There are many variables which new fire officers must consider when they take the front seat for the first time: Listening to the radio, ensuring the crew knows their assignments, gloves, camera, etc. You want to get off the rig and prove there is a reason that you were chosen to lead firefighters. But much of what you prepare for as a fire officer begins before the rig even starts. Being proactive enough to self-educate on things that may not always be immediately problematic is one of the ways that new fire officers can ensure they are not only well informed, but that their decision making is rooted in sound information. One of the most important topics that new fire officers need to consider is building construction, particularly lightweight construction.
So, what is lightweight construction? Many say that it’s lighter, cheaper, and weaker than conventional structural elements like dimensional, sawn, or milled lumber, that it fails in 5–10 minutes, and that it kills firefighters! Let’s take a step back though. Lightweight construction is born of engineered building components, assemblies, and structural elements that use materials of less mass, with the use of connections to act as a composite structural element. This is achieved by arranging the materials in a way that substitutes mass for geometry. (1) I know, it’s a mouthful. Really, it’s just taking smaller building components, connecting them, and arranging them in a way that uses shapes to transmit loads. Lightweight construction has equal or greater strength than dimensional lumber, can be engineered to span longer distances without the use of columns and without deflection, and is specifically engineered to a specific building. These structural elements aim to enhance the speed of construction, limit site fabrication, and save money by eliminating the need for reinforced concrete or steel elements for large open spaces.
Engineered wood products (2) encompass such things as
- Parallel-chord wood trusses
- Truss joist I-beam (TJI)
- Roof trusses
- Metal web truss
- Pin and bar truss
- Bowstring truss
- Laminated veneer lumber (LVL)
- Cross laminated timber (CLT)
- Particle board
- Plywood
- Oriented strand board (OSB)
- Glue laminated lumber (GLULAM)
- Parallel strand lumber (PSL)
- Laminated strand lumber (LSL)
Sometimes you may hear this term used synonymously with lightweight construction, but they are not always the same. Yes, every lightweight wood product is engineered, but not every engineered wood product is lightweight. A parallel-chord wood truss is a lightweight engineered wood product. A bowstring truss, however, is considered a heavy timber engineered wood product.
Parallel-Chord Wood Truss
Probably the most prolific lightweight structural element, the parallel-chord wood truss, has been around since the mid-1950s. This product has been used as the premiere structural element in both commercial and private applications that require long spanning floors. The top and bottom members of a wood truss are called chords. The top chord is in compression, and the bottom chord is under tension. The compressive connecting members are called struts. The tensile connecting members are called ties. Connections are called panel points (3).
As a group, the struts, ties, and panel points are called the web. One type of connection is called a metal plate connector, also known as a gusset plate. Other types of parallel-chord wood trusses include the metal web truss, in which the entire web is constructed with lightweight sheet metal. Another type is called open web pin-connected truss, or pin and bar truss, which has chords made of either solid-sawn or engineered wood, and tubular steel webs using pinned connections (4).
The finger-jointed wood truss, which is coming into prominence more and more of late, is made of a top chord and bottom chord of 2×3 with the web members comprised of smaller dimensions. The finger joints are precut into the chords and web members. They are later seated tightly into the joints and secured via adhesive (5).
Because parallel-chord wood truss systems are open web, they have a high surface-to-mass ratio. This means there is more area to burn, and ultimately the structural member can burn on both sides unimpeded. Fire travels through what is known as a truss loft, the open space created by the open web of all the trusses in a floor or roof system. Think of it as a cockloft that exists on all floors instead of just the top floor. When any part of a wood truss fails, the entire system can become compromised. For example, the bottom chord of a truss is like a piece of rope under tension. How many cuts can you make to that rope before it fails?
Trusses have no redundancy, which is why they are so problematic when under attack by fire. They use geometry, as previously noted, in the form of triangular shapes and oblique angles to capture loads. This is great in theory because these oblique directions distribute loads very well, but think about it: If you remove one part of a triangle, what happens to the remainder of the shape? It collapses.
