"Deflection" has a specific meaning in engineering. In speech, we sometimes say say a question is "deflected," when the trajectory of the conversation is shifted slightly. "Deflection" in the physical world, however, is to deform an object from its natural form.
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Loads and Deflection
Deflection is distinct from from the stress or "load" that causes it. Deflection is the actual change in a form due to load. Here's an example. Imagine a 2 x 10 board or joist is laid flat, spanning from one fixed point such as the top of a wall to another fixed point. Now, imaging walking halfway across the span. The board will sag. How much it sags is the "deflection." The load causing the deflection is you, standing at the center of the board.
The purpose of a truss is to carefully design a single member that can withstand significant and very specific loads by assembling multiple, relatively weak members. The members are generally reinforced at their weakest point. The reinforcement then shifts the weakest point elsewhere. Generally, it is the weakest point of the truss that has the greatest deflection. But trusses can be designed to be "tuned" to have a certain amount of deflection.
It is important for engineers to understand deflection. Generally, deflection is modeled mathematically and predicted based on careful estimates of loads. Inspecting an installed truss or testing a truss in a controlled environment provides empirical evidence of loads; it indicates whether calculations, designs and construction were done properly.
Deflection to Failure
Simply put, the most important aspect of deflection is that it indicates where the truss will break if the current load is magnified. Most materials first bend in the places they will break. That's why engineers build trusses and break or "fail" them. Observing the deflection prior to and during failure teaches engineers how to build better trusses. Deflection provides critical feedback on truss design. If a design is known to be good, but has excessive deflection, it points to another cause. There might be a defect in materials or manufacturing, or an excessive load may be placed on the truss.