By Salvatore Ancona
Behind the failure of many structural engineering elements lies the very instrument designed to hold them together. Engineered wood products are mostly composite structural members that rely on two or more separate materials to achieve strength and ultimately carry a design load. Many fire service professionals recognize gussets in the field as indicative of such a lightweight or engineered structural element. Properly identified as metal plate connectors, “gussets” allow assembly of wood products quickly and economically. They have been adopted as one of the primary routes for designing and connecting engineered systems such as parallel chord timber trusses and roof trusses. Failure of the metal plate or “gusset” connector signifies the compromise of that individual structural element. A recent fire in a private dwelling was revisited and the toll was truly astonishing. It opened the eyes of some of our firefighters and provided great insight into what was really going on during the fire that was unseen at the time. The photos provided will show you the dangers of these connections in a timber truss system, and the lessons learned will provide insight into observations based on experience instead of relying on test methods in a controlled environment. We’ll take a look…
A metal plate connector or “gusset plate” originated in Fort Lauderdale, Florida in 1958. Originally dubbed a “truss grip plate”, they are light gauge steel metal plates that are hydraulically pressed from a side, forming small spikes or teeth on the other side (Figures 1 and 2). The metal plate connector teeth are approximately 1/4-3/8 inch. The plates are first fixed by hand to the truss elements, then are transmitted to be fixed by a hydraulic press. There is a tolerance of a maximum gap of 1/8 inch between the web and rope members.
Experience has shown us that the weakest part of a timber truss is the connection or panel point. This is where web members and chords meet by, you guessed it, connecting via a metal plate connector. As stated earlier, they penetrate wood fibers 1/4 to 3/8 inch (Figure 3). This connection is limited to the principle that there is no mishandling of the truss during transport and installation. If a truss takes a heavy impact on the edge of the panel and the penetrations are dislodged, they cannot be properly replaced. It was observed on one occasion that the metal plate connector that was dislodged was reattached with ½ inch steel. clip. Metal plate connectors are made from light gauge cold formed galvanized steel. This type of steel undergoes compromises at 800°.
Following a recent fire, we examined the conditions of a daylight parallel chord wood beam floor system. It was during this structural integrity assessment that we were able to draw certain conclusions based on actual events. The metal plate connectors were the weakest link in the truss system (Figure 4). Hundreds of metal plate connectors littered the debris, almost if it had rained from the gussets. The fire damage to the farms was extensive. The charred trusses released the bottom chord once the metal plate connector dislodged and fell off. The remaining metal-plate connectors didn’t stay buried in the wood; they moved away when moving the floor system after several trusses collapsed. Contrary to many opinions (including this author’s, in the past), metal plate connectors did not peel, warp, or bend. Once the wood surrounding and embedded in the plate connector burned away, the plate fell in one straight piece, releasing its limbs and precipitating further plate failures. Rust on the galvanized plate tells us that it has been overheated, eventually deglazing the zinc coating. This is the proof for us of its conductive transmission of heat in the wood. This confirms the account of the induced pyrolytic decomposition of the truss wood fibers from the teeth of the metal plate connector. Since the teeth of the metal plate connector were overheated and therefore conducted heat into the penetration, the teeth loosened and eventually allowed the plate to fall out. This challenges the theory that heat is reflected from the plate, which benefits the point of the panel.
Structural collapse under fire conditions
As you can see from the photos provided, the top truss chord remained attached to the bottom of the second story decking due to being nailed from above. You can also see a significant deviation from the ground of 10-12 inches, with no structural integrity underneath (Figure 5). It also proves that due to the massive tensile forces exerted on the bottom chord of the truss by fire damage, the metal plate connectors could not withstand the compression from above, thus releasing the plate, as evidenced by the teeth bent all in one direction.
The trusses were found only 10 minutes after the fire. When this was communicated, the members evacuated the second floor and opened a wall between the first and second floors and extinguished the fire in the trussed attic on one side of the house. Amazingly, post-fire evaluation showed that this allowed the truss elements to retain their integrity, with little damage to the system, but once the heated metal plate connectors were extinguished, there have been a number who have become detached from their connection.
Luckily, the members didn’t stay on the second floor for long. If the loading of several firefighters had continued, including the loading of white water, it is obvious that the ground would have completely collapsed. The trusses burned freely throughout the truss attic, and the first floor had only a mist at times. It’s very dangerous. The gypsum board prevented firefighters from seeing that the structural compromise was significant. It really is a lesson learned.
There have been lab tests regarding metal plate connectors, including the NISTIR 7393. In the opinion of this author, this test is not conducive to your health. The test uses two individual one-foot sections of wood connected by metal plate connectors. The test does not induce fire heating, but rather a radiant heat panel – and oh yes, only on one side (trusses don’t just burn on one side). Additionally, the disclaimer reads: “The load carrying capacity of the mesh metal plate connections was not measured in these tests.” It’s a big problem. Metal plate connectors experience a load during a fire and tensile forces pull the elements apart. This is why the plates remained attached to the wood in this test, and not in the real fires. The test did not reproduce compartment heat, movement due to fire damage, impact loads, point loads or tensile forces.
There are many factors that ultimately lead to the collapse of structural elements. Connections within engineered wood materials are the triggering factor for the majority of these cases. Will a truss always collapse in five to ten minutes? No. But that’s not always what worries us. We should be concerned about the weakening of materials and connections, as with the metal plate connector, which may mask a compromised structural system such as a truss floor, where the only precipitating factor needed to complete the collapse process is… us. (Figure 6)
SALVATORE ANCONA is Assistant Chief Fire Instructor at the Nassau County (NY) Fire Service Academy; a member of the Seaford (NY) Fire Department; a former captain and training officer for the Bellmore (NY) Fire Department; and a paramedic supervisor in Queens, New York. He holds an AS degree in fire science from Nassau Community College and a bachelor’s degree in fire and emergency services administration from the John Jay College of Criminal Justice. Ancona is the author of “The Sons of Brannigan” building construction page on Facebook and was the recipient of the 2019 FDIC International Honeywell DuPont Fellowship.
Combustible Construction: Podium Private Wood Frame Homes
Veneer vs. Facade: what’s the difference?
Grenier versus Cockloft: what’s the difference?
Firefighters and Construction: Beams and Beams: How They Fail