Video of several incidents involving explosions during structural firefighting operations have been posted to YouTube in the last several weeks. Two of these videos, one from New Chicago, IN and the other from Olathe, KS involve residential fires. The other is of a commercial fire in Wichita, KS.

When a video shows some sort of spectacular fire behavior there is generally a great deal of speculation amongst the viewers about what happened. Was it a smoke (fire gas) explosion, backdraft, flashover, or did something else happen? Such speculation is useful if placed in the framework of the conditions required for these phenomena to occur and the Building, Smoke, Air Track, Heat, and Flame (B-SAHF) indicators that provide cues of to current fire conditions and potential fire behavior.

Occasionally, what happened is fairly obvious such as flashover resulting from increased ventilation under ventilation controlled conditions. However, the phenomena and its causal factors are often much more of a puzzle.

Download and print three copies of the B-SAHF Worksheet.

Residential Fire-Olathe, KS

Limited information was posted along with this pre-arrival video of a residential fire in Olathe, KS. The home was unoccupied when the fire occurred.

Watch the thirty seconds (0:30) of the video. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions (based only on what you observe during the first thirty seconds of the video)?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes

Watch remainder of the video and consider the following questions:

1. Did fire conditions progress as you anticipated?
2. What changes in the B-SAHF indicators did you observe?
3. What may have caused the explosion (consider all of the possibilities)?
4. Were there any indications that may have given warning of this change in conditions?

Residential Fire-New Chicago, IN

Companies from New Chicago and Hobart were dispatched to a reported house fire at 402 Madison in New Chicago, IN on February 17, 2012.

Watch the thirty seconds (0:30) of the video. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions (based only on what you observe during the first thirty seconds of the video)?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes

Watch remainder of the video and consider the following questions:

1. Did fire conditions progress as you anticipated?
2. What changes in the B-SAHF indicators did you observe?
3. What may have caused the explosion (consider all of the possibilities)?
4. Were there any indications that may have given warning of this change in conditions?

Commercial Fire-Wichita, KS

Wichita Fire Department on scene of a working building fire in large, non-combustible commercial building. Extreme heat and fire conditions cause an unknown cylinder to explode.

Keep in mind that gas cylinders and other closed containers can result in explosions during structural firefighting operations. Unlike backdraft and smoke explosion, the only clue may be building factors related to occupancy (and this may not be a good indicator when operating at a residential fire).

Wichita Fire Department on scene of a working building fire in a large metal structure. Extreme heat and fire conditions cause an unknown cylinder to explode. If you listen close, you can hear it vent before it goes off. Concussion actually cuts out my audio for just a couple seconds. No one was injured.

Video by Sean Black Photography http://seanblackphotography.smugmug.com/

Firefighter Safety

Potential for explosions related to extreme fire behavior such as backdraft and smoke explosion may be recognized based on assessment and understanding the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) indicators. Other types of explosions such as those resulting from failure of closed containers (e.g., containing liquids or gases) may be a bit more difficult as this potential is likely to be present in most types of occupancies. However, commercial and industrial occupancies present greater risks.

Recognizing that even with sound experienced judgment, there may be undetected hazards on the fireground. Managing the risk requires developing a solid knowledge base and skills and operating within sound rules of engagement such as the IAFC Rules of Engagement for Structural Firefighting. However, considering the hazards presented by rapid fire progression and potential for changes in conditions following explosive events, I would add the following:

• Base your strategies and tactics on current and anticipated fire behavior and structural stability.
• Ensure that members correctly wear complete structural firefighting clothing and SCBA when working in the hazard zone and practice good air management. Buddy check before entry!
• Crews operating on the interior should have a hoseline or be directly supported by a crew with a hoseline. If conditions deteriorate, a hoseline allows self-protection and provides a defined egress path.
• Have well practiced battle drills for tactical withdrawal and abandoning the building (depending on conditions). See Battle Drill, Battle Drill Part 2, and Battle Drill Part 3.

Next…

My next post will address the impact of a closed door on tenability during a residential fire as the ninth tactical implication identified in the UL study on the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction.

Subsequent posts will come back to the Olathe, KS and New Chicago, IN residential fires to examine potential impacts on fire behavior and explosions that resulted during these incidents.

Ed Hartin, MS, EFO, MIFIreE, CFO

Seven Firefighters Injured

Seven firefighters were tragically injured in Prince George’s County Maryland on Friday, February 24, 2012. The fire broke out in the basement of a single-family, one-story house located at 6404 57^th Avenue in Riverdale, MD shortly after 21:00 hours.

Note: View from Alpha-Bravo Corner street side. Photo by Billy McNeel.

On arrival, Engine 807B reported a two-story, single family dwelling with fire showing from the basement level on Side Bravo. Seven members from Companies 807 (Riverdale) and 809 (Bladensburg) entered Floor 1 of the building on Side A (East Side) and within eight seconds were enveloped by untenable, wind driven fire conditions. Preliminary reports indicate that firefighters had initiated an interior attack on the fire when a sudden rush of air, fanned by high winds, entered from the rear of the house either from a door or window being opened or broken out. (Brady, 2012). A report on Monday, February 27 indicated that some of the firefighters ran to the back of the one-story home, then entered through a basement door while other members of their company opened the front door in search of a victim (FirefighterNation, 2012).

In a statement to Washington Post reporter J. Freedom du Lac (2012), Chief Marc Bashoor indicated that strong winds were gusting out of the west at up to 40, 45 mph, blowing directly into the burning basement, which had a west-facing door. “As soon as the guys opened the front door and advanced, it blew from the basement, up the steps and right out the front door,” Bashoor said. “It was like a blowtorch coming up the steps and out the door… Without that wind, the hot air and gases would have been venting out of the rear of the house,” he said. “The current of air essentially produced a chimney right up the steps and out the front door.” (Washington Post, 2012).

Firefighters Ethan Sorrell and Kevin O’Toole from Bladensburg Volunteer Fire Department remain in critical condition at Washington Hospital Center. Riverdale Volunteer, Michael McLary also remains hospitalized for injuries. The other injured firefighters were released and sent home Saturday evening according to the latest reports.

The wind-fueled fireball that injured seven Prince George’s County firefighters when it blew through the burning house they had just entered was “a freak occurrence,” a department spokesman, Mark Brady, said Saturday (du Lac, 2012).

Chris Naum at Command Safety has an excellent post examining the fire building and weather conditions at the time of the incident. See Residential Fire Injures Seven Firefighters: Wind Driven Conditions Suspected.

Freak Occurrence?

Dealing with an accident involving a serious injury or fatality is extremely difficult, particularly when the complete circumstances and eventual outcome is unknown. What may appear to be obvious in retrospect may also have been not so clear to the individuals engaged in emergency operations. However, one might ask if the fire behavior encountered at 6404 57^th Avenue in Riverdale, MD was in fact a freak occurrence. A freak is defined as a thing or occurrence that is abnormal, markedly unusual or irregular.

The conditions encountered were markedly different than usually encountered in fires occurring in single family dwellings. However, the conditions described in this incident are not unusual when considered in light of the building configuration and wind conditions at the time of the incident. Wind, flow path, and burning regime (fuel or ventilation controlled) have a tremendous impact on fire behavior and potential for rapid fire progression resulting in untenable conditions.

Wind Driven Fires

On April 16, 2007 Technician Kyle Wilson of the Prince William Fire & Rescue lost his life in a wind driven fire occurring in a large, single family dwelling. In the introduction to the investigative report produced by Prince William Fire & Rescue examining this incident, Chief Kevin J. McGee states:

First, the impact the wind had on this event was significant. While weather conditions, and specifically wind, are often discussed in the firefighting environment of wildland fires, it does not receive the same attention and consideration in structure fires. This incident showed the dramatic and devastating effect the wind can have on the spread of fire in a building. The wind forced the fire into the building and caused the sudden change in fire conditions inside, including the “blowtorch” effect witnessed by the crews on the scene (Prince William County Fire Rescue, 2008)

In January, the National Institute of Standards and Technology (NIST) released Simulation of the Dynamics of a Wind-Driven Fire in a Ranch-Style House-Texas (Barowy & Madrzykowski, 2012) examining fire behavior in the incident that took the lives of Houston Fire Department Captain James Harlow and Firefighter Damion Hobbs on April 12, 2009 while engaged in firefighting operations in a single family dwelling. This report emphasized that potential for wind driven fire conditions can occur in all types of buildings, including single-family residential structures.

NIST research (Madrzykowski & Kerber.(2009a, 2009b) has identified that wind driven fire conditions can be created with wind speeds as low as 4.5 m/s (10 mph) and that while structural fire departments have recognized the impact of wind on fire behavior, in general, standard operating guidelines (SOG) have not changed to address the risk of wind driven fires (Barowy & Madrzykowski, 2012).

Previous posts have examined NISTs research on the issue of wind driven fires:

• Wind Driven Fires
• NIST Wind Driven Fire Experiments: Establishing a Baseline
  
• NIST Wind Driven Fire Experiments: Anti-Ventilation-Wind Control Devices
  
• NIST Wind Driven Fire Experiments: Wind Control Devices & Fire Suppression

Flow Path

On May 30, 1999, Firefighters Anthony Phillips and Louis Matthews of the District of Columbia Fire Department (DCFD) died and two others were severely injured as a result of rapid fire progression while engaged in firefighting operations at 3146 Cherry Road, NE. The fire occurred in the basement of a two-story, middle of building, townhouse apartment. Crews entered on Floor 1, Side A and were caught in the flow path of hot smoke and flames when a sliding glass door was opened at the Basement Level on Side C. Previous posts examined this incident in detail:

• Fire Behavior Case Study Townhouse Fire: Washington, DC
  
• Townhouse Fire: Washington, DC What Happened
• Townhouse Fire: Washington, DC Extreme Fire Behavior
  
• Townhouse Fire: Washington, DC: Computer Modeling
• Townhouse Fire: Washington, DC Computer Modeling-Part 2

More recently, the City of San Francisco Fire Department released an investigative report examining the circumstances surrounding the deaths of Lieutenant Vincent Perez and Firefighter/Paramedic Anthony Valerio on June 2, 2011 while operating at a fire in the basement of a two story home with two levels below grade. Failure of a basement window placed the Lieutenant and Firefighter in the flow path between the basement window and their entry point on Floor 1. The investigative report produced by the San Francisco Fire Department details their findings and recommendations related to this incident.

