Multiple firefighter fatalities at large commercial fires attract the attention and scrutiny they warrant. However, the fire service must also recognize that most line-of-duty injuries and deaths occur in single- and multiple-family dwellings during routine, "bread-and-butter" fires. A review of...
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- Residential fires may actually pose commercial fire risks. Many of today's "typical" house fires are in buildings that, based on size and interior volume, can and should be categorized as commercial structures with commercial fuel loads. Combined with modern synthetic fuel loads, fires in large, unprotected and un-firestopped voids made of lightweight engineered building materials can be catastrophic.
- Thermal imagers (TIs) do not provide an adequate indication of a weakened floor or pending collapse. There is a potentially dangerous misconception in the fire service that TIs can detect fire on the floors below or above a firefighter. TIs detect variations in surface temperatures for objects in the camera's field of vision. They cannot detect temperatures in areas that are thermally shielded from the camera's view by the finish materials of a floor or ceiling. During these tests, average temperatures below the assembly were in excess of 1,200°F, while average temperatures on top of the carpet were less than 100°F. The application of water during suppression operations will further mask or eliminate the thermal signatures available to the TI's sensor.
- Floor collapse can occur in six minutes. Engineered wood floor assemblies have the potential to collapse very quickly under well-ventilated fire conditions. When it comes to lightweight construction, there is no margin of safety. There is less wood to burn, and therefore less time before the assembly fails. During this research, the shortest failure time was noted during the unfinished/unprotected engineered wooden I-joist test. This assembly experienced a total structural collapse at six minutes.
Tested assemblies weaken significantly well before they completely collapse. The ASTM E119 definition of collapse requires that the floor totally collapse. Several fire-test standards recognized outside the United States, such as ISO 834:1: Fire-Resistance Tests, Elements of Building Construction, Part 1, look instead at how long a test sample is able to maintain its ability to support the applied load during the fire test, taking into account when a floor is progressively deflecting or failing prior to a complete structural collapse — clearly a critical piece of information for the fire service. The bottom line: If the ISO standard was applied to the unprotected engineered wooden I-joist assembly, the accepted failure time would change from 6:03 to four minutes. The engineered I-joist assembly clearly demonstrated the significance of the difference between the ASTM E119 and the ISO 834:1 standards. At 3:30, the engineered I-joist floor assembly began to vibrate vertically approximately eight inches for 90 seconds.
All lightweight assemblies studied in the tests exhibited similar differences in time-to-failure based on the standard applied, but none exhibited as severe a vibration period prior to collapse. Fire service instructors must reinforce that the dramatic vibrations exhibited by the engineered I-joist assembly were a very late indicator of a pending structural collapse and that this behavior will not be exhibited by all floor systems prior to structural collapse.
- Plastic ridge vents can mask fire intensity. In the early stages of the modern roof-assembly test, a significant amount of smoke was emitting from the continuous plastic ridge vent. As the temperatures increased, the ridge vent melted and collapsed upon itself, sealing the natural opening. The heavy smoke emitting from the continuous ridge vent diminished to a light smoke trail, although the fire was still raging below. Temperatures in the attic space went from approximately 200°F to 1,400°F in less than 60 seconds. These are flashover conditions and can cause global failure of the ceiling. Firefighters conducting size-up and attempting to read smoke conditions from the exterior may be deceived by this change in the volume and velocity of the smoke venting from the roof, and roof teams may conclude that the roof is safe to operate on, when in fact it may be rapidly approaching the point of collapse.
- Collapse times of the tested assemblies. The collapse times for all of the assemblies are shown in the chart above.
In addition to the collapse times, a large amount of significant useful data for the fire service was obtained during these fire tests to include: the observation of the conditions of the ceiling and floor or roof from both sides of the assembly, temperatures in the concealed spaces, deflections of the floor and roof surfaces prior to collapse, thermal imager video from the top of the assembly, and video and audio recordings from the simulated firefighters' perspectives.
Over the past few decades, millions of single-family homes have been built with truss-constructed roofs that create large, undivided attic spaces and unfinished basements that have unprotected lightweight wood floor systems above them. This study established that while all unprotected wood floor assemblies are susceptible to early failure when exposed to fire, modern lightweight assemblies fail significantly sooner and the failures are more global. Unprotected combustible wood construction also poses the threat of accelerated fire development.
Following are some tactical considerations for firefighting operations in lightweight constructed structures.