As construction materials change over time, so too must the fire service’s approach to residential fires. New research from UL underscores the importance of revising tactics used during basement fires.
The two-year UL study, funded by a grant from the National Institute of Standards and Technology’s (NIST’s) American Recovery and Reinvestment Act, focused on examining the hazards of basement fires involving various residential flooring systems.
UL has performed other research for the fire service, so why focus on basement fires? “At least once a year, we gather a group of firefighters from around the country and ask them about their pressing issues and concerns,” explains Stephen Kerber, a research engineer at UL who worked on the project. “Most [of the discussion of basement fires] was anecdotal, and we felt we could put some numbers together on different floor materials and construction.”
Kerber and his team worked with other organizations to perform extensive experiments with different types of floor joists, including dimensional lumber, engineered I-joists, metal plate connected wood trusses, steel C-joists, castellated I-joists and hybrid trusses.
For the first phase of the experiments, researchers worked with Michigan State University’s structural engineering program and in their own lab to burn the various materials in furnaces, before moving on to record results in larger-scale fires. “We went to the fire training academy in Delaware County, Pa.,” Kerber explains. “We built two simulated basements and tested different floor systems, different fuel loads and different ventilation and loading.” Floor collapse times ranged from 3:28 to 12:45; the dimensional lumber experiments collapsed at an average of 11:57 while the engineered floor systems collapsed at an average of 7:00.
The team then recreated identical structures inside their Northbrook lab and ran the same tests, but without environmental factors, such as wind. They tested variables, including applied load, ventilation, fuel load, span and protection methods in numerous burns.
The final phase of testing involved full-scale burns. Two homes in a nearby Chicago suburb were acquired—one constructed in the 1940s and other in the 1990s. The UL researchers worked with the Chicago Fire Department lighting basement fires in each and letting them burn to collapse. “We recorded everything,” Kerber says. In total, researchers recorded results during 17 full-scale fire experiments.
Significant Safety Issues
The most significant finding of the research: the universal instability of all types of floor construction during a basement fire. According to the research summary, “During all of these controlled experiments where the variables were systematically controlled, there were no reliable and repeatable warning signs of collapse.” In addition, it was found that all basement fires can lead to a change in flow paths, resulting in unexpected fire behavior. “One of the biggest findings is that there is no way to know when it’s safe to operate on top of a basement fire,” Kerber says. “You need to have a really good reason to be there.”
Kerber admits he was surprised at how quickly some of the floors collapsed, and explains that it’s not about the type of flooring in a newer home, rather it’s about how flooring is constructed—with fewer supports and longer spans. “The material itself doesn’t matter so much; it’s that there’s not enough of it,” he says.
Take Extra Precautions
Asked how the research findings should be used, Kerber says, “The fire service really needs to question when to send someone in when there’s a basement fire. Departments should look at their training and their standard operating procedures for basement fires, and take [this research] into consideration. You need to approach the fire and control it before you commit to the top of it.”
Summaries and a full report on UL’s research findings are available on their website at www.ul.com/global/eng/pages/offerings/industries/buildingmaterials/fire/fireservice/basementfires/. A full online course about tackling basement fires safely can be found at http://content.learnshare.com/courses/73/356711/player.html.
Sidebar: UL Recommendations
Following are just a few of the findings and recommendations from the UL research report summary. Departments that adhere to the recommendations will immediately improve firefighters’ “understanding, safety and decision-making when sizing up a fire in a one- or two-family home.”
- Collapse times of all unprotected wood floor systems are within the operational time frame of the fire service, regardless of response time.
- Size-up should include the location of the basement fire as well as the amount of ventilation. Collapse always originated above the fire and the more ventilation available, the faster the time to floor collapse.
- When possible, the floor should be inspected from below prior to operating on top of it. Signs of collapse vary by floor system: Dimensional lumber should be inspected for joist rupture or complete burn-through; engineered I-joists should be inspected for web burn-through and separation from subflooring; parallel chord trusses should be inspected for connection failure; and metal C-joists should be inspected for deformation and subfloor connection failure.
- Sounding the floor for stability is not reliable and therefore should be combined with other tactics to increase safety.
- Thermal imagers may help indicate that there is a basement fire, but they shouldn’t be used to assess structural integrity from above.
- Attacking a basement fire from a stairway places firefighters in a high-risk location due to 1) being in the flow path of hot gases flowing up the stairs and 2) working over the fire on a flooring system that has the potential to collapse due to fire exposure.
- The thought that if a firefighter quickly descended the basement stairs, cooler temperatures would be found at the bottom was determined inaccurate. The experiments in this study showed that temperatures at the bottom of the basement stairs where often worse than the temperatures at the top of the stairs.
- Coordinating ventilation is extremely important. Ventilating the basement created a flow path up the stairs and out through the front door of the structure, almost doubling the speed of the hot gases and increasing temperatures of the gases to levels that could cause injury or death to a fully protected firefighter.