Safety Investigation Report Line-of-Duty Deaths, 133 Berkley Way, June 2, 2011, Box 8155, Incident #11050532

Structural Firefighting Under Wind Conditions

Research and fireground experience point to the following:

• Building configuration including windows, doors, and open interior stairways can have a significant impact on development of a flow path from the fire to one or more exhaust points.
• Introduction of additional air to a ventilation controlled fire (without concurrent fire suppression) will quickly result in increased heat release rate.
• Creation of openings at and above the fire level which result in a flow path with an exhaust opening above the inlet will result in a rapid increase in heat release rate.
• Thermal conditions in the flow path above the fire and/or downstream from the fire location or will quickly become untenable.
• Even limited wind conditions can result in wind driven fire conditions.
• These factors in combination are even more likely to result in rapid fire progression and untenable conditions in the downstream flow path.

It is essential that Firefighters and Fire Officers recognize the influence of ventilation on fire behavior and potential for wind driven fire conditions and adjust their strategies and tactics accordingly. The following guidance is based on recommendations developed through the NIST wind driven fires research as well as data from National Institute for Occupational Safety and Health (NIOSH) death in the line of duty reports and incident investigative reports by the Texas State Fire Marshals Office.

Potential for wind driven conditions increases directly with wind speed. When wind speeds exceed a gentle breeze (8-12 mph) consider the potential for wind driven fire conditions and apply the following strategic and tactical considerations (CWIFR District Board, 2011):

• If potential for wind driven fire conditions is identified, this should be communicated to all companies and members working at the incident as a safety message.
• When possible, operate from the exterior and apply water from upwind directly into the involved compartments prior to interior attack. Even low flow exterior streams applied from upwind can have a significant impact on controlling the fire prior to interior operations).
• In a wind-driven fire, it is most important to use the wind to your advantage and attack the fire from the upwind side of the structure, especially if the upwind side is the burned side. Note that this may be contrary to conventional offensive tactics that place hoselines between the hazard presented by the fire and potential occupants and uninvolved property.
• Avoid pressurization of the building without first establishing adequate exhaust openings (2-3 times larger than the inlet). Remember that wind can create the same (or greater) positive pressure as a blower used in positive pressure ventilation (PPV). Pressurization without adequate exhaust can result in extreme fire behavior. Note: This is particularly important when the fire is on the leeward (downwind) side of the building and entry is made from the windward (upwind) side of the building.
• Consider controlling the flow path by using anti-ventilation such as door control and limiting the use of (horizontal and vertical) tactical ventilation prior to fire control. However, it is essential to remember that unplanned ventilation resulting from fire effects can have a significant impact on the ventilation profile and subsequent flow path(s).
• Avoid working in the exhaust portion of the flow path (between the fire and exhaust opening) or potential flow paths (between the fire and potential exhaust openings). Unplanned ventilation from fire effects can suddenly change the interior thermal conditions.
• Identify potential refuge areas, escape routes, and safety zones prior to and during interior operations. Taking refuge in a compartment with an intact and closed door may temporarily provide tenable conditions and a place of refuge until the fire can be controlled or another avenue of egress established.

References & Additional Reading

Brady, M. (2012). Seven firefighters injured battling Riverdale house fire. Retrieved February 26, 2012 from http://pgfdpio.blogspot.com/2012/02/seven-firefighters-injured-battling.html

Central Whidbey Island Fire & Rescue (CWIFR) District Board. (2011). Board minutes February 9, 2012. Coupeville, WA: Author. [Adoption of Purpose, Policy, and Scope of SOG 4.3.6 Structural Firefighting Under Wind Conditions]

District of Columbia (DC) Fire & EMS. (2000). Report from the reconstruction committee: Fire at 3146 Cherry Road NE, Washington DC, May 30, 1999. Washington, DC: Author.

du Lac, J. (2012). Blaze that injured 7 Prince George’s firefighters called ‘freak occurrence’. Retrieved February 26, 2012 from http://www.washingtonpost.com/local/blaze-that-injured-7-prince-georges-firefighters-called-freak-occurrence/2012/02/25/gIQAdGJMaR_story.html?hpid=z3

FirefighterNation. (2012). Critically burned in Maryland house fire, firefighters face long recovery. Retrieved February 28, 2012, from http://www.firefighternation.com/article/news-2/critically-burned-maryland-house-fire-firefighters-face-lengthy-recovery.

Madrzykowski , D. &  Barowy, A. (2012). Simulation of the dynamics of a wind-driven fire in a ranch-style house – Texas, TN 1729. Retrieved February 8, 2012 from http://www.nist.gov/customcf/get_pdf.cfm?pub_id=909779

Madrzykowski, D & Kerber, S. (2009a). Fire fighting tactics under wind driven conditions: Laboratory experiments, TN 1618. Retrieved February 8, 2012 from http://fire.nist.gov/bfrlpubs/fire09/PDF/f09002.pdf

Madrzykowski, D & Kerber, S. (2009b). Fire fighting tactics under wind driven fire conditions: 7-story building experiments, TN 1629. Retrieved February 8, 2012 from http://fire.nist.gov/bfrlpubs/fire09/PDF/f09015.pdf

Madrzykowski, D. & Vettori, R. (2000). Simulation of the Dynamics of the Fire at 3146 Cherry Road NE Washington D.C., May 30, 1999, NISTR 6510. August 31, 2009 from http://fire.nist.gov/CDPUBS/NISTIR_6510/6510c.pdf

National Institute for Occpational Safety and Health (NIOSH). (2008). Death in the line of duty…2007-12. Retrieved February 9, 2012 from http://www.cdc.gov/niosh/fire/pdfs/face200712.pdf

National Institute for Occpational Safety and Health (NIOSH). (2009). Death in the line of duty…2009-11. Retrieved February 9, 2012 from http://www.cdc.gov/niosh/fire/pdfs/face200911.pdf

National Institute for Occpational Safety and Health (NIOSH). (2009). Death in the line of duty…2007-29. Retrieved February 9, 2012 from http://www.cdc.gov/niosh/fire/reports/face200729.html

National Institute for Occupational Safety and Health (NIOSH). (1999). Death in the line of duty, Report 99-21. Retrieved August 31, 2009 from http://www.cdc.gov/niosh/fire/reports/face9921.html

Prince William County Department of Fire & Rescue. (2007). Line of duty death investigative report. Retrieved February 9, 2012 from http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCgQFjAB&url=http%3A%2F%2Fwww.iaff.org%2Fhs%2FLODD_Manual%2FLODD%2520Reports%2FPrince%2520William%2520County%2C%2520VA%2520-%2520Wilson.pdf&ei=b3dKT8LyGfHSiALt5tnrDQ&usg=AFQjCNFBBTfVkWIREXw0-wbd978fWSoP8w&sig2=y6_OEeJvhFSggiKioMESaw

San Francisco Fire Department. (2012). Safety Investigation Report Line-of-Duty Deaths, 133 Berkley Way, June 2, 2011, Box 8155, Incident #11050532 Retrieved February 26, 2012 from http://statter911.com/files/2012/02/Safety-Investigation-133-Berkeley-Way-Printable.pdf

Texas State Fire Marshal’s Office. (2007). Firefighter fatality investigation, Investigation Number FY 07-02. http://www.tdi.texas.gov/reports/fire/documents/fmloddnoonday.pdf

Texas State Fire Marshal’s Office. (2009). Firefighter fatality investigation, Investigation Number FY 09- http://www.tdi.texas.gov/reports/fire/documents/fmloddhouston09.pdf

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

Residential Fire

This post examines fire development during a residential fire in Lyons, New York.

Download and the B-SAHF Worksheet.

Watch the first minute and thirty seconds (1:30) of the video. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes

In addition, consider how the answers to these questions impact your assessment of the potential for survival of possible occupants.

Now watch the video clip from 1:30 until 2:00. Now answer the following questions:

1. Did fire conditions progress as you anticipated?
2. What changes in the B-SAHF indicators did you observe?
3. What indications of fire stream effectiveness did you observe?
4. What potential avenues of fire extension would you consider based on the type of construction and building design?

As you watch the remainder of the video, consider the changes in observed conditions and what information this might provide the Incident Commander. What information should interior crews report to Command during this stage of incident operations.

More on Reading the Fire

See the following posts for more information on reading the fire:

• Reading the Fire” How to Improve Your Skills
• Reading the Fire: Building Factors
• Reading the Fire Building Factors Part 2
• Reading the Fire: Building Factors Part 3
• Reading the Fire: Smoke
• Reading the Fire: Smoke Part 2
• Reading the Fire: Air Track
• Reading the Fire: Air Track Part 2
• Reading the Fire: Heat
• Reading the Fire: Heat Part 2
• Reading the Fire: Heat Part 3
• Reading the Fire: Flames
• Reading the Fire: Flames Part 2
• Reading the Fire: Putting it All Together
• Incipient Stage Fires: Key Fire Behavior Indicators
• Growth Stage Fires: Key Fire Behavior Indicators
• Fully Developed Fires: Key Fire Behavior Indicators
• Decay Stage Fires: Key Fire Behavior Indicator

Ed Hartin, MS, EFO, MIFireE, CFO

The eighth and tenth tactical implications identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) are the answer to the question, can you vent enough and the influence of pre-existing openings or openings caused by fire effects on the speed of progression to flashover.

