Series of articles on Firefighter Issues
that have appeared in Boston Firefighter’s Local 718 Digest.
- Flashover Information
- Truss Construction
FLASHOVER TRAINING IN THE FIREHOUSE
It is more important than ever for all members of the fire service to discuss dangers of flashover.
- Fires today are growing faster than 20 years ago because of plastic which burns hotter.
- Houses are often insulated allowing a faster build up of heat inside the room of origin.
- Bunker gear often makes it possible for us to go further into a fire without noticing dangerous conditions.
- Smoke alarms are allowing the fire service to be notified sooner, allowing us to arrive before flashover as opposed to after flashover.
As you can see from the diagram below, in a typical fire flashover is likely to occur just about the time we may be arriving at a fire.
A flashover occurs at the stage of a fire where all surfaces and objects within a space have been heated to their ignition temperature, and flame breaks out almost at once over the surface of all objects in the space. The temperature is between 932 to 1,112 degrees Fahrenheit, although flashover temperatures can peak at about 2,000 degrees Fahrenheit.
The first phase of flashover results from the ignition of flammable gases produced by combustion (primarily carbon monoxide) that have accumulated in the upper parts of the fire area.
As this is happening, the radiant heat of the original fire is heating nearby combustibles--the walls, furniture, anything in the room--and these also begin giving off flammable gases.
This is called "pyrolysis." In this phase, smoke banks down quickly, reducing visibility dramatically.
The next phase of flashover is the rollover or flaming of these gases near the ceiling. This may appear as small flashes of flame in the dense smoke, or as rollover, a flame front rolling out across the ceiling. The fire has now changed from a steady-state fire to an aggressive, fast moving fire.
The rapid fire development is complete as conditions move into final phase: thermal collapse. Intense radiant heat pours down from all around. At this point a firefighter can no longer get under the thermal balance (the stratified layer of heat and smoke). Even in bunker gear this is extremely dangerous. We must try to recognize the warning signs that precede flashover. Unfortunately, by the time firefighters experience this phenomena often enough to recognize it, they have put their lives in danger many times. I have tried to provide examples of how information readily available on website can be used to develop training programs that allows firefighters to recognize the warning signs before getting caught in a flashover.
Source One – Near Miss Reports
I started by going to the National Fire Fighter Near-Miss Report website.
The National Fire Fighter Near-Miss Reporting System is a voluntary, confidential, non-punitive and secure reporting system with the goal of improving fire fighter safety. Submitted reports are reviewed by fire service professionals. Identifying descriptions are removed to protect your identity. The report is then posted on this web site for other fire fighters to use as a learning tool.
National Fire Fighter Near-Miss Report
Report Number: 09-0000300
Report Date: 03/18/2009 1239
FFs narrowly escape flashover.
Our department was dispatched to a residential structure fire. Upon arrival, light brown smoke and flames were showing from the B side of the structure, Entry was made into the structure via the A side with a 1-3/4 inch handline. One person was at the door feeding hose, a firefighter and I entered with a thermal imager, handline, and set of irons. The house was set up as an apartment style building, with a hallway and entry to the apartment directly to the right upon entry. Entry was forced into the apartment, and no noticeable smoke changes were noted. While progressing further into the living area, the temperature of the room, which was about 350 degrees F, rapidly climbed to approximately 800 degrees. Smoke conditions rapidly darkened, and we proceeded to immediately exit the structure. Heat conditions worsened and I began to feel burning on my arms, legs and neck. I turned around and noticed a wall of orange flame as the room began to flash. The firefighter and I made it to the hallway and rapidly exited the structure. Once outside, I noted that a layer of tar had covered my mask from the smoke, and my helmet had melt marks and bubbling in the paint. My SCBA had slight charring on the regulator, and my gloves had burned partially away.
We learned to be prepared. Even in the most routine of structure fires the situation can rapidly change.
A RIT team was not in place in this event, and had something happened, we may have been trapped in that room with no outside assistance. Even with automatic mutual aid agreements, we were still short handed on this call. Smoke conditions gave no indication that a flashover was imminent, and when cues did indicate flashover was coming, it happened rapidly. Had we not been paying attention or had "tunnel vision," we may have been badly burned or worse.
Source Two - NIOSH Fire Fighter Fatality Reports
I can almost always find a report on the NISOH website to add to a drill.
Fire Fighter Fatality Investigation and Prevention Program
Each year an average of 105 fire fighters die in the line of duty. To address this continuing national occupational fatality problem, NIOSH conducts independent investigations of fire fighter line of duty deaths. This web page provides access to NIOSH investigation reports and other fire fighter safety resources.