Truss Joist I-Beam (TJI)
Truss joists, or wood I-joists, are members of the engineered family, as we all know. They are composite materials, which means they use different components to achieve strength and resists forces and loads. Let’s break it down to individual parts. I-Joists have a top flange and a bottom flange with plywood or OSB web members. (6). They are combined by inserting the web member into a precut channel using a thermoset adhesive. The web member is inserted into the channel about ½ inch. Within the web there are prepunched knockout holes for utilities. All these factors contribute to the strength and failure of the material. The top flange resists compressive forces, the bottom flange resists tensile forces, and the web distributes the load evenly throughout. The deeper the web gets directly correlates to the amount of weight or loads the member can support. The flanges can be of dimensional lumber or engineered to specific sizes using pressed and laminated wood materials.
When under attack by fire, the unprotected I-joist performs poorly. Once fire reaches the I-joist, the plywood or OSB burns at a high rate, and fire extends through openings within the web, impacting both sides of the joist. Remember, we cannot assume that contractors did not compromise the web by cutting in openings that exceed the manufacturers’ recommendation—this has been observed dozens of times by this author. As soon as the web starts to burn, the I-joist loses strength. There is no redundancy, no margin of safety. The OSB web is also subject to chipping and moisture, which can influence load-carrying capability long before a fire even starts. When gypsum boards are added to the underside of I-joists for ceiling assemblies, rooms can withstand up to 40 minutes of fire resistance.
The I-joist fails quicker than any other wood light weight material used in floor and roof assemblies. The National Engineered Lightweight Construction Research Project reveals that the wood I-joist fails within five minutes of direct fire attack. When one joist fails, loads are superimposed onto other I-joists, which can cause the rapid failure of an entire floor system. Collapse is often without warning. Firefighters should look for sagging and spongy floor systems, although this is not always seen. One way to assess the integrity of the I-joist system is to look at the location where the wall meets the floor. The floor system can be seen inches beneath the edge molding still tacked to the wall. The top flange can remain attached to the underside of the floor after the I-joist has completely burned away, giving an appearance that the floor is still intact, but the sudden loading of a crawling firefighter will rapidly cause collapse.
Connections
A metal plate connector or “gusset plate” hails originally from Fort Lauderdale, Florida, in 1958. Originally dubbed “grip plate for truss,” these connectors are made of galvanized steel sheet plates that are hydraulically pressed on one side, forming small spikes or teeth on the other side. The teeth of the metal plate connector are about 1/4 to 3/8 inches. The plates are then secured, first by hand to truss members, and then by a hydraulic press. There is an allowance for a 1/8-inch max space between web and chord members (7).
Experience has shown us that the weakest part of a lightweight component or any engineered product is the connection. This is where web members and chords are connected by, you guessed it, the metal plate connector or a simple adhesive. They may be effective connections in a laboratory test, however experience suggests otherwise. Metal plate connectors often fall away from the truss at the panel point where all the web members meet, causing failure of the entire truss (8). This connection is limited to the premise that there is no mishandling of the truss during transport and installation. Should a truss undergo a significant impact to the panel point and the penetrations are dislodged, they cannot be set back in appropriately. It has been observed on one occasion that the metal plate connector that was dislodged was re secured with a ½-inch steel staple. Metal plate connectors are made from galvanized, light-gauge, cold-formed steel. This type of steel experiences buckling and warping at 800F. Pinned connections, such as used in pin and bar truss systems, also use cold formed steel.
Adhesives have long been thought of as flammable; this is not the case. But older adhesives used in lightweight construction have been known to rapidly delaminate, which can occur sooner than the release of plate connections or cold-formed steel connections, again without warning. Today, lightweight components use thermoset adhesives that char, but if you are a firefighter who can somehow ascertain what adhesive is being used during a job, you deserve a medal.
The Bowstring Truss
The bowstring truss was initially introduced as a bridge design, invented by Squire Whipple in the early 1800s. The truss gets its name from its obvious shape and layout, where there is a curved upper chord and flat bottom chord (9). In construction, the design was later used as roof trusses for large factories, warehouses, bowling alleys, or really any location requiring large, open spans that could withstand significant snow loading. The truss uses a curved-arch top chord and a linear or flat bottom chord. There are number of different web configurations and names for the bowstring truss, however the three most common are the lattice bowstring truss (also known as a Belfast truss), which is the earliest known design of bowstring trusses used within structures (Figure 10). Other types include the Summerbell and Tim/TECO bowstring trusses (same manufacturer).