The ninth implication; the effects of closed doors on tenability for victims and firefighters, will be addressed in the next post.

Photo Credit: Captain Jacob Brod, Pineville (NC) Fire Department

Kerber (2011) indicates that firefighters presume that if you create enough ventilation openings that the fire will return to a fuel controlled burning regime. I am not so sure that this is the case. Until fairly recently, the concept of burning regime and influence of increased ventilation on ventilation controlled fires was not well recognized in the US fire service. However, there has been a commonly held belief that increased ventilation will improve interior conditions and reduce the potential for extreme fire behavior phenomena such as flashover. In either case, the results of the experiments conducted by UL on the influence of horizontal ventilation cast considerable doubt on the ability to accomplish either of these outcomes using horizontal, natural ventilation.

The Experiments

In order to determine the impact of increased ventilation, Kerber (2011) compared changes in temperature with varied numbers and sizes of ventilation openings. The smallest ventilation opening in the experiments conducted in both the one and two story houses was when the door on Side A was used to provide the only opening. The largest number and size of ventilation openings was in the experiments where the front door and four windows were used (see Figures 1 and 3)

The area of ventilation openings in experiments conducted in the one-story house ranged from 1.77 m² (19.1 ft²) using the front door only to 9.51 m² (102.4 ft²) with the front door and four windows. In the two-story house the area of ventilation openings ranged from 1.77 m² (19.1 ft²) with front door only to 14.75 m² (158.8 ft²) using the front door and four windows.

The most dramatic comparison is between Experiments 1 and 2 where a single opening was used (front door) and Experiments 14 and 15 where five openings were used (door and four windows).

One Story House

Experiment 1 was conducted in the one-story house using the door on Side A as the only ventilation opening. The door was opened eight minutes after ignition (480 seconds). Experiment 14 was also conducted in the one-story house, but in this case the door on Side A and four windows were used as ventilation openings. Windows in the living room and bedrooms one, two, and three were opened sequentially immediately after the door was opened, providing more than five times the ventilation area as in Experiment 1 (door only).

Figure 1. Ventilation Openings in the One-Story House

In both Experiment 1 (door only) and Experiment 14 (door and four windows), increased ventilation resulted in transition to a fully developed fire in the compartment of origin (see Figure 2). In Experiment 1, a bi-directional air track developed at the door on Side A (flames out the top and air in the bottom). In Experiment 14, a bi-directional air track is visible at all ventilation openings, with flames visible from the door and window in the Living Room on Side A and flames visible through the window in Bedroom 3. No flames extended out the ventilation openings in Bedrooms 1, 2, and 3. The upper layer in Bedroom 3 is not deep, as such there is little smoke visible exiting the window, and it appears to be serving predominantly as an inlet. On the other hand, upper layer in Bedroom 2 is considerably deeper and a large volume of thick (optically dense) smoke is pushing from the window with moderate velocity. While a bi-directional air track is evident, this window is serving predominantly as an exhaust opening.

Figure 2. Fire Conditions at 600 seconds (10:00)

As illustrated in Figure 3, increased ventilation resulted in a increase in heat release rate and subsequent increase in temperature. It is important to note that the peak temperature in Experiment 14 (door and four windows) is more than 60% higher than in Experiment 1 (door only).

Figure 3. Living Room Temperature 0.30 m(1’) Above the Floor One-Story House

Note. Adapted from Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (p. 298), by Steve Kerber, 2011, Northbrook, IL: Underwriters Laboratories.

Based on observed conditions and temperature measurement within the one-story house, it is evident that increasing the ventilation from 1.77 m² (19.1 ft²) using the front door to 9.51 m² (102.4 ft²) with the front door and four windows did not return the fire to a fuel controlled burning regime and further, did not improve interior conditions.

It is important to note that these experiments were conducted without coordinated fire control operations in order to study the effects of ventilation on fire behavior. Conditions changed quickly in both experiments, but the speed with which the fire transitioned from decay to growth and reached flashover was dramatically more rapid with a larger ventilation area (i.e., door and four windows).

Two Story House

Experiment 2 was conducted in the two-story house using the door on Side A as the only ventilation opening. The door was opened ten minutes after ignition (600 seconds). Experiment 15 was also conducted in the two-story house, but in this case the door on Side A and four windows were used as ventilation openings. One window in the Living Room (Floor 1, Side A, below Bedroom 3) Den (Floor 1, Side C, below Bedroom 2) and two windows in the Family Room (Side C) were opened sequentially immediately after the door was opened, providing more than eight times the ventilation area as in Experiment 2 (door only).

Figure 4. Ventilation Openings in the Two-Story House

In both Experiment 2 (door only) and Experiment 15 (door and four windows), increased ventilation resulted in transition to a fully developed fire in the compartment of origin. Flames were seen from the family room windows in Experiment 15 (see Figure 5). However, in Experiment 2, no flames were visible on the exterior (due to the distance between the fire compartment and ventilation opening) and a bi-directional air track developed at the door on Side A (smoke out the top and air in the bottom). In Experiment 15, a bi-directional air track is visible at all ventilation openings, with flames visible from the windows in the family room on Side C. No flames extended out the ventilation openings on Side A or from the Den on Side C (see Figure 5). The upper layer is extremely deep (particularly considering the ceiling height of 16’ in the family room and foyer atrium. The velocity of smoke discharge from ventilation openings is moderate.

Figure 5. Fire Conditions at 780 seconds (13:00)

As illustrated in Figure 6, increased ventilation resulted in a increase in heat release rate and subsequent increase in temperature. It is important to note that the peak temperature in Experiment 15 (door and four windows) is approximately 50% higher than in Experiment 2 (door only).

Figure 6. Living Room Temperature 0.30 m(1’) Above the Floor One-Story House

Note. Adapted from Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (p. 299), by Steven Kerber, 2011, Northbrook, IL: Underwriters Laboratories.

Another Consideration

Comparison of these experiments answers the questions if increased horizontal ventilation would 1) return the fire to a fuel controlled state or 2) improve interior conditions. In a word, no, increased horizontal ventilation without concurrent fire control simply increased the heat release rate (sufficient for the fire to transition through flashover to a fully developed stage) in the involved compartment.

Examining thermal conditions in other areas of the building also provides an interesting perspective on these two sets of experiments. Figure 7 illustrates temperatures at 0.91 m (3’) during Experiment 1 (door only) and Experiment 14 (door and four windows) in the one-story house.

Figure 7. Temperatures at 0.91 m (3’) during Experiments 1 and 14

Note. Adapted from Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (p. 99, p. 162), by Steven Kerber, 2011, Northbrook, IL: Underwriters Laboratories.

Thermal conditions not only worsened in the fire compartment, but also along the flow path (for a more detailed discussion of flow path, see UL Tactical Implications Part 7) and in downstream compartments. Temperature in the hallway increased from a peak of just over 200^o C to approximately 900^o C when ventilation was increased by opening the four additional windows.

Unplanned Ventilation

Each of the experiments in this study were designed to examine the impact of tactical ventilation when building ventilation was limited to normal leakage and fire conditions are ventilation controlled (decay stage). In each of these experiments, increased ventilation resulted in a rapid increase in heat release rate and temperature. Even when ventilation was increased substantially (as in Experiments 14 and 15), it was not possible to return the fire to a fuel controlled burning regime.

It is also possible that a door or window will be left open by an exiting occupant or that the fire may cause window glazing to fail. The impact of these types of unplanned ventilation will have an effect on fire development. Creation of an opening prior to the fire reaching a ventilation controlled burning regime will potentially slow fire progression. However, on the flip side, providing an increased oxygen supply will allow the fire to continue to grow, potentially reaching a heat release rate that will result in flashover. If the opening is created after the fire is ventilation controlled, the results would be similar to those observed in each of these experiments. When the fire is ventilation controlled, increased ventilation results in a significant and dramatic increase in heat release rate and worsening of thermal conditions inside the building.

If the fire has self-ventilated or an opening has been created by an exiting occupant, the increased ventilation provided by creating further openings without concurrent fire control will result in a higher heat release rate than if the openings were not present and will likely result in rapid fire progression.

What’s Next?

I will be at UL the week after next and my next post will provide an update on UL’s latest research project examining the influence of vertical ventilation on fire behavior in legacy and contemporary residential construction.

Two tactical implications from the horizontal ventilation study remain to be examined in this series of posts: the impact of closed doors on tenability and the interesting question can you push fire with stream from a hoseline?

The last year has presented a challenge to maintaining frequency of posts to the CFBT Blog. However, I am renewing my commitment to post regularly and will be bringing back Reading the Fire, continuing examination of fundamental scientific concepts, and integration of fire control and ventilation tactics.

References

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

The seventh tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is the influence of changes in ventilation on flow path.

“Every new ventilation opening provides a new flow path to the fire and vice versa. This could create very dangerous conditions when there is a ventilation limited fire” (Kerber, 2011).

Air Track and Flow Path

Air track and flow path are closely related and provide an excellent framework for understanding the influence of changes in ventilation on fire development and flow path.

Air Track: Closely related to flow path, air track is the movement of air and smoke as observed from the exterior and inside the structure. Air track is used to describe a group of fire behavior indicators that includes direction of smoke movement at openings (e.g., outward, inward, pulsing), velocity and turbulence, and movement of the lower boundary of the upper layer (e.g., up, down, pulsing).

