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What's Going On (continued)

All structural action consists of tension and/or compression. In these structures, this means that every straw is either in tension or compression. It is interesting to go through a few structures with your students to try to figure out which straws are in tension and which are in compression. You can determine if you are correct by pulling out a pin on one end of the straw. If the two points where the ends of the straw had been attached move apart, the straw was under tension. If the two points move toward each other, the straw was under compression. It is interesting to compare where the tension and compression elements are in bridges versus cantilevers. In compression bridges, which are supported (held up) at both ends, the line of straws along the top are in compression and the line of straws along the bottom are in tension. In cantilevers, which are supported (held up) at one end, the opposite is true. Here, the line of straws along the top are in tension and the line of straws along the bottom are in compression.

Many engineering principles emerge from these activities. Your students will readily volunteer that "triangles are strong," and you will most likely see examples of diagonal bracing put in to stabilize square sections. Pin joints are common in real structures. For instance, rivets are used with steel beams and nails are used with wood planks in the same way that pins are used with straws in this activity.

• Students should be encouraged to test the strength, stability, and durability of their structures. In this way, they gain insights into the behavior of structures that are subjected to outside forces, and they can see how to improve their structures so as to strengthen weak points. In addition to hanging weights on structures, they can be "wind tested" using a large piece of cardboard as a fan. You can also leave the structures intact overnight, to discover which ones are still standing in the morning.

• An additional interesting extension is a search for unnecessary straws. Ask the builders if they think there is a straw they could cut that would not make the whole thing fall down. Have them try it. If it works, could they cut another straw? How many cuts do they think it will take before the structure collapses? Could they build a structure that could not stand many cuts?

• Have the students take a set of materials home to build a structure with their family. They can draw or photograph their structure and write about what they did.