For 'In Straws We Trussed', we will build structures of optimum height and strength, measured by the number of marbles that can be held. In order to understand the process, we must first experiment with different structural designs. We will begin by examining the bridge design.
To begin, let's understand the terms compression and tension, then apply their meanings to making our bridge.
Compression-the process of something being pressed together, or squished
Tension-the process of pulling or stretching something
In order to imagine this in relation to a bridge, imagine a floating piece of foam. Try stretching it and pressing it together. Not much seems to happen, right? Next, put a grid in the middle of the piece of foam, or follow the pictures below.
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| stretched | |
pressed together |
Now, as you stretch or press the piece of foam together, you may be able to slightly see some changes in the distance between the lines on the foam. As the lines become closer together, the foam is being compressed; as the lines become further apart, the foam is experiencing tension.
Next, imagine this same piece of foam as a bridge. Holding the foam on one end, let the other end rest on top of a solid surface, such as a table or the edge of a chair. Keeping your hand in the same position, use your other hand to press down slightly on the middle of the piece of foam towards the ground. What do you see?
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Perhaps, the first thing you noticed was the grid in the middle of the piece of foam. As you pressed towards the ground, the top of the grid became compressed (the lines became closer together). But, what happened to the bottom of the grid? The lines grew further apart, indicating the tension of the bottom of the foam.
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| The top | |
The bottom |
Try pressing the middle of the foam up rather than down. What seems to happen here? Similar to pressing down on the foam, the tensions and compressions are reversed. Here, the top of the foam experiences tension, while the bottom of the foam is compressed.
Try pressing in the middle of the piece of foam, and pay attention to the end of the foam resting on the table. What happens to it? It moves horizontally, or across, rather than up and down.
As illustrated by the piece of foam, a bridge also experiences tension and compression when it has a force from either above, below, or the sides of the bridge. Forces may come from strong wind, traffic, earthquakes, etc.
We can now use the Truss Applet to further experiment with these forces.
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