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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. 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.

Ratings given in the ASTM E119 Tests for lightweight trusses can be very deceiving. Some of the notable deficiencies in these test ratings are:

  • The time/temperature curve used in the testing process is out of date. Due to materials such as petroleum based plastics and synthetics that are used in the make up of products contained in fire loads, fires burn at higher temperatures through the free burning stage and reach these temperatures much sooner than indicated on the time/temperature curve presented in the test that uses natural materials.
  • The ASTM E119 Test is also conducted inside of a structure that is 100% fire stopped and does not contain any void spaces. All buildings contain some type of void space due to breaching of walls for utilities and other work. In real fire situations, void spaces between trusses hold and hide carbon monoxide and other hazardous products of combustion. This presents a hazardous condition that is not easily seen by fire fighters because the signs of backdraft and flashover are not able to be felt or seen in the void space created by the trusses without the use of thermal imaging cameras. The environment during testing is also negatively pressurized which unlike real conditions will allow hazardous products of combustion to be ventilated as opposed to accumulating in the voids.
  • The ASTM E119 Test does not consider the point of origin of a fire as being inside the truss space itself, which could be common with faulty wiring and utilities that run through truss spaces. The testing procedures also assume that fire will only burn in an upward pattern and does not consider that fallen debris can extend fire into void spaces beneath a floor as well as sideways into wall voids.
  • The ASTM E119 Test is conducted with weight loads that are static and not equivalent to the dynamic load of multiple fire fighters, tools, hose and flowing water that is placed on trusses. This leads to false security when analyzing testing results.

With these factors in mind, firefighters should not be convinced that a lightweight truss assembly will give sufficient warning prior to failure. The ASTM E119 Test only provides a theoretical basis for analyzing the effects of fire on a truss assembly and does not take the full extent of conditions into consideration that can be encountered outside the laboratory.

Lightweight trusses present an obvious threat to firefighter safety on the fireground and should be viewed as a major factor when performing risk analysis by company officers and fireground commanders. Proper pre-planning and recognition of fireground conditions will go a far way when formulating an initial plan of action. Knowing where these buildings are located in your district may turn out to be the factor that keeps all members on the fireground operating in a safe manner.


 

Jeffrey Pindelski is a 16 year plus student of the fire service and currently a Battalion Chief with the Downers Grove Fire Department in Illinois. He previously served for 12 years as a Firefighter and Lieutenant on the Truck and Heavy Rescue Company. Jeff is a staff instructor at the College of Du Page and also instructs courses at the Downers Grove Fire Academy. He has been involved with the design of several training programs dedicated to firefighter safety and survival and is the coauthor of the text R.I.C.O., Rapid Intervention Company Operations.

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