Understanding the Dangers of Lightweight Truss 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. ASTM E119 is used to establish time ratings of wood structures such as lightweight trusses.


Lack of recognition of key aspects related to building construction is one of the top five common threads when analyzing firefighter fatality case studies. ASTM E119 is used to establish time ratings of wood structures such as lightweight trusses. These ratings however are based on laboratory...


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Lack of recognition of key aspects related to building construction is one of the top five common threads when analyzing firefighter fatality case studies. ASTM E119 is used to establish time ratings of wood structures such as lightweight trusses. These ratings however are based on laboratory testing and are not assurances of performance under actual fire conditions. With relevance to actual fires there seems to be several deficiencies that exist with the ASTM E119 Test in regards to the safety of fire fighters.

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.

Lightweight wood truss construction is being used more and more with new construction because it offers builders cost savings, easier access to run utilities and ventilation, and can support a weight load equivalent to a solid structural member under normal conditions.

 

Lightweight truss construction consists of top and bottom members that run parallel. 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. All wood usually consists of 2x4's or 2x3's. The wood members are connected together with a fastener made of stamped sheet metal containing spikes ("gusset plates").

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.

Another hazard of lightweight trusses exists with the connection points of the wood members themselves. These are often referred to as "gusset plates" or "gang nails". They are usually made of 18 gauge stamped steel and penetrate the wood only 1/2 to 3/8 of an inch. New technology has even introduced gusset plates that are now fabricated of plastic.

Firefighters and fireground commanders should understand the risks involved in each scenario that they are likely to encounter with buildings within their district. Lightweight wood truss construction may be difficult to determine once construction of a building is completed unless companies have prior knowledge before an incident takes place. Developing solid pre-fire plans is the place to start. By preplanning and analyzing possible fire situations within building that may cause problems, we increase our success rate while diminishing the possibilities of injury or death to our people.

False security is sometimes developed by firefighters from a lack of understanding of ratings assigned to construction materials by testing agencies. Relevance to actual fires is often cited as being a key issue in structural fire safety research. One structural test that is highly regarded by the U.S. building codes for fire safety is the ASTM E119 Test which is conducted by the American Society for Testing and Materials. ASTM E119 is used to establish time ratings of wood structures such as lightweight trusses. These ratings however are based on laboratory testing and are not assurances of performance under actual fire conditions. With relevance to actual fires there seems to be several deficiencies that exist with the ASTM E119 Test in regards to the safety of fire fighters.

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