Truss Truce: Part 3

truss — A framework of triangulated forms in which all loads are carried by compression or tension in each member of the frame.I will now introduce you to three simple and effective tools that will demonstrate to your fire officers and firefighters how...


truss —   A framework of triangulated forms in which all loads are carried by compression or tension in each member of the frame. I will now introduce you to three simple and effective tools that will demonstrate to your fire officers and firefighters how a truss works. Photo 5 shows my...


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Have students disconnect a diagonal yellow web member from the top chord panel point as shown in photo 14. After having students stress the truss, ask if the yellow web member would be in tension or compression (if still connected). The visible strain provides evidence that the yellow web member would be in compression. Inform students that they will occasionally hear web members referred to as "struts" (compression) and "ties" (tension). Students will also find truss diagrams that show compressive web members drawn as bold lines and tensile web members drawn as thin lines.

IMPORTANT CONCEPT: Students have just revealed (and demonstrated) the magic of lightweight truss construction. Increased strength and rigidity was the result of adding pieces that created geometry (triangles), not by increasing the dimension of the pieces. Remind students that to support more load or to increase clear span legacy conventional construction required making things bigger and relying on compression.

K'Nex flat truss (parallel chord truss) — Have students assemble the trapezoid shown in photo 15.

For the photos 15-20 for this article, please use this link: http://www.firehouse.com/stateprovince/other/truss-truce-part-3-remaining-photos

As students manipulate (twist, squeeze) the trapezoid, discuss whether there is any structural value. Students will quickly recognize that the four-sided structure is weak and unstable and has no structural value.

 

Now have students triangulate the trapezoid using blue rods (photo 16). As students manipulate the five-panel truss, they will quickly recognize that it is rigid, stable and strong. Although the truss is more rigid and much stronger than the trapezoid, no piece of the structure got bigger (legacy, conventional thinking); instead, more pieces were added to create triangles (contemporary, lightweight thinking).

Now have students disconnect a panel point anywhere along the top chord. Have them simulate the forces that would stress and strain the truss should a heavy load be applied (photo 17). Point out that the top chord strain at the disconnected panel point is pushing/shortening; thus, the stress is compression.

After reconnecting the top chord, have students disconnect a panel point anywhere along the bottom chord. Again, have them "load" the truss. Students should observe that the bottom chord strain is lengthening/pulling, thus the stress is tension (photo 18).

Finally, ask students how to make the truss stronger and more rigid. Remind them that the length can't change, the depth of the truss can't change, and no piece can get bigger. The secret is to add additional triangles (shown by the vertical orange lines shown in photo 19). Again, emphasize that this additional triangulation decreases the effective length of the top and bottom chords.

The Yardstick Truss

Not only is my yardstick truss an interesting training tool, it is cheap, portable, and reliable. To assemble the yardstick truss you'll need three yardsticks, a saw, one hot glue gun, some string, one nail, and a quarter-inch drill. The purpose of the Yardstick Truss is to demonstrate the importance of the bottom chord. Granted the yardstick truss is not "engineered," but it is dramatic.

Yardstick truss anatomy (photo 20):

• One 30-inch top chord

• Eight nine-inch web members

• Five hot-glue panel points (spaced seven inches apart)

• Three or four feet of quarter-inch string

• One finishing-type nail

• Four to six 10-pound weightlifting plates

 

For the photos 21-25 for this article, please use this link: http://www.firehouse.com/stateprovince/other/truss-truce-part-3-photos-20-25

To assemble the yardstick truss (photo 21):

  1. While maintaining a one-inch overhang at each end, use a pencil to mark each of the seven-inch panel points.
  2. Drill a quarter-inch hole at each end of the top chord (in the middle of each one-inch overhang at the ends).
  3. Use the hot glue gun to connect the web members at the panel points. (The panel points don't require a lot of glue.)
  4. At one end secure the string with a knot tied around a nail (so the string can't pull through) as shown in photo 22.
  5. To serve as the bottom chord, stretch the string along the bottom of the truss and through the hole at the opposite end.
  6. Pull the string into tension so that the top chord deflects (arches) slightly upward (depending on mass and rigidity of yardstick, about a two- to three-inch top chord camber as shown in photo 23.)
  7. While holding tension to maintain the top chord deflection, secure the opposite end by wrapping the string two or three times and securing with a non-slip knot (the knot doesn't have to be fancy). Pre-stressing a slight arch in the top chord will ensure that the top chord flattens when loaded; when loaded, the bottom of the web members should rest on the string. (Again, see photo 23.)
  8. Do not hot-glue the string to the bottom of the web members!