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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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 firstname.lastname@example.org or access his website www.competentcommand.com.