and Pins--Building Out
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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.
What's Going On
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.
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.