Firefighter Safety Report: Truss System Failures
NIOSH | John Howard M.D.
More than 60% of the roof systems in the United States are built using a truss system. By design, wooden truss systems contain a significant fuel load and are often hidden from sight. Fires in truss systems can burn for long periods before detection and can spread quickly across or through the trusses. Steel trusses are also prone to failure under fire conditions and may fail in less time than a wooden truss under the same conditions.
The number of fire fighter fatalities related to structural collapse could be significantly reduced through proper education and information concerning truss construction. Fire fighters should be discouraged from risking their lives solely for property protection activities.
Unfortunately, fires are not predictable: conditions often deteriorate quickly, and fire-damaged building components, including trusses, can collapse with little warning. Engineering calculations provide data for an approximate time of failure under specified fire conditions; however, under uncontrolled fire conditions, the time to truss failure is unpredictable.
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 • Employing a defensive strategy once burning of truss members is identified • Broadly disseminating new tactical safety concepts learned at each fire