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.