truss — A framework of triangulated forms in which all loads are carried by compression or tension in each member of the frame. It is time for the fire service to make peace with an old nemesis: the truss . For too long, the fire service has vilified this structural engineering marvel. Trusses...
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On page 80, figure A illustrates the components of a typical truss. Virtually all trusses used to support structures have the same components and behave the same as the truss illustrated. All trusses are derivatives of three basic truss configurations:
- The flat/parallel truss (figure B)
- The triangular truss (figure C)
- The arch/bowstring truss (figure D)
Each truss configuration relies on the same system of chords, connections and web members that create triangles: three panel points create a triangle; the more triangles, the stronger the truss. Because it does not rely on triangulation, note that the original "bowstring" is not a truss (figure E). Over the years, the arch truss has assumed the bowstring moniker.
Command-O-Quiz: Of the trusses listed below, which represents the most danger to firefighters?
- King post
Answer: Sorry, that was a trick question. Actually, each truss represents approximately the same risk to firefighters. As mentioned above, all trusses work the same — chords, web members, panel points, triangles, compression, tension; the point of the question is to emphasize that the failure of a single bowstring truss can be more catastrophic than the failure of 10 parallel trusses.
Allow me to explain: Let's say that a single bowstring timber truss has a 50-foot span and is spaced 30 feet from adjacent bowstring trusses. The failure of this single bowstring truss would compromise more than 3,000 square feet of roof! Compare that with lightweight parallel trusses spaced four feet apart.
Visualize a Type III tilt-up warehouse featuring a flat panelized roof: A section of roof is comprised of 10 parallel chord truss purlins; each truss purlin spans 50 feet and is spaced four feet from adjacent trusses. Should all 10 of these trusses suddenly fail, 2,200 square feet of roof would be compromised. The failure of the single bowstring truss would compromise 800 more square feet of roof than the failure of the 10 lightweight parallel chord trusses!
Because of its mass and bolted panel points, the timber bowstring truss should be more fire resistive than its lightweight parallel truss cousins; thus, the timber bowstring should resist fire for a longer period than its lightweight parallel relatives. Although it is a "truss," the conventional timber arch truss is not "lightweight."
Next time, you will get to know the truss — up close and personal. You will understand the components of a truss. You will understand the principles of how and why trusses work; you will understand how, with the minimum amount of material (mostly empty space framed by triangles), these lightweight marvels can be so incredibly rigid and strong. I will describe tools that you can use to train your firefighters about how trusses work, including how to build a model truss made of yardsticks and string that will support 40 pounds.
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 may be contacted at firstname.lastname@example.org or access his website www.competentcommand.com.