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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Prepare for the Big Event
- Using the backs of two chairs (or the edge of two tables) to support each end of the top chord, position the truss between the chairs.
- To protect the floor when the weight plates fall, place a folded floor mat or flattened boxes under the truss.
- The truss should easily support 40 pounds (the truss shown in the photos could have supported 60 pounds).
Load the top chord with 40 pounds (photo 24). Make sure the plates are positioned gently (minimize impact) and distribute the load as much as possible. (I don't recommend concentrating the plates in the center of the top chord.) Keeping toes and feet clear, use a butane lighter to heat the bottom chord. The resulting failure is pretty cool. Although the truss shown could have easily supported 60 pounds, it's better to be conservative so the truss doesn't fail because of overloading.
IMPORTANT CONCEPT: Using a separate 30-inch yardstick (pre-cut for a future truss), position the yardstick (flat, not on edge) as a simple beam bridging the span between the chairs. Ask students: Could the 30-inch yardstick simple beam support 40 pounds? (Of course, the answer is no, it couldn't support 10 pounds.) Now ask: The 30-inch simple beam bridge must support 40 pounds. What are your options?
There are a few options: inserting columns to turn the simple beam into a continuous beam; make the beam deeper (increasing the mass, like a header); turn the yardstick on edge and add adjacent on-edge yardsticks to share (distribute) the load; suspend the center of the beam with a cable or steel rod (as shown on page 98 of the September 2009 issue of Firehouse®); use a steel I-beam; or some combination of the first four options. Remind students that the best option is to replace the beam with the yardstick truss.
Call to Action
A marine biologist who has studied and understands shark behavior does not consider a shark to be an "evil" monster or a vocational "enemy." To the informed marine biologist, a shark is simply an interesting fish. However, because marine biologists respect the power and grace of sharks, they would not do something stupid that would make them vulnerable to shark attack. Likewise, fire officers who understand the grace, power and behavior of trusses do not consider them evil. Trusses are incapable of thinking; trusses are not aware. On the fireground, fire officers are capable of thinking and making decisions. Fire officers must understand truss behavior, identify and factor the presence of lightweight construction, determine who and what has the most value, and make appropriate decisions.
A truss is an assembly of pieces that are engineered to resist dead and live loads; nothing more and nothing less. Trusses are not evil predators that hunger for firefighter prey; trusses are predictable, trusses are reliable. Fire officers are not nearly as predictable or reliable as a truss. Trusses have been around since the Roman Empire, so they aren't going away any time soon.
Should wood trusses become involved in fire (or steel trusses exposed to high heat) the master craftsman fire officer — an informed strategist — will recognize and factor this hazard and the risk will not be ignored. Your call to action is to acknowledge that you do not possess magical powers that make you invincible.
Your final call to action is to ensure that your firefighters will not be harmed because you did not understand the principles of building construction, and in particular the behavior of lightweight structural components once they are exposed to heat — including glued-together wood I-joists.
The informed strategist acknowledges that when the failure of a truss harms a firefighter it's not because the truss is evil. It's not because fire is evil. It's not because gravity is evil. It is because a fire officer allowed the firefighter to be there where and when the truss failed — or worse, didn't know where the firefighter was, what he was doing, and why he was there. Make sure your firefighters aren't there when and where it happens in your community.