1 FF Dies and 2 FFs Injured in a Flashover During a House Fire – Ohio (click for report)
On March 21, 2003, a 25-year old male career fire fighter (the victim) was fatally injured in a flashover during a house fire. The victim and two other fire fighters were on an interior attack crew and had just gone through the front door of a single family residence. The hose line was uncharged and the crew was calling for water when a flashover occurred. From the time the victim arrived on scene until the flashover was approximately four minutes. After the flashover, fire fighters on the front porch witnessed the victim walk toward the front door then turn and retreat into the structure. The two other fire fighters on the interior crew exited through the front door. They were injured and transported to the hospital where they were treated and released. The victim was located and removed from the structure within 10 minutes. He was transported via ambulance to the hospital where he was pronounced dead.
NIOSH investigators concluded that, to minimize the risk of similar occurrences, fire departments should:
Additionally, emergency dispatchers should:
- review and revise existing standard operating procedures (SOPs) for structural fire fighting to ensure fire fighters enter burning structures with charged hose lines
- ensure that a Rapid Intervention Team (RIT) is established and in position prior to initiating an interior attack
- ensure that ventilation is closely coordinated with interior operations
- ensure that crew continuity is maintained on the fireground
- ensure that fire command always maintains close accountability for all personnel operating on the fireground
- obtain as much information as possible from the caller and report it to the responding fire fighters
Source Three –Videos from NIST
(The National Institute for Standards and Technology)
This website is often technical but it also contains some excellent information. The photos below show flashovers from NIST experiments both in the laboratory and in the field.
This video series contains video of the Dry Scotch Pine Tree test, a Sofa/Living Room test, and an Open Floor Plan Office Workstation Test.
Other Websites with Flashover Information
BE PRO-ACTIVE WITH TRAINING.
YOUR LIVES DEPEND UPON IT.
FIREFIGHTER SAFETY AND TRUSS CONSTRUCTION
Wooden trusses are used in roof systems in more than 60% of all buildings in the United States. Truss and related engineered wooden floor systems are also becoming more common. Today, more engineered structures use lighter weight materials, producing larger spans and clear openings. Trusses can be designed to carry expected loads, be produced economically, be safely handled, and reduce construction costs.
Engineered building components may provide adequate strength under normal loading; but under fire conditions, these truss systems can become weakened and fail, leading to the collapse of roofs, floors, and possibly the entire structure. Truss systems are usually hidden, and fires within truss systems may go unnoticed for long periods of time, resulting in loss of integrity. Structural design codes often do not factor in this decreased system integrity, as fire degrades the structural members. Fire fighters typically rely on warning signs to indicate imminent truss failure such as roofs and floors that feel spongy or are visibly sagging. Quite often, these warning signs are not good predictors of truss system failures. Fifteen separate incidents investigated by NIOSH identified at least 20 fatalities and 12 injuries that have occurred from 1998–2003 during fire-fighting operations in buildings containing truss systems.
At least three scenarios can occur in which fire fighters suffer fatalities and injuries while operating at fires involving truss roof and floor systems [Dunn 1992]:
- While fire fighters are operating above a burning roof or floor truss, they may fall into a fire as the sheathing or the truss system collapses below them.
- While fire fighters are operating below the roof or floor inside a building with burning truss floor or roof structures, the trusses may collapse onto them.
- While fire fighters are operating outside a building with burning trusses, the floor or roof trusses may collapse and cause a secondary wall collapse.
The following case reports describe incidents involving fire fighter injuries and deaths due to fires involving truss system failures.
On March 8, 1998, one male career fire fighter, a captain (victim), died when the wooden-bowstring trussed roof of a building collapsed and blocked his exit route. The first company on the scene reported light smoke showing from a one-story commercial building. While fire companies waited for the security doors to be opened, fire conditions changed dramatically on the roof. Heavy fire was coming from the ventilation holes opened by the ventilation crew. When the doors were opened, the fire fighters encountered heavy smoke with near-zero visibility approximately 15 feet inside the door. The engine crews advanced until deteriorating conditions forced them to withdraw. During this time, the victim became separated from his crew and did not exit from the building. Approximately 20 minutes after the engine crews entered the building, the roof partially collapsed, blocking the front entry and hampering rescue operations. The victim was later located by the Rapid Intervention Team, and cardiopulmonary resuscitation was performed immediately and enroute to the hospital, where the victim was pronounced dead.
On September 5, 1998, a 54-year-old male career fire fighter (the victim) died when an exterior brick parapet wall collapsed on him. The warehouse was constructed of brick masonry walls with heavy timber trusses supporting the roof. The front and rear masonry walls extended above the peak of the roof, forming parapet walls. The first responding officer ordered an exterior-only attack using deck guns and hose lines. The incident commander called dispatch to request mutual aid from three additional fire departments. A career department (including the victim) arrived on the scene approximately 15 minutes later and was positioned at the north end of the building and prepared for an exterior attack. The victim approached the building to open a set of large doors (each 15 feet high by 6 feet wide) so that hose lines could be directed through the doors. The doors closed as the victim returned to the hose lines. The victim was approaching the building a second time to prop the doors open when the brick parapet wall suddenly collapsed outward, killing him instantly. Fire fighters at the east side of the warehouse reported a partial roof collapse at approximately the same time the parapet wall on the north side collapsed.
