During a fire, any building may become inherently unsafe for occupants and fire service personnel. However, some building construction features present unique or unexpected hazards. This section discusses these hazards.
Trusses are widely used in construction to span wide areas without the need for vertical supports, reducing both material and construction costs (Figure 2.24). Under ordinary conditions, trusses work well and building codes have permitted this type of construction for many years. However, trusses often fail suddenly and totally during fires. Both wood and metal trusses are made of interdependent members which all fail if one member fails. Adjacent trusses, in their weakened state, are then unable to carry the additional load and these also fail in quick succession. It is impossible for crews operating at a fire to predict the time or extent of a collapse since they cannot see how many trusses are affected, which components, and to what extent.
Wood trusses have less mass than solid lumber, which greatly reduces the “extra” wood compared to solid joists that burn through more slowly and provide indications to firefighters of an impending collapse. The higher surface area-to-volume ratio of trusses compared to joists allows trusses to burn more quickly. In addition, the metal gusset plates that hold wood truss components together may fail quickly as fire consumes the wood in which the gusset teeth are shallowly embedded.
Many firefighters have been killed in collapses attributed to trusses, particularly wooden ones, since the 1970s. Incident commanders and/or safety officers typically consider the presence of trusses in their fireground risk analysis. Marking these buildings that include trusses makes this information immediately available to firefighters. The State of New Jersey requires this as a direct result of five firefighters losing their lives in Hackensack in 1988 (Figure 2.25).
Another component used to maximize construction efficiency is the wooden I-beam. Similar to trusses, I-beams eliminate extra wood, thereby providing less warning prior to failure under fire conditions. However, they lack the metal gusset connection plates that appear to be at the root of many wood truss failures.
Wherever these lightweight construction techniques are used, serious consideration should be given to providing sprinkler protection throughout the building, if not already required. Sprinkler protection of combustible concealed spaces is an important feature for firefighter safety. Further discussion about lightweight construction can be found in “Building Construction for the Fire Service,” published by the NFPA.
Vertical shafts within buildings sometimes have exterior openings accessible to firefighters. Any such doors or windows should be marked “SHAFTWAY” on the exterior with at least 6 inch high lettering (Figure 2.26) as required by the IFC and NFPA 1. This warns firefighters that this would be an unsafe entry point. If properly marked, time will not be wasted attempting entry at these points.
Normally, interior openings to shafts are readily discernable. Ordinary elevator doors are not likely to be mistaken for anything else. However, other interior shaft openings that could be mistaken for ordinary doors or windows should also have shaft-way marking.
Without special precautions, roof-mounted skylights obscured by heavy smoke or snow may collapse under the weight of a firefighter. Skylights should be designed to bear the same weight load as the roof. The same applies to coverings over unused skylights. If this is not practical, mount barriers around skylights to prevent firefighters from inadvertently stepping on them.