Observation of air track indicators may provide clues as to the potential flow path of air and hot gases inside the fire building. As discussed in previous posts in this series (Part 1, Part 2, Part 3, Part 4, Part 5, Part 6), movement of air to the fire has a major impact on fire development. Movement of hot gases away from the fire is equally important!

Flow Path: In a compartment fire, flow path is the course of movement hot gases between the fire and exhaust openings and the movement of air towards the fire.

Both of these components of flow path are important! Movement of hot gases between the fire an exhaust openings is a major factor in heat transfer outside the compartment of origin and presents a significant thermal threat to occupants and firefighters. When the fire is in a ventilation controlled burning regime, movement of air from to the fire provides the oxygen necessary for fire growth and increased heat release rate (impacting on conditions in the flow path downstream from the fire.

Flow path can significantly influence fire spread and the hazard presented to occupants and firefighters.

Reading the Fire

Before engaging in the meat of this UL Tactical Implication, quickly review essential air track indicators used in the Building, Smoke, Air Track, Heat, and Flame (B-SAHF) fire behavior indicators organizing scheme.

Figure 1. Air Track Indicators

As illustrated in Figure 1, key indicators include wind direction and velocity (consider this before you even arrive on-scene), directions in which the air and smoke are moving, and the velocity and flow of smoke and air movement.

Take a look at Figure 2. Consider all of the B-SAHF indicators, but pay particular attention to Air Track. What is the current flow path? How might the flow path change if one or more windows on Floor 2 Side A are opened prior to establishing fire control?

Figure 2. Residential Fire in a 1 ½ Story Wood Frame Dwelling

Photo courtesy of Curt Isakson, County Fire Tactics

UL Focus on Flow Path

Tactical implications related to flow path identified in Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) focus on creation of additional openings and changes in flow path as a result of “crews venting as the go” (p. 296). This is only one issue related to flow path!

The UL experiments showed that increasing the number of flow paths resulted in higher peak temperatures, a faster transition from decay to growth stage and more rapid transition to flashover. However, this is not the only hazard!

As previously discussed in the series of posts examining the fire in a Washington DC townhouse that took the lives of Firefighters Anthony Phillips and Louis Matthews, operating in the flow path presents potential for significant thermal hazard.

In this incident, the initial attack crew was operating on the first floor of a two-story townhouse with a daylight basement. When crews opened the sliding glass doors in the basement (on Side C), a flow path was created between the opening at the basement level on Side C, up an open interior stairway to the first floor, and out the first floor doorway (on Side A). Firefighters working in this flow path were subjected to extreme thermal stress, resulting in burns that took the lives of Firefighters Phillips and Mathews and serious injuries to another firefighter.

Figure 1. Perspective View of 3146 Cherry Road and Location of Slices

Note: From Simulation of the Dynamics of the Fire at 3146 Cherry Road NE Washington D.C., May 30, 1999, NISTR 6510 (p. 15) by Dan Madrzykowski and Robert Vettori, 2000, Gaithersburg, MD: National Institute for Standards and Technology.

Figure XX illustrates thermal conditions, velocity and oxygen concentration at various locations within the flow path.

Figure 10. Perspective Cutaway, Flow/Temperature, Velocity, and O₂ Concentration

The temperature of the atmosphere (i.e., smoke and air) is a significant concern in the fire environment, and firefighters often wonder or speculate about how hot it was in a particular fire situation. However, gas temperature in the fire environment is a bit more complex than it might appear on the surface and is only part of the thermal hazard presented by compartment fire.

Convective heat transfer is influenced by gas temperature and velocity. When hot gases are not moving or the flow of gases across a surface (such as your body or personal protective equipment) is slow, energy is transferred from the gases to the surface (lowering the temperature of the gases, while raising surface temperature). These lower temperature gases act as an insulating layer, slowing heat transfer from higher temperature gases further away from the surface. When velocity increases, cooler gases (which have already transferred energy to the surface) move away and are replaced by higher temperature gases. When velocity increases sufficiently to result in turbulent flow, hot gases remain in contact with the surface on a relatively constant basis, increasing convective heat flux.

For a more detailed discussion of this incident and the influence of radiative and convective heat transfer in the flow path, see the prior posts on the Washington DC Townhouse Fire Case Study.

• Fire Behavior Case Study Townhouse Fire: Washington, DC
• Townhouse Fire: Washington DC-What Happened
• Townhouse Fire: Washington DC-Extreme Fire Behavior
• Townhouse Fire: Washington DC-Computer Modeling
• Townhouse Fire: Washington DC-Computer Modeling Part 2

Wind Driven Fires & Flow Path

While operating in the flow path presents serious risk, when fire behavior is influenced by wind, conditions in the flow path can be even more severe. In experiments conducted by the National Institute of Standards and Technology (NIST) demonstrated that under wind driven conditions, both temperature and heat flux, which were twice as high in the “flow” portion of the corridor as opposed to the “static” portion of the corridor (where there was no flow path). See the previous posts on Wind Driven Fires for more information on flow path hazards under wind driven conditions:

• Wind Driven Fires
• NIST Wind Driven Fire Experiments: Establishing a Baseline
• NIST Wind Driven Fire Experiments: Anti-Ventilation and Wind Control Devices
• NIST Wind Driven Fire Experiments: Anti-Ventilation and Wind Control Devices Part 2

Discussion

The sixth and seventh tactical implications identified in the UL Horizontal Ventilation Study are interrelated and can be expanded to include the following key points:

• Heat transfer (convective and radiative) is greatest along the flow path between the fire and exhaust opening.
• Exhaust openings located higher than the fire will increase the velocity of gases along the flow path (further increasing convective heat transfer).
• Flow of hot gases from the fire to an exhaust opening is significantly influenced by air flow from inlet openings to the fire (the greater the inflow of air, the higher the heat release rate and flow of hot gases to the exhaust opening).
• Flow path can be created by a single opening that serves as both inlet and exhaust (such as an open door or window).
• Thermal conditions in the flow path can quickly become untenable for both civilian occupants and firefighters. As noted in an earlier NIST Study examining wind driven fires, under wind driven conditions this change can be extremely rapid.
• Closing an inlet, exhaust opening, or introducing a barrier (such as a closed door) in the flow path slows gas flow and reduces the hazard downstream from the barrier.
• When the fire is ventilation controlled, limiting inflow of air (e.g., door control) can slow the increase in heat release rate and progression to a growth stage fire.
• Multiple openings results in multiple flow paths and increased air flow to the fire, resulting in more rapid fire development and increased heat release rate.

What’s Next?

The next tactical implication identified in the UL Horizontal Ventilation study examines an interesting question: Can you vent enough (to return the fire to a fuel controlled burning regime)? This question may also be restated as can you perform sufficient natural horizontal ventilation to improve internal conditions. The answer to this question will likely be extended through the Vertical Ventilation Study that will be conducted by UL in early 2012!

References

District of Columbia (DC) Fire & EMS. (2000). Report from the reconstruction committee: Fire at 3146 Cherry Road NE, Washington DC, May 30, 1999. Washington, DC: Author.

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

Madrzykowski, D. & Kerber, S. (2009). Fire Fighting Tactics Under Wind Driven Conditions. Retrieved (in four parts) February 28, 2009 from http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part1.pdf; http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part2.pdf;http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part3.pdf;http://www.nfpa.org/assets/files//PDF/Research/Wind_Driven_Report_Part4.pdf.

Madrzykowski, D. & Vettori, R. (2000). Simulation of the Dynamics of the Fire at 3146 Cherry Road NE Washington D.C., May 30, 1999, NISTR 6510. August 31, 2009 from http://fire.nist.gov/CDPUBS/NISTIR_6510/6510c.pdf

National Institute for Occupational Safety and Health (NIOSH). (1999). Death in the line of duty, Report 99-21. Retrieved August 31, 2009 from http://www.cdc.gov/niosh/fire/reports/face9921.html

The sixth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) identifies potential hazards and risks related to the tactic of Vent Enter Search (VES).

Kerber (2011) provides a straightforward explanation of this tactic.

It can be described as ventilating a window, entering through the window, searching that room and exiting out the same window entered…A primary of objective of VES is to close the door of the room ventilated to isolate the flow path…

Origins and Context

While it is difficult to identify or isolate the origins of many fireground tactics, VES has been practiced by FDNY for many years and is described in detail in the Firefighting Procedures Volume 3, Book 4: Ladder Company Operations at Private Dwellings manual (FDNY, 1997). As described in Ladder Company Operations at Private Dwellings, FDNY truck companies are staffed with an officer, apparatus operator, and four firefighters and are divided into two teams; inside team and outside team. While tactics are dependent on the type of structure and fire conditions; VES is performed by the outside team while the inside team works in conjunction with an engine company, supporting fire attack and searching from the interior. In this context, VES is part of a coordinated tactical operation.

It is also important to recognize the impact of changes in the fire environment since the development of this tactic (likely in the 1960s). Changes in the speed of fire development are graphically illustrated in the Underwriters Laboratories (UL) test of fire development with modern and legacy furnishings.

The tremendous fuel load, development of ventilation limited conditions, and rapid transition from tenable to untenable conditions for firefighters following increased ventilation (without initiation of fire control), reduce the time for Firefighters to make entry and control the door when performing VES in the modern fire environment.

Influence on Fire Behavior

The following incident illustrates the rapid changes in conditions that can result during VES operations. This information was originally presented in the post titled Criticism Versus Critical Thinking. This incident involved VES at a residential structure where rapid fire progress required the Captain conducting the search to perform emergency window egress from a second floor window onto a ladder.

Companies were dispatched to a residential fire at 0400 hours with persons reported. On arrival, cars were observed in the driveway and neighbors reported the likely location of a trapped occupant on the second floor.