On December 28, 2000, four career fire fighters were injured when a section of a church roof collapsed, trapping them inside. The roof system was formed using lightweight wooden trusses with gusset plate connectors. Two different truss systems were used in the different construction phases. The exterior peaked roof was covered with standard asphalt shingles. The interior ceiling consisted of ½-inch drywall attached to the bottom of the trusses. Three 1¾-inch hose lines were advanced into the building by three crews. The incident commander ordered the first crew to enter the structure for an aggressive fire attack, cautioning them not to enter the structure very far. Two subsequent crews were sent into the building through a different set of doors. Approximately 7 minutes after the first crew entered, the second and third crews met in a classroom. They noticed intense fire in the ceiling/truss void area where a small piece of ceiling in the classroom had fallen. Soon afterward, the roof collapsed in the classroom area, trapping and injuring four fire fighters. Three were able to escape by breaking through an exterior window. The fourth fire fighter located the classroom door, and the incident commander led him into the hallway and out of the structure. The lightweight truss roof collapsed less than 10 minutes after the fire fighters entered the building.
Fire Fighters’ actions that can improve safety
Use extreme caution when operating on or under truss systems.
Immediately open ceilings and other concealed spaces whenever a fire is suspected in a truss system.
- Notify the incident commander whenever truss construction is discovered.
- Communicate interior conditions to the incident commander as soon as possible and provide regular updates.
- Use a defensive fire-fighting strategy once burning of truss members is identified (unless someone is trapped).
- Expect imminent collapse once lightweight truss roofs or floors are involved in a fire [Klaene and Sanders 2000].
- If possible, avoid cutting the truss chords when cutting holes for roof ventilation. Cuts can weaken the roof.
- Avoid roof areas loaded by air conditioning units, air handlers, and other heavy objects.
- Be aware of alternative exit routes at all times when working above or below a truss.
- Consider using roof ladders or working from aerial ladders or platforms instead of walking or standing directly on the roof [Brannigan 1999; Dunn 1998].
- Be aware that fires can be in the truss void or other concealed areas (see Figure 2). Once the fire enters a concealed space, it can travel to remote locations rapidly, since the wooden web members surrounded by open air space provide an excellent fuel source [Brannigan 1999].
- When a truss is suspected to be above a ceiling, use a pike pole or other tools to open up the ceiling and check for truss construction [Brannigan 1999]. If there is a fire barrier in the void, use the same procedure on the opposite side.
- Be aware of the possibility of flashover or back draft when opening concealed spaces and take the appropriate safety precautions.
- When opening ceilings or other concealed spaces, have charged hose line(s) ready.
- Be aware of the nearest exit and of other fire fighters in the area. [Klaene and Sanders 2000].
Early detection of fires involving truss systems is important for safe fireground operations. Pre-incident planning is an important tool for identifying the type of building, the building contents, the load-bearing and interior wall locations, and the presence of trusses. This information will aid incident commanders in managing the multiple hazards in a fire. Today's construction methods incorporate lightweight building components, and this trend is expected to grow. Learning about trusses and their performance under fire attack can greatly enhance fire fighter safety. Lives will continue to be lost unless fire departments make appropriate fundamental changes in fire-fighting tactics involving trusses. These fundamental changes include the following:
- Venting the roof using proper safety precautions.
- Opening concealed spaces quickly to determine fire location.
- Being constantly aware of the time the fire has been burning.
- Providing continuous feedback on changing conditions to the incident commander.
- Watching for signs of structural deterioration.
This material is abridged from NIOSH.
More information is available on the web at www.cdc.gov/niosh/firehome.html.
What is a Truss?
A truss can be defined as structural members (such as boards, timbers, beams, or steel bars) joined together in a rigid framework. They are most often in the shape of a triangle or series of triangles. Some trusses are rectangular. Trusses can be built of wood, steel, wood and steel, or aluminum. Concrete trusses are not common but do exist, usually in very large structures. The truss framework is usually arranged in a single plane so that loads applied at points of intersecting members will cause only direct stress (compression or tension). Three-dimensional trusses (space frames) are very light in weight. The design of a truss, which separates compressive and tensile stresses, allows for a minimum of materials to be used, resulting in economic benefit. The unique characteristic of a truss is the inherent stability of the triangle.
All parts and connections of a truss are vital to the stability of the truss system. The bottom chord of a truss is under tension. A tension member acts like a rope. If the bottom chord of the truss breaks, the truss system may fail by pulling apart. Conversely, the top chord of a truss is under compression. The top chord acts like a column. Failure of a compression member reduces the overall load-bearing capacity of the truss. An often overlooked hazard is found where interior trusses or wooden beams extend beyond the exterior wall to provide a balcony or a stairway landing. Fire burning inside the building can degrade the truss or beam, resulting in collapse of the cantilevered balcony or stairway landing. Fire fighters standing on or under the collapsing exterior landing may be injured or killed.