I submitted this as part of my Fire Science degree program.
Building construction has had an impact on the outcomes of the firefighter’s ability to survive in a hostile fire environment. The types of construction have evolved over the years, with residential construction be built using lighter and lighter materials. Additionally, the contents of residential occupancies have changed, both in quantity, but also in the what those materials are made of. The increase in contents made of highly volatile components, has a higher impact on the structure due to increased temperatures, which are made with lighter weight materials. Fires in today’s “modern” residential buildings pose greater risks than their “legacy” predecessors. Although there are many types of engineered products being used in modern construction, this paper will focus on the use of trusses, and truss systems. These structures are subject to rapid fire spread through areas of unprotected wood construction, the collapse of unprotected dimensional lumber, and the collapse of lightweight engineered wood components. The increased use of trusses in both floor and roof assemblies in residential structures is having a negative impact on firefighter safety.
Trusses are defined as a series of structural members joined together to form a rigid framework. Modern trusses are 2 inch by 4 inch pieces of lumber held together by connector plates. In most cases these connector plates are made of stamped lightweight steel that is coated with zinc. The metal connector plates have teeth punched into them that are from 5/16 inch to 9/16 protruding from the plate which is what holds them to the wood, and for joining two pieces of wood together (Harmon & Lawson 2007). Modern trusses are designed by computers using established engineering practices to develop strength to withstand the anticipated loads of normal use, for the location being used. The computers calculate the needs of the trusses according to the building code required for the anticipated loads, to include finish materials, winds and snow loads. Under normal, non-fire, conditions trusses perform well for the purposes of building a strong cost efficient structure in a quick time frame.
The design and shapes of trusses has been around since the beginning of building structures. In 1952, a builder in Florida began experimenting with trusses, using plywood gusset plates with various connection types like glue, staples, nails and screws, and then the metal plate connection was invented and the wood truss industry began (WoodCon). The use of trusses in residential construction expanded because they are lower cost and faster to install than traditionally framed roofs and floor systems. They do not require the same level of skilled craftsman to build these systems as they are built in a faraway factory and shipped to the construction site. Trusses allow for more wide open floor plans, and just about any configuration of roof can be built quickly. These factors provide for cost savings and construction speed, which in turn saves the consumer money. Most builders today and they will tell you that engineered roof trusses are the only way to go and are far better than the old roof frames. The primary benefits of using pre-fabricated roof trusses are cost efficiency and construction speed. Additional benefits are the opportunities to use the open void space created to run ventilation trunk lines, plumbing, and electrical wiring.
Trusses fail in a wide range of ways. Trusses are engineered to be in compression and suspension at the same time. They are held in both conditions through the use of the connecting plates. Truss components rely on all of the parts to be held together, the failure of any one part will cause the whole truss to fail (Corbett, & Brannigan, 2013). Remember, trusses are used as part of a greater system, as one fails, the adjacent truss comes under greater stress and the potential to fail is increased as each part is taken away. Comparing legacy construction with modern lightweight construction has been tested in the laboratory. Legacy construction of floors and roofs with typical gypsum board finishes outperform modern floor trusses by over 30 minutes (Dalton, Backstrom, & Kerber, 2009). The time factor before failure should be enough to show that modern lightweight construction poses a greater risk for firefighter safety. The metal plates with their teeth penetrating the surface of the wood, transfer heat beneath the surface, and the area surrounding the plate. The increase in temperature, and the deeper penetration of temperature hastens the failure of the trusses joints.
Remember that trusses are just a component of the entire structure. In most cases, trusses represent the horizontal members, tying the vertical members together. Once the horizontal members begin to fail, the vertical members will succumb to gravity and either fall inward or outward, thus having leading to complete structural collapse. When it comes to lightweight construction, there is little margin for safety under fire conditions. There is less wood to burn and, therefore, potentially less time to collapse.
Additional stresses are applied to the structural members when we apply live load of firefighters during firefighting operations. Building codes call for live loads of 40 to 60 pounds per square foot in residential construction for floors. Roof loads are calculated at for dead loads at 8 pounds per square foot and 30 pounds per square foot of live load to account for snow. When adding firefighters in full personal protective clothing, you are adding significant concentrated live loading in a volatile environment on potentially compromised structural members due to fire conditions. A typical 200 pound fire fighter in full protective clothing and tool which adds another 75 pounds which makes for an approximately 275 pound live load. Using a 275 pound firefighter, when kneeling the load is distributed to four points of contact of the knees and toes within a 2 square foot area, creating 34 pounds of live load at each point of contact. When standing, the load of each point is concentrated further by having 137.5 pounds of concentrated live load. If at any point, the fire has compromised the strength of the truss beneath the firefighter, the live load of the firefighter will exceed the design, and lead to failure (Harvel, 2011).
By the very nature and design, trusses create void spaces between floors. The open web of the truss is good for running utilities such as wiring and HVAC. The open space provides for continuous fire and superheat smoke spread. These conditions weaken the truss and consequently the floor being supported. Once firefighters begin working on top or below the space, it creates a hazard of collapse (Dunn).
The fire service needs to react to the changes in building construction methods and the materials used that increase fuel loads and reduce burn time to collapse and or failure when exposed to fire. Firefighters and building occupants are at a greater risk than ever before. It is clear that education alone will not facilitate the need for earlier fire detection for occupant safety or provide firefighters the necessary tools for responding too and attacking fires where firefighters must attempt to make aggressive interior attacks to save lives. The length of time the fire has burned will have significant effect on structural stability. The decrease in strength due to the failure of one component in the system is what poses the hidden dangers to the uninformed firefighter.
Corbett, G. P., & Brannigan, F. L. (2013). Brannigan's building construction for the fire service. Jones & Bartlett Publishers.
Dalton, J. M., Backstrom, R. G., & Kerber, S. (2009). Structural Collapse: The Hidden Dangers of Residential Fires. Fire Engineering.
Dunn, V. (2010). Collapse of burning buildings: A guide to fireground safety. PennWell Books.
Harman, K. A., & Lawson, J. R. (2007, January). A study of metal truss plate connectors when exposed to fire. National Institute of Standards and Technology. doi:10.6028/nist.ir.7393
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Kerber, S. (2012, 10). Analysis of changing residential fire dynamics and its implications on firefighter operational timeframes. Fire Technology, 48, 865-891. doi:http://dx.doi.org.ezproxy2.apus.edu/10.1007/s10694-011-0249-2
NIOSH. (2005, May 06). Preventing Injuries and Deaths of Fire Fighters Due to Truss System Failures. Retrieved August 25, 2016, from https://www.cdc.gov/niosh/docs/2005-132/
WoodCon. (n.d.). History of wood roof trusses | Woodcon. Retrieved September 05, 2016, from http://timber-trusses.com/history-of-wood-roof-trusses.html