Given fire conditions on Floor 1, the Captain of the first in truck, a 23 year veteran, determined that Vent, Enter, and Search (VES) was the best option to quickly search and effect a rescue.

The following video clip illustrates conditions encountered at this residential fire:

Find more videos like this on firevideo.net

In his, vententersearch.com post Captain Van Sant provided the following information about his observations and actions:

When we vent[ed] the window with the ladder, it looks like the room is burning, but the flames you see are coming from the hallway, and entering through the top of the bedroom doorway. Watch it again and you’ll see the fire keeps rolling in and across the ceiling.

When I get to the window sill, the queen-sized bed is directly against the window wall, so there is no way to “check the floor” … Notice that you continue to see my feet going in, because I’m on the bed.

Believe me, in the beginning, this was a tenable room both for me and for any victim that would have been in there…

My goal was to get to the door and close it, just like VES is supposed to be done. We do it successfully all the time.

When I reached the other side of the bed, I dropped to the floor and began trying to close the door. Unfortunately, due to debris on the floor, the door would not close [emphasis added].

Conditions were still quite tenable at this point, but I knew with the amount of fire entering at the upper level, and smoke conditions changing, things were going to go south fast…

I kept my eyes on my exit point, and finished my search, including the closet, which had no doors on it. Just as I was a few feet from the window, the room lit off…

Tactical Considerations

Is VES an appropriate tactic for primary search in private dwellings? This question must be placed into operational context bounded by fire dynamics, resources and staffing, experience level of the firefighters involved, potential for survivable occupants, and the fireground risk management philosophy of the department. Consider the following:

• There have been instances where VES has resulted in saving of civilian life.
• There have been instances where VES has resulted in significant thermal injury to firefighters.
• The UL ventilation tests (Kerber, 2011) demonstrate that conditions rapidly become untenable for civilian occupants in rooms with open doors. Rooms with closed doors remain tenable for civilian occupants for a considerable time.
• VES may result in rapid search of specific threatened areas.
• VES is a high risk tactic that involves working alone (but if a second firefighter remains at the entry point this is similar to oriented search).
• VES (as normally practiced) involves working without a hoseline.
• VES changes the ventilation profile and places firefighters in the flow path between the fire and an exhaust opening (unless or until the door to the compartment is closed)
• As demonstrated in the UL ventilation tests (Kerber, 2011), thermal conditions change from a tenable operating environment for firefighters to untenable and life threatening in a matter of seconds.

Based on these factors, you may determine that VES is not an appropriate tactic for primary search under any circumstances, or you may determine that it might be appropriate under specific circumstances. The following tactical scenarios may provide a framework for discussion of these issues.

Scenario 1:You have responded to a fire in a medium sized, two-story, wood frame, single-family dwelling at 02:13 hours. You observe a smoke issuing at moderate velocity from the eaves and condensed pyrolizate on the inside of window glazing. A dull reddish glow can be observed through several adjacent windows on the Charlie Side (back of the house), Floor 1.

Given your normal first alarm assignment and staffing Is VES an appropriate option for primary search given the conditions described and potential for possible occupants? Why or why not?

Scenario 2: You have the same building, smoke, air track, heat, and flame indicators as in Scenario 1, but a female occupant meets you on arrival and reports that her husband is trying to rescue their daughter who was sleeping in a bedroom on Floor 2 at the Alpha/Bravo corner of the house.

Given your normal first alarm assignment and staffing Is VES an appropriate option for primary search given the conditions described and reported occupants?  Why or why not?

Scenario 3: You have the same building, smoke, air track, heat, and flame indicators as in Scenario 1, and observe two occupants, an adult male and a female child in a window on Floor 2 at the Alpha/Bravo corner of the house. Smoke at low velocity is issuing from the open window above the occupants. However, before you can raise a ladder to rescue the occupants in the window, they disappear from view and the volume and velocity of smoke discharge from the window increases.

Given your normal first alarm assignment and staffing Is VES an appropriate option for primary search given the conditions described and initial observation of occupants? Why or why not?

Vertical Ventilation Study

UL has commenced a study on the Effectiveness of Vertical Ventilation and Fire Suppression Tactics using the same legacy and contemporary residential structures used in their study of horizontal ventilation. This research project will examine a range of vertical ventilation variables including the size, location, and timing of openings. In addition, further research will be conducted on the effectiveness of exterior streams and their impact on interior conditions.

Preliminary design parameters for the study were developed in conjunction with a technical panel representing a wide range of jurisdictions and types of fire service agencies, including:

• Atlanta Fire Department (GA)
• Central Whidbey Island Fire & Rescue (WA)
• Chicago Fire Department (IL)
• Cleveland Fire Department (OH)
• Coronado Fire Department (CA)
• Fire Department of the City of New York (FDNY) (NY)
• Loveland-Symmes Fire Department (OH)
• National Institute of Standards and Technology (NIST) (MD)
• Northbrook Fire Department (IL)
• Milwaukee Fire Department (WI)
• Lake Forest Fire Department (IL)
• Phoenix Fire Department (AZ)

Full scale tests are anticipated to begin in January 2012. I will provide updates as this research project progresses.

References

Fire Department of the City of New York (FDNY). (1997) Firefighting prodedures volume 3 book 4: Ladder company operations at private dwellings. New York: Author.

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

The fifth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is described as failure of the smoke layer to lift following horizontal natural ventilation and smoke tunneling and rapid air movement in through the front door.

In the experiments conducted by UL, both the single and two story dwellings filled rapidly with smoke with the smoke layer reaching the floor prior to ventilation. This resulted in zero visibility throughout the interior (with the exception of the one bedroom with a closed door). After ventilation, the smoke layer did not lift (as many firefighters might anticipate) as the rapid inward movement of air simply produced a tunnel of clear space just inside the doorway.

Put in the context of the Building, Smoke, Air Track, Heat, and Flame (B-SAHF) fire behavior indicators, these phenomena fit in the categories of smoke and air track. Why did these phenomena occur and what can firefighters infer based on observation of these fire behavior indicators?

Smoke Versus Air Track

There are a number of interrelationships between Smoke and Air Track. However, in the B-SAHF organizing scheme they are considered separately. As we begin to develop or refine the map of Smoke Indicators it is useful to revisit the difference between these two categories in the B-SAHF scheme.

Smoke: What does the smoke look like and where is it coming from? This indicator can be extremely useful in determining the location and extent of the fire. Smoke indicators may be visible on the exterior as well as inside the building. Don’t forget that size-up and dynamic risk assessment must continue after you have made entry!

Air Track: Related to smoke, air track is the movement of both smoke (generally out from the fire area) and air (generally in towards the fire area). Observation of air track starts from the exterior but becomes more critical when making entry. What does the air track look like at the door? Air track continues to be significant when you are working on the interior.

Smoke Indicators

There are a number of smoke characteristics and observations that provide important indications of current and potential fire behavior. These include:

• Location: Where can you see smoke (exterior and interior)?
• Optical Density (Thickness): How dense is the smoke? Can you see through it? Does it appear to have texture like velvet (indicating high particulate content)?
• Color: What color is the smoke? Don’t read too much into this, but consider color in context with the other indicators.
• Physical Density (Buoyancy): Is the smoke rising, sinking, or staying at the same level?
• Thickness of the Upper Layer: How thick is the upper layer (distance from the ceiling to the bottom of the hot gas layer)?

As discussed in Reading the Fire: Smoke Indicators Part 2, these indicators can be displayed in a concept map to show greater detail and their interrelationships (Figure 1).

Figure 1. Smoke Indicators Concept Map

Air Track

Air track includes factors related to the movement of smoke out of the compartment or building and the movement of air into the fire. Air track is caused by pressure differentials inside and outside the compartment and by gravity current (differences in density between the hot smoke and cooler air). Air track indicators include velocity, turbulence, direction, and movement of the hot gas layer.

• Direction: What direction is the smoke and air moving at specific openings? Is it moving in, out, both directions (bi-directional), or is it pulsing in and out?
• Wind: What is the wind direction and velocity? Wind is a critical indicator as it can mask other smoke and air track indicators as well as serving as a potentially hazardous influence on fire behavior (particularly when the fire is in a ventilation controlled burning regime).
• Velocity & Flow: High velocity, turbulent smoke discharge is indicative of high temperature. However, it is essential to consider the size of the opening as velocity is determined by the area of the discharge opening and the pressure. Velocity of air is also an important indicator. Under ventilation controlled conditions, rapid intake of air will be followed by a significant increase in heat release rate.

As discussed in Reading the Fire: Air Track Indicators Part 2, these indicators can be displayed in a concept map to show greater detail and their interrelationships (Figure 2).

Figure 2. Air Track Indicators Concept Map

air t

Discharge of smoke at openings and potential openings (Building Factors) is likely the most obvious indicator of air track while lack of smoke discharge may be a less obvious, but equally important sign of inward movement of air. Observation and interpretation of smoke and air movement at openings is an essential part of air track assessment, but it must not stop there. Movement of smoke and air on the interior can also provide important information regarding fire behavior.

An Ongoing Process

Reading the fire is an ongoing process, beginning with reading the buildings in your response area prior to the incident and continuing throughout firefighting operations. It is essential to not only recognize key indicators, but to also note changing conditions. This can be difficult when firefighters and officer are focused on the task at hand.

UL Experiment 13

This experiment examined the impact of horizontal ventilation through the door on Side A and one window as high as possible on Side C near the seat of the fire. The family room was the fire compartment. This room had a high (two-story) ceiling with windows at ground level and the second floor level (see Figure 3).

Figure 3. Two-Story Dwelling

In this experiment, the fire was allowed to progress for 10:00 after ignition, at which point the front door (see Figure 3) was opened to simulate firefighters making entry. Fifteen seconds after the front door was opened (10:15), an upper window in the family room (see Figure 3) was opened. No suppression action was taken until 12:28, at which point a 10 second application of water was made through the window on Side C using a straight stream from a combination nozzle.

