the evolution of light and heat in varying intensities.”
  • Webster’s Dictionary: “A fire is an exothermic chemical reaction that emits heat and light”

Fire can also be explained in terms of the Fire Tetrahedron – a geometric representation of what is required for fire to exist, namely, fuel, an oxidizing agent, heat, and an uninhibited chemical reaction.

Measuring Fire

Heat Energy is a form of energy characterized by vibration of molecules and capable of initiating and supporting chemical changes and changes of state (NFPA 921). Heat energy is measured in units of Joules (J), however it can also be measured in Calories (1 Calorie = 4.184 J) and BTU’s (1 BTU = 1055 J).

Temperature is a measure of the degree of molecular activity of a material compared to a reference point. Temperature is measured in degrees Fahrenheit (melting point of ice = 32 º F, boiling point of water = 212 º F) or degrees Celsius (melting point of ice = 0 º C, boiling point of water = 100 º C).

 

º F

Response

98.6

Normal human oral/body temperature

111

Human skin begins to feel pain

131

Human skin receives a second degree burn injury

162

Human skin is instantly destroyed

212

Water boils and produces steam

284

Glass transition temperature of polycarbonate

446

Melting temperature of polycarbonate(Mask)

>572

Charring of modern protective clothing fabrics begins

>1112

Temperatures inside a post-flashover room fire

 

Heat Transfer

Heat transfer is a major factor in the ignition, growth, spread, decay and extinction of a fire. It is important to note that heat is always transferred from the hotter object to the cooler object - heat energy transferred to and object increases the object’s temperature, and heat energy transferred from and object decreases the object’s temperature.

CONDUCTION

Conduction is heat transfer within solids or between contacting solids.

courtesy of NIST

 

courtesy of NIST

 

 

 

 

 

 

CONVECTION

Convection is heat transfer by the movement of liquids or gasses.

 

courtesy of NIST (convection on firefighter)

 

 

 

 

 

 

 

RADIATION

Radiation is heat transfer by electromagnetic waves.

 

courtesy of NIST (Radiation on the firefighter)

Fire Phenomena

Fire Development is a function of many factors including: fuel properties, fuel quantity, ventilation (natural or mechanical), compartment geometry (volume and ceiling height), location of fire, and ambient conditions (temperature, wind, etc).

Traditional Fire Development The Traditional Fire Development curve shows the time history of a fuel limited fire. In other words, the fire growth is not limited by a lack of oxygen. As more fuel becomes involved in the fire, the energy level continues to increase until all of the fuel available is burning (fully developed). Then as the fuel is burned away, the energy level begins to decay. The key is that oxygen is available to mix with the heated gases (fuel) to enable the completion of the fire triangle and the generation of energy.

Link:

http://www.youtube.com/watch?v=KhjuvI1c2oE&list=PLCDE5594A65DF3034&index=18&feature=plpp_video

 

 

Fire Behavior in a Structure The Fire Behavior in a Structure curve demonstrates the time history of a ventilation limited fire. In this case the fire starts in a structure which has the doors and windows closed. Early in the fire growth stage there is adequate oxygen to mix with the heated gases, which results in flaming combustion. As the oxygen level within the structure is depleted, the fire decays, the heat release from the fire decreases and as a result the temperature decreases. When a vent is opened, such as when the fire department enters a door, oxygen is introduced. The oxygen mixes with the heated gases in the structure and the energy level begins to increase. This change in ventilation can result in a rapid increase in fire growth potentially leading to a flashover (fully developed compartment fire) condition.

 

Changes in Today’s fires:

Modern Building Construction + More Plastics = Extreme Fire Behavior

 

 

 

Did you notice that fire development has changed? There is early decay now! We as firefighters need to share this with all firefighters especially ones that havn’t been to fire behavior class in some time.

 

 

 

 

 

 

 

 

 

Energy Efficient Modern Building Construction:

Effective Insulation

Properly installed and inspected insulation in floors, walls, and attics ensures consistent temperatures with less energy use. The result is lower utility costs and a quieter, more comfortable home.

High Performance Windows

Energy-efficient windows use advanced technologies to keep heat in during the winter and out during the summer. They also block damaging ultraviolet sunlight that can discolor carpets and furnishings.

Tight Construction and Ducts Homebuilders Making a Difference:

Advanced techniques for sealing holes and cracks in a home’s “envelope” and in heating and cooling ducts help reduce drafts, moisture, dust, pollen, pests, and noise. A tightly sealed home improves comfort and indoor air quality while lowering utility and maintenance costs.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The tactical considerations include:

  • Stages of fire development:The stages of fire development change when a fire becomes ventilation limited.
    • It is common with today’s fire environment to have a decay period prior to flashover which emphasizes the importance of ventilation
  • Forcing the front door is ventilation: Forcing entry has to be thought of as ventilation as well.
    • While forcing entry is necessary to fight the fire it must also trigger the thought that air is being fed to the fire and the clock is ticking before either the fire gets extinguished or it grows until an untenable condition exists jeopardizing the safety of everyone in the structure.
  • No smoke showing:A common event during the experiments was that once the fire became ventilation limited the smoke being forced out of the gaps of the houses greatly diminished or stopped all together.
    • No some showing during size-up should increase awareness of the potential conditions inside.
  • Coordination: If you add air to the fire and don’t apply water in the appropriate time frame the fire gets larger and safety decreases.
  • DON’T FORCE DOOR UNTILL YOU HAVE A CHARGED HOSELINE IN PLACE!

 

 

For more information:

http://www.ul.com/global/eng/pages/offerings/industries/buildingmaterials/fire/fireservice/ventilation/

 

http://www.nist.gov/fire/fire_behavior.cfm

 

http://cfbt-us.com/index.html

 

http://buildingsonfire.com/

 

Stay Safe and please train on the new changes to fire development. It may save your life.

Thanks

Lt. John Shafer

 

 

 

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John Shafer

An 18 year veteran of the fire service, and currently a Lieutenant and the Training Officer for the Greencastle Fire Department. An Indiana regionally recognized instructor on building construction, fireground search and command management, he has traveled throughout the State of Indiana delivering specialized training programs on building construction, fireground search and firefighter safety.