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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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 yardstick truss and two trusses assembled using a child's construction toy called K'Nex (www.knex.com).

 

K'Nex Trusses as Training Tools

I recommend you buy individual pieces rather than invest in a prepackaged K'Nex set. For example, I ordered enough individual pieces to create 30 student activity "kits." Each kit has enough pieces to assemble one triangular and one flat truss. I store the pieces for each kit in plastic freezer bags. Each kit costs about $4. I have students first assemble the triangular truss, disassemble and use the same pieces to assemble the flat truss. I recommend tossing in a few extra rods and connectors into each kit.

During your presentation, guide and discuss step-by-step assembly so that students experience the structure getting stronger and more rigid as geometry is added. (Let me know if you figure out how to assemble an arch truss.) During your presentation, emphasize that trusses are not dangerous; rather, trusses are unthinking, unaware, inanimate objects. Instead, encourage firefighters to understand and respect trusses for the engineering marvels that they are. Trusses cause harm only when they are not identified and not factored by fire officers.

K'Nex triangular truss — Start with the K'Nex triangular truss. Have students build the simple, single-panel truss shown in photo 8. Have students twist, squeeze and manipulate the truss. Ask students to identify the weakest part of this truss. They should identify the center of the truss.

Ask students how to make the single-panel truss stronger and more rigid, with the following four conditions:

  1. The length of the truss (heel to heel) must stay the same.
  2. The depth of the truss must stay the same.
  3. No piece of the truss can increase in mass.
  4. Use only the remaining contents of the K'Nex kit.

Given those conditions, the only option is to add geometry by adding pieces between the top and bottom chords.

Have students add a kingpost (the vertical red rod shown in photo 9). Discuss how this two-panel truss is somewhat stronger but still offers little structural value. Before adding more pieces, have students disconnect the king post from the bottom chord panel point. Simulate truss loading by artificially stressing the truss at the heels. Ask students, if still connected to the bottom chord, would the king post be in tension (pulled down) or in compression (pushed up).

Photos 10 and 11 show evidence (strain) that the bottom chord panel point would be pulling down on the king post; thus, the king post would be in tension. Ask students what would happen if a column were positioned under the same panel point, thus resisting the pulling force. (Answer: The king post would be in compression.) Reconnect the bottom chord panel point and disconnect a top chord panel point. Again, apply stress and notice that the king post is pulled down by the bottom chord panel point, further evidence that, if still connected, the king post would be pulled into tension.

After reconnecting the king post to the bottom chord, ask students to identify where the weakness is now; students will identify that the weakness is now between the kingpost and the heels. To resolve this weakness, add a diagonal web member (yellow rod) to each side of the king post (photo 12). Again, have students manipulate the truss and discuss the increased strength and rigidity. Students will identify two remaining weak spots along the bottom chord. Ask students how to resolve this weakness.

The bottom chord weakness can be resolved by adding two vertical blue rod web members (photo 13). Ask students to manipulate the finished truss (twist, squeeze, stress) and discuss how strong and rigid the truss has become although — this is important to emphasize — no piece of the truss got bigger; in other words, pieces were added, but the mass of the truss did not change.

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