Bowstring trusses are not lightweight, however they are engineered wood products that use multiple components to achieve load-carrying ability. When one of these components fails, the entire system is compromised. The connections, loading, and misuse of the truss loft have been just a few reasons why these systems have fatally crashed down onto unsuspecting firefighters with no apparent signs or symptoms. Again, due to the design of these systems, they allow for direct fire impingement on all sides. Fire also attacks the connections at the panel points of these systems, which are often made of steel.
Again, there have been many variations of the bowstring truss, and there are many who point out those different nomenclatures in the firefighting community. The point is that these arched systems require substantial support to remain upright, and when attacked by fire and lateral bracing and/or an unmaintained arched truss systems have become rotted out at the connections, the outcome has historically been tragic. Let’s focus less on what we call it when we pull up to a job, and just recognize that these roof systems are inherently unstable during fire conditions.
Failure
By far, the greatest modality of failure of lightweight elements is the reduction in mass, which is characterized by a higher surface-to-mass ratio. Simply stated, there is more available material to be attacked by fire. Secondly, the open concept of many of the lightweight components permits unimpeded fire travel. Think of a truss loft, the open space created by the design of many trusses, and how this promulgates lateral fire spread at enormous rates. Miami-Dade (FL) Fire Captain Bill Gustin described this design as “horizontal balloon frame.” This is significantly enlightening when you consider why so many of these systems fail when attacked by fire. When it comes to lab testing, many know that I am very skeptical of these tests. As firefighters, we must consider the facts. Engineered wood products, specifically lightweight elements, are manufactured and shipped in controlled environments. The facilities conducting the tests are sent the very best product, which has not been subjected to improper storage on the jobsite, moisture, ultraviolet exposure, improper installation, and long-term expansion and contracting from thermal environments within a home. These factors significantly weaken the systems over the years, such that when a fire occurs, they are already susceptible to early failure.
With dimensional lumber of mass and closed joist bays we see local failure, and a redundancy of the remaining floor system (Figure 11). Firefighters attempting to operate above a fire where truss systems are present are taking a significant risk. The excessive weight caused of members in full gear can undoubtedly compromise wood trusses that are burning. A lot of the warning signs or sounds usually mentioned are not always present, and wood trusses fail without warning.
Engineered wood products are susceptible to environmental damage such as mold, fungal attack, or pests. Water damage, concentrated loading, renovation, and repair are all common over the lifespan of these elements (Figure 12). We frequently look at lightweight or engineered system collapses as occurring because of fire damage alone, whereas the prescription could be there long before the fire, which ends up merely being the precipitating factor of global failure.
When a bowstring truss system is attacked by fire, the connections are where we see the early failure. Steel bolts and plates fail or become ineffective, releasing the structural members. Because these truss systems are so widely spaced, when a bowstring truss releases, a large, open span supporting the roof is dismantled, releasing wood joists above and causing a significant local collapse of that area. Bowstring trusses use cross ties and bracing, since they are very unstable laterally when not connected to a system. When the bracing and ties fail, it can cause a large truss to topple over laterally. The occupants also tend to use the truss loft within a truss as storage space, often loading up the bottom chord with excessive weight, which contributes to unpredictable collapses, such as that seen in the Hackensack fire.
A firefighter’s education never stops. Keeping up with information about lightweight construction is necessary to contemplate extreme risk and better allow fire officers to make informed decisions. Just being aware of the existence of lightweight elements allows us to limit excessive personnel, effectively manage our suppression and ventilation efforts, and monitor conditions with a heightened sense of awareness. Many of the tragedies surrounding lightweight construction could have potentially been avoided, but the threat was not known at the time. We are now in a more informed position, and this education has undoubtedly saved firefighters’ lives. Know the building, as Frank Brannigan said—the building is your enemy.
SALVATORE ANCONA is a deputy chief fire instructor at the Nassau County (NY) Fire Service Academy. He is also a captain with the Seaford (NY) Fire Department and a fire inspector with the North Bellmore (NY) Fire Department. He has a bachelor’s degree in Fire & emergency services administration from John Jay College of Criminal Justice, an associate’s degree in fire science from Nassau Community College, and attends Georgetown University for his master’s in emergency and disaster management. Ancona was a recipient of the 2019 FDIC International Honeywell DuPont scholarship, is a contributor to Fire Engineering, and will be presenting Lightweight Construction, Heavyweight Implications: The New Fire Officers Guide to Engineered Wood Products at FDIC International in 2025.