As with all the other experiments in this series fire development followed a consistent path. The fire quickly consumed much of the available oxygen inside the building and became ventilation controlled. At oxygen concentration was reduced, heat release rate and temperature within the building also dropped. Concurrently, smoke and air track indicators visible from the exterior were diminished. Just prior to opening the door on Side A, there was little visible smoke from the structure (see Figure 4).

Figure 4. Experiment 13 at 00:09:56 (Prior to Ventilation)

As illustrated in Figure 5, a bi-directional air track was created when the front door was opened. Hot smoke flowed out the upper area of the doorway while air pushed in the bottom creating a tunnel of clear space inside the doorway (but no generalized lifting of the upper layer.

Figure 5. Experiment 13 at 00:10:14 (Door Open)

As illustrated in Figure 6, opening the upper level window in the family room resulted in a unidirectional air track flowing from the front door to the upper level window in the family room. No significant exhaust of smoke can be seen at the front door, while a large volume of smoke is exiting the window. However, while the tunneling effect at floor level was more pronounced (visibility extended from the front door to the family room), there was no generalized lifting of the upper layer throughout the remainder of the building.

Figure 6. Experiment 13 at 00:10:21 (Door and Window Open)

With the increased air flow provided by ventilation through the door on Side A and Window at the upper level on Side C, the fire quickly transitioned to a fully developed stage in the family room. The heat release rate (HRR) and smoke production quickly exceeded the limited ventilation provided by these two openings and the air track at the front door returned to bi-directional (smoke out at the upper level and air in at the lower level) as shown in Figure 7.

Figure 7. Experiment 13 at 00:11:22 (Door and Window Open)

What is the significance of this observation? Movement of smoke out the door (likely the entry point for firefighters entering for fire attack, search, and other interior operations) points to significant potential for flame spread through the upper layer towards this opening. The temperature of the upper layer is hot, but flame temperature is even higher, increasing the radiant heat flux (transfer) to crews working below. Flame spread towards the entry point also has the potential to trap, and injure firefighters working inside.

Gas Velocity and Air Track

A great deal can be learned by examining both the visual indicators illustrated in Figures 4-7 and measurements taken of gas velocity at the front door. During the ventilation experiments conducted by UL, gas velocities were measured at the front door and at the window used for ventilation (see Figure 3). Five bidirectional probes were placed in the doorway at 0.33 m (1’) intervals. Positive values show gas movement out of the building while negative values show inward gas movement. In order to provide a simplified view of gas movement at the doorway, Figure 8 illustrates gas velocity 0.33 m (1’) below the top of the door, 0.33 m (1’) from the bottom of the door, and 0.66 m (2’) above the bottom of the door.

A bidirectional (out at the top and in at the bottom) air track developed at the doorway before the door was opened (see Figure as a result of leakage at this opening. It is interesting to note variations in the velocity of inward movement of air from the exterior of the building, likely a result of changes in combustion as the fire became ventilation controlled. The outward flow at the upper level resulted in visible smoke on the exterior of the building. While not visible, inward movement of air was also occurring (as shown by measurement of gas velocity at lower levels in the doorway.

Creation of the initial ventilation opening by opening the front door created a strong bidirectional air track with smoke pushing out the top of the door while air rapidly moved in the bottom. Had the door remained the only ventilation opening, this bidirectional flow would have been sustained (as it was in all experiments where the door was the only ventilation opening).

Opening the upper window in the family room resulted in a unidirectional flow inward through the doorway. However, this phenomenon was short lived, with the bidirectional flow reoccurring in less than 60 seconds. This change in air track resulted from increased heat release rate as additional air supply was provided to the fire in the family room.

Figure 8. Front Door Velocities

Note: Adapted from Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (p. 243), by Stephen Kerber, Northbrook, IL: Underwriters Laboratories, 2011.

While not the central focus of the UL research, these experiments also examined the effects of exterior fire stream application on fire conditions and tenability. Each experiment included a 10 second application with a straight stream and a 10 second application of a 30^o fog pattern. Between these two applications, fire growth was allowed to resume for approximately 60 seconds.

The straight stream application resulted in a reduction of temperature in the fire compartment and adjacent compartments (where there was an opening to the family room or hallway) as water applied through the upper window on Side C (ventilation opening) cooled the compartment linings (ceiling and opposite wall) and water deflected off the ceiling dropped onto the burning fuel. As the stream was applied, air track at the door on Side A changed from bidirectional to unidirectional (inward). This is likely due to the reduction of heat release rate achieved by application of water onto the burning fuel with limited steam production.

When the fog pattern was applied, there was also a reduction of temperature in the fire compartment and adjacent compartments (where there was an opening in the family room or hallway) as water was applied through the upper window on Side C (ventilation opening) cooled the upper layer, compartment linings, and water deflected off the ceiling dropped onto the burning fuel. The only interconnected area that showed a brief increase in temperature was the ceiling level in the dining room. However, lower levels in this room showed an appreciable drop in temperature. Air track at the door on Side A changed from bidirectional to unidirectional (outward) when the fog stream was applied. This effect is likely due to air movement inward at the window on Side C and the larger volume of steam produced on contact with compartment linings as a result of the larger surface area of the fog stream.

The effect of exterior streams will be examined in more detail in a subsequent post.

Important Lessons

The fifth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is described as failure of the smoke layer to lift following horizontal natural ventilation and smoke tunneling and rapid air movement in through the front door.

Additional lessons that can be learned from this experiment include:

• Ventilating horizontally at a high point results in higher flow of both air and smoke.
• Increased inward air flow results in a rapid increase in heat release rate.
• The rate of fire growth quickly outpaced the capability of the desired exhaust opening, returning the intended inlet to a bi-directional air track (potentially placing firefighters entering for fire attack or search at risk due to rapid fire spread towards their entry point).

Tactical applications of this information include:

• Ensure that the attack team is in place with a charged line and ready to (or has already) attack the fire (not simply ready to enter the building) before initiating horizontal ventilation.
• Cool the upper layer any time that it is above 100^o C (212^o F) to reduce radiant and convective heat flux and to limit potential for ignition and flaming combustion in the upper layer.

Note that this research project did not examine the impact of gas cooling, but examination of the temperatures at the upper levels in this experiment (and others in this series) point to the need to cool hot gases overhead.

What’s Next?

I am on the hunt for videos that will allow readers to apply the tactical implications of the UL study that have been examined to this point in conjunction with the B-SAHF fire behavior indicators. My next post will likely provide an expanded series of exercises in Reading the Fire.

The next tactical implication identified in the UL study (Kerber, 2011) examines the hazards encountered during Vent Enter Search (VES) tactical operations. A subsequent post will examine this tactic in some detail and explore this tactical implication in greater depth.

References

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

The fourth tactical implication identified in the Underwriters Laboratories study of the Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction (Kerber, 2011) is that fire attack and (tactical) ventilation must be coordinated. This recommendation has been repeated in National Institute for Occupational Safety and Health (NIOSH) Death in the Line of Duty Reports for many years. In fact, most reports on firefighter fatalities related to rapid fire progression contain this recommendation.

Importance of Coordination

Coordination of (tactical) ventilation and fire attack as a tactical implication is closely related to the first two tactical implications identified in the UL study; potential changes in fire behavior based on stages of fire development, burning regime, and changes in ventilation profile that increase oxygen supplied to the fire.

If air is added to the fire and water is not applied in the appropriate time frame the fire gets larger and the hazards to firefighters increase. Examining the times to untenability provides the best case scenario of how coordinated the attack needs to be. Taking the average time for every experiment from the time of ventilation to the time of the onset of firefighter untenability conditions yields 100 seconds for the one-story house and 200 seconds for the two-story house. In many of the experiments from the onset of firefighter untenability until flashover was less than 10 seconds. These times should be treated as very conservative. If a vent location already exists because the homeowner left a window or door open then the fire is going to respond faster to additional ventilation openings because the temperatures in the house are going to be higher at the time of the additional openings (Kerber, 2011, p. 289-290)

The Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction Underwriters Laboratories (UL) on-line course and report provide an example of firefighters are at risk when ventilation is performed prior to entry, fire attack is delayed, and other tactical operations such as primary search are initiated.

In UL’s hypothetical example, the firefighters make entry into the one-story house, search the living room (fire compartment), the kitchen, and dining room shortly after forcing the door and ventilating a large window in the fire compartment. Consider a somewhat different scenario, with the same fire conditions.

Companies respond to a residential fire with persons reported during the early morning hours. A truck and engine arrive almost simultaneously and while the engine lays a supply line from a nearby hydrant, the truck company forces entry, ventilates a window on Side A, and begins primary search (anticipating that the engine crew will be right behind them to attack the fire). The engine completes a forward lay and begins to stretch an attack line after the search team has made entry.

Figure 1. Timeline and Progression of Primary Search

Figure 2. View of the Living Room (Fire Compartment) from the Door on Side A

As illustrated in Figure 3, visible flaming combustion when the door is opened at 08:00 is limited to a small flame from the top of the couch just inside the door on Side A. However, in the 30 seconds that it takes for the search team to make entry, flaming combustion has resumed and flames are near or at the ceiling above the couch. The search team may estimate that they have time to complete a quick search of the bedrooms (likely location of the reported persons). However, fire development progresses to untenable conditions within a minute, trapping the crew on Side D of the house.

