Unlike conventional construction, lightweight wood truss construction does not obtain its strength from the size of the materials used but rather from compression and tension of the materials used in its construction.
Lack of recognition of key aspects related to building construction is one of the top five common threads when analyzing firefighter fatality case studies. Buildings that contain lightweight wood truss construction are susceptible to collapse from fire exposure in a very short amount of time. This is a known fact to the fire service, no matter what anyone or any organization says otherwise.
As fire service members, we can not place 100% of the blame on the building industry though. Why are we still losing firefighters due to collapse in commercial structures that are made of lightweight truss construction after business hours when our rescue profile is very low? (see figure 1) It is imperative that we take the steps necessary to change our tactics and the way that we act on the fireground, especially when it comes to risk/benefit analysis. The time and temperature curves for reaching high temperatures indicative of flashover conditions are much shorter than they were in the past due to the composition of materials and structural components. They no longer consist of the mass that they once did.
Lightweight wood truss construction is being used more and more with new construction because it offers builders savings in cost, easier access to run utilities and ventilation, larger spans without obstacles such as columns for support and they can support a weight load equivalent to a solid structural member under normal conditions. According to the truss manufacturing industry, it is estimated that 65% of roofs on new commercial construction are constructed of trusses and 10% of new commercial buildings utilize trusses for floors. Just as alarming, 65% of roofs and 25% of floors in new residential construction feature lightweight truss construction.
In tests done by the building industry, a 2,600-square-foot home was constructed within 148 labor hours using truss components as opposed to 401 hours using conventional construction methods. In that test, the truss construction also utilized 26% less lumber than the conventionally built home. Numbers such as these should raise the awareness level of every firefighter when discussing the hazards of trusses under fire conditions and risk-benefit analysis.
Lightweight truss construction consists of top and bottom members that run parallel to each other. These are referred to as chords and are made of wood. These chords are cross-connected for support by wood that forms a web-like pattern. Trusses use a series of triangles that are connected and are designed to transfer the load that they carry to load-bearing walls in the structure. All wood usually consists of 2-by-4's or 2-by-3's. Unlike conventional construction, lightweight wood truss construction does not obtain its strength from the size of the materials used but rather from compression and tension of the materials used in its construction. The top chord is supported by load bearing walls.
It acts as a bridge between these walls. With this being under a load, the top chord is being placed under compression while the bottom, unsupported chord provides tension.
Conventional construction techniques do not rely on a sum of the total members for structural stability whereas lightweight truss construction does. Because of the bottom chord providing tension, a failure of any one connection point ("gusset plate") will cause the load of that truss to be transferred to another which may already be weakened thus causing a collapse of multiple trusses.