As we all know, building construction has changed drastically over the last 20-25 years especially with the evolution of the increasingly popular use of lightweight construction and its engineered structural components. The reason for its popularity is simple; the load carrying capacity increases in these structures and its cost efficient.
Originally in this post, I said this has changed over the last 20-25 but in reality the changes started to have an impact on the fire service around 50-65 years ago with the development of the first part of the structural component, floor and roof truss construction which were both and to an extent are today held together with glue.
So let’s start off with the basic types wood truss construction.
Trusses are easily identifiable by their triangular framework of multiple structural elements. This is what distinguishes them from other structural products. Due to their structural efficiency this element is a cost-effective solutions for many structures we see all over the world today (bridges & buildings). When talking about residential construction, wood truss held together but metal plates are the primary type used and are fabricated from 2x4 or 2x6 dimension lumber. Any trusses built from larger dimension lumber is usually found in custom built homes and due to the popularity of these type of homes today, this is fairly common in newer suburban developments.
In roof truss construction the three sides of the triangle are known as “chords” and the pieces connecting the top and bottom chords together are known as the “webs”. The “connectors” that join the chords and webs in the modern truss system together are usually done by metal-toothed plates and is most common in truss roof assembly. In truss roof assembly or otherwise known as pitch chord truss, the top chord is sloped and the bottom chord is typically horizontal because it will directly support the ceiling.
Another type of truss found in roof truss assembly is parallel chord truss but this is normally found to form floor assemblies. In this type of truss assembly, the top and bottom chords run parallel with the top chord in compression and the bottom chord in tension.
In both parallel and pitched chord trusses metal tooth plate connectors (MPC) are used extensively to join the chords and webs together. These connectors are multi-tooth plated and are embedded into the wood fibers using a hydraulic press.
So how do these trusses carry a load?
As we all know from high school geometry, a simple triangle is stable in nature and all 3 sides are equal. Meaning, any force applied to it will be transferred around all three sides with limited movement or change of shape. As previously mentioned, the top chord is in compression and the bottom is under tension when the system is under what is known as gravity loads (i.e. live loads). Live loads are not to be confused with the assembly itself.
Redistributing the load
The performance of wood truss construction, whether exposed to outside forces such as hurricanes, earthquakes or fire can be attributable to 2 factors.
Structural redundancy & load redistribution across the floor or roof
This is found within each truss. When one truss member fails, the load it is carrying will redistribute itself to the remaining truss members. Also, should one of the truss lose its strength or stiffness, the entire assembly – floor or roof – will redistribute the loads through sheathing and/or bracing to the adjacent trusses.
When a single member of a truss is cut, the structural integrity is in fact compromised. However, this alone will not normally cause a catastrophic collapse. In most cases the truss will still carry the most of the normal load that has been originally applied. The cut member of the truss however, will cause a glaring defect that will need inspection. When looking at a total collapse of the system, this is dependent on many factors. These factors will include the amount of the load, span of the truss, & roof and floor integrity all under fire conditions or not.
In the next part of this multi-part series, we will discuss other types of more advanced wood truss construction.
Until next time; work hard, stay safe and live inspired.
About the Author
NICHOLAS J. HIGGINS is a firefighter with 14 years of service all within departments in Piscataway, NJ. Nick has held the ranks of Lieutenant and Captain as well as being a township elected District Fire Commissioner for 1 term (3 years) in Piscataway, NJ. He is also a NJ State certified level 2 fire instructor. He holds a B.S. in Accounting from Kean University working in Corporate Taxation and is the founder/contributor of the Firehouse Tribune website.