truss — A framework of triangulated forms in which all loads are carried by compression or tension in each member of the frame. With apologies to the legacy of the late, great Professor Frank Brannigan, do not fear trusses. Trusses are not geometric predators that hunger for firefighter prey...
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— A framework of triangulated forms in which all loads are carried by compression or tension in each member of the frame.
With apologies to the legacy of the late, great Professor Frank Brannigan, do not fear trusses. Trusses are not geometric predators that hunger for firefighter prey. Trusses are not aware; trusses are incapable of thinking or making a decision. On the other hand, the alert, poised, and confident fire officer is capable of thinking and making decisions.
To dovetail with part one of "Truss Truce" (Firehouse®, February 2010), rather than "beware the truss," a more constructive maxim may be "beware the fire officer"; to be more specific, beware the fire officer who does not understand truss behavior and does not factor the presence of trusses during a fireground operation.
If a firefighter dies as a result a truss failure, is it the fault of the truss? Should the truss be vilified? Read the lightweight construction-related line-of-duty-death reports from the National Institute for Occupational Safety and Health (NIOSH) — and, before NIOSH, the U.S. Fire Administration (USFA) fatality investigation reports. If lightweight construction was involved in the fatality, the reports tend to portray trusses as the problem. However, by carefully reading these reports, you will notice that enough information is provided for the reader to peel the layers of the proverbial onion. As your mind peels the layers of information and timelines, you arrive at an important question that the reports refrain from asking: Why was the firefighter there when the truss failed? Granted, 20/20 hindsight is easy; nonetheless, the answer will reveal a pattern that includes no size-up, problems not identified, no strategy, no tactical accountability, freelancing, fire officers at task level, nobody watching the clock and more (see "13 Fireground Indiscretions," Firehouse®, April 2006).
Part one of this article offered a brief overview of truss anatomy and truss history. This article will:
- Review basic truss anatomy,
- Review how a typical truss works,
- Introduce the most dangerous truss that you have never seen,
- Offer training tools that you can use to demonstrate truss behavior to your firefighters,
- Using these training tools, demonstrate how a typical truss works.
Let's start by reviewing the anatomy of a typical truss. Using the following diagram, identify the basic components (anatomy) of a truss:
A = _____
B = _____
C = _____
D = _____
E = _____
F = _____
A = King post (also a web member), B = Panel points (connections), C = Top chord, D = Truss panel, E = Web member, F = Bottom chord
How a Truss Works
Whether it is a triangular truss , a flat truss or an arch truss, the business portion of a truss are the top and bottom chords — especially the bottom chord. Granted, every element of a truss is important, but the chords do most of the work; in fact, three to four times the work compared to a given web member (rule of thumb). For example, the top chord of a triangular truss may need to resist a compressive force of around 2,400 pounds, while within the same truss a compressive web member may experience a force of just 500 pounds.
Because its entire length is in tension, the bottom chord is the most critical truss component. When loaded, the bottom chord strains to resist three to four times the tension as compared to a shorter web member that is in tension. Conversely, when loaded, the top chord strains to resist three to four times the compression as compared to a shorter web member that is in compression.
Because it is in compression you could (theoretically) cut through the top chord of a truss and it will not come apart. Because it is in tension I do not recommend that you cut through the bottom chord of a loaded truss. I strongly recommend that you never disturb a top or bottom chord panel point. The more pieces being held together at a panel point increases the importance of that particular panel point. Should the panel point shown in photo 1 fail, the energy of five truss components would be released — including the bottom chord.
With load applied, a truss distributes the forces generated by the load through a series of web members; web members deliver these forces, alternately as tension or compression, to the top and bottom chords (Figure B). Web members are connected to the chords at connections called panel points. Typical lightweight panel point connectors include metal pins, metal plates, (referred to as gusset plates or gang nails), glued finger joints, welds and other connection methods.
The Most Dangerous Truss
Photos 2, 3 and 4 show the most dangerous truss that I'm aware of. At first glance, these trusses (or what's left of them) don't look all that special; there are lots of lightweight open web trusses held together by plywood "gusset plates." Look closely at photo 2; notice that there are no metal connectors — not a bolt, not a nail, not a staple, not even a thumbtack. All truss members are held together by glue. These trusses supported the roof of an unsprinklered community center in Woodinville, WA. The trusses failed due to a modest live load of wet snow. (The auto-pilot truck company that would stampede directly onto this roof and begin vertical ventilation "over the seat of the fire" should be placed on around-the-clock suicide watch.)
These glued gusset plate trusses were manufactured by a company in Everett, WA, in the 1960s. Although the company no longer exists, there are more out there.
The majority of trusses are designed to be supported the same as a conventional simple beam, at two bearing points at each distal end; these bearing points are referred to as the "heels." (Note: A truss is not a beam; a truss is a precision-engineered structural component that replaces a conventional solid beam. Beams deflect, or bend; due to the engineered direct stressing of each member, truss deflection is minimal.) There should be no "continuous" compressive support members between the two bearing points. If a truss requires an intermediary structural support, such as a column between the bearing points, a red flag should be raised: there is likely a serious problem with the truss. Supports installed between the bearing points effectively reengineers the truss and voids any design safety factor. (Important note: there are exceptions to this rule, such as the "tri-bearing" truss.)
Like all engineered wood products, trusses can be designed with a slight camber. This camber assures that the cords will not deflect over time. After a couple of years, this camber will relax and flatten, rather than sag.
Reality Check: Truss Engineer As Fireground Strategist
I once asked an engineer who designs steel trusses what happens to the safety factor of a steel bar joist parallel truss should a single web member be removed. (Considering that a steel bar joist could have a dozen or more web members, you would think that the safety factor would perhaps be reduced by a single-digit percent.) His answer: There would no longer be any safety factor; the factor of safety would be zero. He elaborated that his company would no longer be responsible for the performance of the truss. To ensure it sinks in, read that last sentence again. In other words, since the truss has been "modified," it is no longer their truss.
A contents fire can quickly modify an unprotected steel truss; all that is required is ample heat — direct flame exposure is not required. Any fire officers out there want to own the performance of steel trusses that have been modified during your fireground operation?
The steel-truss engineer asked why the failure of a web member would be a concern to the fire service. I explained that firefighters may consider cutting a hole so that heat and smoke could be vented through the roof. With jaw dropped and eyebrows raised, he said he couldn't believe that humans would consider being supported above a fire by trusses that he designs and knows intimately. (Since this conversation, I consider the combination of firefighters supported by lightweight steel trusses operating above a fire is akin to meat on the grill.)
MARK EMERY, EFO, is a shift battalion chief with the Woodinville, WA, Fire & Life Safety District. He is a graduate of the National Fire Academy's Executive Fire Officer program and an NFA instructor specialist. Emery received a bachelor of arts degree from California State University at Long Beach and is a partner with Fire Command Seattle LLC in King County, WA. He is in no way affiliated with or an advocate for the truss manufacturing or building construction industries. He may be contacted at email@example.com or access his website www.competentcommand.com.