Figure 3. Fire Progression in the Living Room 00:08:00 to 00:10:00

As the search team completes primary search of Bedroom 2 and moves towards Bedroom 3 in the hallway, conditions have deteriorated to an untenable level. Figure 4 illustrates the change in temperature at the 3’ level in the Living Room (fire compartment). Shortly before the search team reached Bedroom 2, fire conditions in the living room began to change dramatically, with temperature at the 3’ level transitioning from ordinary to extreme, quickly becoming untenable in the living room, hallway and adjacent compartments. In addition to this significant change in temperature, flames (with temperatures higher than the gas temperature at the 3’ level) significantly increase radiant heat transfer (flux) to the surface of both fuel packages and firefighters protective equipment.

Figure 4. Temperature at the 3’ Level

Note: Figure 4 illustrates temperature conditions starting eight minutes after ignition. The fire previously progressed through incipient and growth stages before beginning to decay due to lack of ventilation.

Why the Dramatic Change in Conditions?

As discussed in UL Tactical Implications Part 1, Fires in the contemporary environment progress from ignition and incipient stage to growth, but often become ventilation controlled and begin to decay, rather than continuing to grow into a fully developed fire. This ventilation induced decay continues until the ventilation profile changes (e.g., window failure due to fire effects, opening a door for entry or egress, or intentional creation of ventilation openings by firefighters. When ventilation is increased, heat release rate again rises and temperature climbs with the fire potentially transitioning through flashover to the fully developed stage (see Figure 4 and 5).

Figure 5. Fire Development in a Compartment

Captain James Mendoza of the San Jose (CA) Fire Department and CFBT-US Lead Instructor demonstrates the influence of ventilation on fire development using a small scale prop developed by Dr. Stefan Svensson of the Swedish Civil Contingencies Agency.

The prop used in this demonstration is a small, single compartment with a limited ventilation opening on the right side (which in a full size building could be represented by normal building leakage or a compartment opening that is restricted such as a partially open door or window). The front wall of the prop is ceramic glass to permit direct observation of fire conditions within the compartment.

As you watch this demonstration, pay particular attention to how conditions change as the fire develops and then enters the decay stage. In addition, observe how quickly the fire returns to the growth stage and develops conditions that would be untenable after the window is opened at 12:17.

Download Doll’s House Plans (or Doll’s House Plans: Metric) for directions on how to construct a similar small scale prop.

Fire development and changes in conditions following ventilation in this demonstration mirror those seen in the full scale experiments conducted by UL. Increasing ventilation to a ventilation controlled fire, results in increased heat release rate and transition from decay to the growth stage of fire development.

The same phenomena can be observed under fireground conditions in the following video clip of a residential fire in Dolton, Illinois (this is a long video, watch the first several minutes to observe the changes in fire behavior).

It appears that the front door was open at the start of the video clip and the large picture window on Side A was ventilated at approximately 00:47. Fire conditions quickly transition to the growth stage with flames exiting the window and door, causing firefighters on an uncharged hoseline that had been advanced into Floor 1, to quickly withdraw.

As discussed in UL Tactical Implications: Part 1:

• Fires that have progressed beyond the incipient stage are likely to be ventilation controlled when the fire department arrives.
• Ventilation controlled fires may be in the growth, decay, or fully developed stage.
• Regardless of the stage of fire development, when a fire is ventilation controlled, increased ventilation will always result in increased HRR.
• Firefighters and fire officers must recognize that the ventilation profile can change (e.g., increasing ventilation) as a result of tactical action or fire effects on the building (e.g., window failure).
• Firefighters and fire officers must anticipate potential changes in fire behavior related to changes in the ventilation profile and ensure that fire attack and ventilation are closely coordinated.

Coordinated Tactical Operations

Understanding how fire behavior can be influenced by changes in ventilation is essential. But how can firefighters put this knowledge to use on the fireground and what exactly does coordination of tactical ventilation and fire attack really mean?

Tactical ventilation can be defined as the planned, systematic, and coordinated removal of hot smoke and fire gases and their replacement with fresh air. Each of the elements of this definition is important to safe and effective tactical operations.

Ventilation (both tactical and unplanned) not only removes hot smoke, but it also introduces fresh air which can have a significant effect on fire behavior.

Tactical ventilation must be planned; these two elements speak to the intentional nature of tactical ventilation. Tactics to change the ventilation profile must be intended to influence the fire environment or fire behavior in some way (e.g., raise the level of the upper layer to increase visibility and tenability). The ventilation plan must also consider the flow path (e.g., vent ahead of, not behind, the attack team; vent in the immediate area of the fire, not at a remote location).

Tactical ventilation must be systematic, exhaust openings should generally be made before inlet openings (particularly when working with positive pressure ventilation or when taking advantage of wind effects).

And as pointed out in the UL Study (Kerber, 2011), tactical ventilation must be coordinated. Coordination of ventilation and other tactical operations requires consideration of sequence and timing:

Sequence: Ventilation may be completed before, during, or after fire attack has been initiated. Sequence will likely depend on the stage of fire development, burning regime, time required to reach the fire.

If the fire is small and staffing is limited, it may be appropriate to control the fire and then effect ventilation (e.g., hydraulic ventilation performed by the attack team). This approach minimizes potential fire growth,

In general, when the fire is ventilation controlled (as those beyond the incipient stage are likely to be), ventilation should not be completed unless the attack line(s) can quickly apply water to the seat of the fire. In a small, single family dwelling this may mean that the attack team is on-air, the line is charged, and the entry door is unlocked or has been forced and is being controlled (held closed). In a larger building, this may mean that the attack line has entered the structure and is in position to move onto the fire floor or into the fire area.

The key questions that must be answered prior to implementing tactical ventilation are:

1. What influence will these ventilation tactics have on fire behavior?
2. Are charged and staffed attack line(s) in place?
3. Will the attack team(s) be able to quickly reach the fire?
4. How will this impact crews operating on the interior of the building?

Coordination requires clear, direct communication between companies or crews assigned to ventilation, fire attack, and other tactical functions that are or will be taking place inside the building.

Important: While not a tactical implication directly raised by the UL study, another important consideration is the hazard of working without or ahead of the hoseline. While a controversial topic in the US fire service (where truck company personnel generally work on the interior without a hoseline), searching with a hoseline provides a means of protection and a defined exit path. Staffing is another key element of the operational context. If you do not have enough personnel to control the fire and search; in most cases it is likely the best course of action to control the fire and ensure a safer operating environment for search operations.

What’s Next?

The next tactical implication identified in the UL study (Kerber, 2011) examines information that may be obtained by reading the air track at the entry point opening. This implication will be expanded with a broader discussion of air track indicators and how related hazards can be mitigated to improve firefighter safety.

References

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf

Developing and maintaining proficiency in reading the Fire using the B-SAHF (Building, Smoke, Air Track, Heat, and Flame) organizing scheme for fire behavior indicators, requires practice. This post provides an opportunity to exercise your skills using a video segment shot during a commercial fire.

Residential Fire

This post examines fire development during a residential fire in New Chicago, Indiana.

Download and the B-SAHF Worksheet.

Watch the first 30 seconds (0:30) of the video. First, describe what you observe in terms of the Building, Smoke, Air Track, Heat, and Flame Indicators; then answer the following five standard questions?

1. What additional information would you like to have? How could you obtain it?
2. What stage(s) of development is the fire likely to be in (incipient, growth, fully developed, or decay)?
3. What burning regime is the fire in (fuel controlled or ventilation controlled)?
4. What conditions would you expect to find inside this building?
5. How would you expect the fire to develop over the next two to three minutes

In addition, consider how the answers to these questions impact your assessment of the potential for survival of possible occupants.

Now watch the video clip from 0:30 until firefighters make entry at 3:05. Now answer the following questions:

1. Did fire conditions progress as you anticipated?
2. What changes in the B-SAHF indicators did you observe?
3. How do you think that the stage(s) of fire development and burning regime will change over the next few minutes?
4. What conditions would you expect to find inside this building now?
5. How would you expect the fire to develop over the next two to three minutes

The crews working in this video appeared to achieve fire control fairly quickly and without incident. However, consider the following tactical and task related questions:

1. It did not appear that any member of the first arriving companies performed a 360o recon and size-up (they may have, but this was not visible in the video). Why might this be a critical step in size-up at a residential fire?
2. It appeared that two lines were run simultaneously (the first line to the door ended up as the back-up line, possibly due to a slight delay in charging the line). How should fire attack and backup roles be coordinated?
3. Fire attack was initiated from the interior (unburned side). What would have been the impact of the first line darkening the fire from the exterior (prior to entry)?
4. Were there any indicators of potential collapse (partial) of the roof? How would you manage this risk when working in a lightweight wood frame residence with observed extension into the trussloft? What factors would influence your decision-making and actions?

Reading the Fire

See the following posts for more information on reading the fire:

• Reading the Fire” How to Improve Your Skills
• Reading the Fire: Building Factors
• Reading the Fire Building Factors Part 2
• Reading the Fire: Building Factors Part 3
• Reading the Fire: Smoke
• Reading the Fire: Smoke Part 2
• Reading the Fire: Air Track
• Reading the Fire: Air Track Part 2
• Reading the Fire: Heat
• Reading the Fire: Heat Part 2
• Reading the Fire: Heat Part 3
• Reading the Fire: Flames
• Reading the Fire: Flames Part 2
• Reading the Fire: Putting it All Together
• Incipient Stage Fires: Key Fire Behavior Indicators
• Growth Stage Fires: Key Fire Behavior Indicators
• Fully Developed Fires: Key Fire Behavior Indicators
• Decay Stage Fires: Key Fire Behavior Indicator

Ed Hartin, MS, EFO, MIFireE, CFO

UL’s third tactical implication is that there may be little smoke showing when a fire initially enters the decay stage as a result of limited ventilation. These fire conditions may present similar indicators to an incipient fire. However, fire conditions and the hazards presented to firefighters are considerably different.

