A modern building technique can mean greater risk for firefighters.
NFPA Journal®, March/April 2009
Structural stability is a critical factor that must be carefully considered when making the offensive/defensive decision at structure fires. Unfortunately, conditions that can lead to structural collapse are often difficult to predict. No building is completely immune to structural failure, but some buildings will withstand a very large and intense fire without catastrophic (total) collapse. Others, however, can experience early collapse under similar fire conditions. For example, fire-resistive buildings will withstand an intense fire for a considerable time, while non-combustible buildings with truss roof construction can fail in just a few minutes once the fire attacks the truss space.
Modern construction methods conserve materials by using lighter-weight structural members that provide the same load-bearing capabilities as earlier methods that used massive structural members. The primary way this is accomplished is through truss construction, where lightweight trusses take the place of large wood beams or steel I-beams. Truss construction is structurally sound under normal conditions. However, these lightweight structural members can be adversely affected by fire much sooner than heavier building materials. Compounding the problem is the fact that the truss loses its load-bearing capacity as it loses its triangular configuration. For firefighters, that can translate into failure with little warning. As noted in a number of reports, a well-involved fire in a truss space may not be detected until the fire breaks through the ceiling or roof, which is why truss roofs and/or concealed spaces have played a major role in firefighter fatalities.
Truss floor assemblies are also common, and truss construction above large, open areas is particularly dangerous. A collapsed truss roof or floor over a compartmented area—like that commonly found in residential occupancies—may be partially suspended by walls separating rooms below. Any roof or floor collapse is a serious condition requiring immediate attention. Truss construction over large, undivided spaces—such as places of assembly, mercantile, business, and industrial and storage occupancies—presents an even more serious threat. Large-area roofs or floors tend to collapse in large sections, falling onto firefighters working below and sometimes resulting in total building collapse.
Simply identifying the problems associated with truss construction is not enough. Likewise, complaining that truss construction places firefighters at increased risk will not eliminate the hazard. Most firefighters and fire departments are aware of the problems associated with lightweight construction. But action is necessary to reduce firefighter on-duty deaths and injuries.
First, departments must be aware of the location of truss construction in their response areas. This is best done during inspections and pre-planning when buildings are under construction. Contingency plans and Standard Operating Procedures (SOPs) must be developed for fires in large-area truss buildings. The use of thermal-imaging cameras can be beneficial in determining if there is a fire in concealed spaces. However, ceiling tiles or other ceiling structures should also be removed to determine if fire is invading this critical area and compromising the truss system. Firefighters also need to watch for outward signs that the fire has entered the truss space, such as the fire self-venting at the roof.
Officers must be trained to conduct a risk-versus-benefit analysis as part of developing an incident strategy. An involved fire in a large undivided truss space is a sure sign that sending firefighters inside the building would place them at extreme risk while, with few exceptions, offering no life-safety benefit. For this reason, if firefighters are already working inside a fire building, they should be evacuated immediately. Beyond these obvious indicators, pre-incident plans can identify specific situations that rule out an offensive attack.
If followed, these suggestions can reduce the number of on-duty fireground fatalities. Far too many firefighters have lost their lives due to the collapse of truss construction.
The column is adapted from the authors' book Structural Fire Fighting, available at www.nfpa.org or (800) 344-3555.