Visible Indications of Fire Development

In Reading the Fire: B-SAHF, I introduced Building, Smoke, Air Track, Heat, and Flame (B-SAHF) as an organizing scheme for fire behavior indicators. Use of a standardized and organized approach to reading the fire can improve our ability to assess current fire conditions and predict likely fire development and changes that may occur.

Station Officer Shan Raffel of Queensland (Australia) Fire Rescue recently published an excellent article titled The Art of Reading Fire on the FirefighterNation website that provides another view of the B-SAHF indicators and Reading the Fire.

Building factors (particularly the normal ventilation profile, size and compartmentation, and thermal characteristics) can have a significant impact on fire development and how fire conditions present from the exterior of the building. However, this UL tactical implication relates most closely to Smoke and Air Track as well as somewhat indirectly to Heat (but this is the key to understanding what is happening). First a quick review of these key indicators

Smoke: What does the smoke look like and where is it coming from? This indicator can be extremely useful in determining the location and extent of the fire. Smoke indicators may be visible on the exterior as well as inside the building.

Air Track: Related to smoke, air track is the movement of both smoke (generally out from the fire area) and air (generally in towards the fire area).

Heat: This includes a number of indirect indicators. Heat cannot be observed directly, but you can feel changes in temperature and may observe the effects of heat on the building and its contents. Visual clues such as crazing of glass and visible pyrolysis from fuel that has not yet ignited are also useful heat related indicators. Important: Temperature influences smoke and air track indicators such as volume and velocity of smoke discharge.

For a more detailed look at B-SAHF and reading the fire, see the following posts:

How to Improve Your Skills
Building Factors
Building Factors Part 2
Building Factors Part 3
Smoke Indicators
Smoke Indicators Part 2
Air Track Indicators
Air Track Indicators Part 2
Heat Indicators
Heat Indicators Part 2
Heat Indicators Part 3
Flame Indicators
Flame Indicators Part 2
Incipient Stage Fires: Key Fire Behavior Indicators
Growth Stage Fires: Key Fire Behavior Indicators
Fully Developed Fires: Key Fire Behavior Indicators
Decay Stage Fires: Key Fire Behavior Indicators

Stages of Fire Development, Burning Regime, Smoke, Air Track & Heat

While the “stages of fire” have been described differently in fire service textbooks the phenomenon of fire development is the same. For our purposes, the stages of fire development in a compartment will be described as incipient, growth, fully developed and decay (see Figure 1). Despite dividing fire development into four “stages” the actual process is continuous with “stages” flowing from one to the next. While it may be possible to clearly define these transitions in the laboratory, in the field it is often difficult to tell when one ends and the next begins.

Understanding the stages of fire development is important, but this only provides a limited picture of fire development in a compartment. Conversion of chemical potential energy from fuel depends on availability of adequate oxygen for the combustion reaction to occur. As the ambient air in the compartment provides adequate oxygen, in incipient stage and early growth stage, heat release rate is limited by the chemical and physical characteristics of the fuel. This condition is known as a fuel controlled burning regime. In a compartment fire, combustion occurs in an enclosure where the air available for combustion is limited by 1) the volume of the compartment and 2) ventilation. Ventilation in a compartment fire is limited (particularly if doors and windows are closed and intact), as the fire grows and heat release rate increases, so too does demand for oxygen. When fire growth is limited by the available oxygen, heat release rate is slowed and then diminishes. This condition is known as a ventilation controlled burning regime.

Many if not most fires that have progressed beyond the incipient stage when the fire department arrives are ventilation controlled. This means that the heat release rate (the fire’s power) is limited by the existing ventilation. If ventilation is increased, either through tactical action or unplanned ventilation resulting from effects of the fire (e.g., failure of a window) or human action (e.g., exiting civilians leaving a door open), heat release rate will increase (see Figure 1)

Figure 1. Fire Development Curve (Fuel and Ventilation Controlled Regimes)

Several things happen as a compartment fire develops: Heat release rate increases, smoke production increases, and pressure within the compartment increases proportionally to the absolute temperature. These conditions result in a number of fire behavior indicators that may be visible from the exterior of the building. As a fire moves from the Incipient to the Growth Stage, an increasing volume of smoke may be visible from the exterior (Smoke Indicator) and the velocity of smoke discharge will likely increase (Air Track Indicator and indirect Heat Indicator).

It is a reasonably logical conclusion that a smaller volume of smoke and lower velocity of smoke discharge will be observed in incipient and early growth stage fires and the volume and velocity of smoke discharge will increase as the fire develops. However, what happens when the fire becomes ventilation controlled?

Influence of Ventilation on Residential Fire Behavior

Earlier this year, Underwriters Laboratories (UL) conducted a series of full-scale experiments to determine the influence of ventilation on fire behavior in legacy and contemporary residential construction (see Did You Ever Wonder? and UL Ventilation Course).

These tests were conducted in full-scale one and two-story, wood-frame structures constructed inside the UL laboratory in Northbrook, IL. Fires in the one-story structure were all started in the living room (see Figure 2) and involved typical contents found in a single-family home.

Figure 2: One-Story Structure and Floor Plan

As discussed in UL Tactical Implications Part 1 and Part 2, each of the fires during these tests quickly became ventilation controlled due to the fuel load within the buildings and limited ventilation provided by closed and intact doors and windows.

As each fire developed, the volume of smoke visible from the exterior and velocity of smoke discharge increased. This is consistent with fire development within the structure and increasing heat release rate, temperature, and volume of smoke production from the developing fire. Figure 3 illustrates exterior conditions at 05:05 during Test 5 conducted in the one-story residence.

Figure 3: Conditions at 05:05 (UL Test 5)

Interestingly, as the fire became ventilation controlled (as determined by both measurement of oxygen concentration and the heat release rate in the building), the volume and velocity of smoke discharge decreased to a negligible level as illustrated in Figure 4.

Figure 4: Conditions at 05:34 (UL Test 5)

This change occurred within a matter of 30 seconds! How might this influence firefighters’ perception of fire conditions inside the building if they arrived at 05:34 rather than 05:04? While presenting much the same as an incipient or early growth stage fire, conditions within the building at 05:34 are significantly ventilation controlled and increased ventilation resulted in rapid fire development and transition through flashover to a fully developed fire.

NIST Phoenix Warehouse Tests

In the Hazard of Ventilation Controlled Fires, I discussed a series of tests conducted by the National Institute for Standards and Technology (NIST) at an ordinary constructed warehouse in Phoenix, AZ. These tests were intended to develop information about performance of ordinary constructed buildings related to structural collapse. However, they also provided some interesting information regarding fire behavior.

One of the tests involved a fire in the front section of the warehouse that measured 50’ x 90’ (15.2 m x 27.4 m) with a height of 15’ (4.6 m) to the top of the pitched truss roof. The fuel load for this test included four stacks of 10 wood pallets and the interior finish and combustible structural elements of the building.  All doors and windows were closed at the start of the test.

Figure 5 illustrates a large volume of dark gray to black smoke discharging from the roof of the structure and flames visible from roof ventilators at 03:57. The B-SAHF indicators visible n this photo indicate a significant growth stage fire within this building.

Figure 5. Conditions at 03:57 (NIST Warehouse Test)

However, at 05:31 conditions visible on the exterior are quite different. The color and volume of smoke discharge (Smoke Indicators) as well as the velocity of discharge (Air Track Indicator) may lead firefighters to believe that this is an incipient or early growth stage fire. Nothing could be further from the truth. This fire is in the decay stage as a result of limited ventilation and any increase in ventilation will result in a rapid and significant increase in heat release rate!

Figure 6. Conditions at 05:31 (NIST Warehouse Test)

For more information on these tests see Structural Collapse Fire Tests: Single Story, Ordinary Construction Warehouse (Stroup, Madrzykowski, Walton, & Twilley, 2003) or view the videos of this series of tests at the NIST Structural Collapse webpage.

The Key

Heat release rate and temperature drop as the fire becomes ventilation controlled. Volume and velocity of smoke discharge are a function of pressure (given a constant opening size). Reduction in temperature and corresponding reduction in pressure will result in a smaller volume and lower velocity of smoke discharge.

When the temperature is the same, the velocity of discharge will likely be similar. Figure 1 shows that the temperature inside a compartment may be the same during the growth and decay stages of the fire. If the ventilation profile (number, size, and location of openings) remains the same, similar Smoke and Air Track indicators can be present.

Decay stage incidators may be subtle. Consider the full range of B-SAHF inciators that may be observed under ventilation controlled, decay stage conditions (see Figure 7.

Figure 7. B-SAHF Decay Stage Indicators.

Durango, Colorado Commercial Fire

CFBT-US developed a case study examining an extreme fire behavior event that occurred during a commercial fire in Durango, CO in 2008 injuring nine firefighters and fire officers. The reporting party indicated that there was a large amount of dark smoke coming from the roof of the building. However, when firefighters arrived, they found nothing showing but a small amount of light colored smoke. Why might this have been the case?

Download a copy of the Fire Behavior Case Study: Durango CO Commercial Fire and see if this may have been a result of similar fire development and presentation of fire behavior indicators as seen in the UL and NIST tests!

Ed Hartin, MS, EFO, MIFireE, CFO

References

Kerber, S. (2011). Impact of ventilation on fire behavior in legacy and contemporary residential construction. Retrieved July 16, 2011 from http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/ventilation/DHS%202008%20Grant%20Report%20Final.pdf.

Stroup, D., Madrzykowski, D., Walton, W., & Twilley, W. (2003). Structural collapse fire tests: Single story, ordinary construction warehouse, NISTIR 6959. Retrieved July 16, 2011 from http://www.nist.gov/customcf/get_pdf.cfm?pub_id=861215