# Structures

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Activity source: Construction Specifications Institute

Experiment 1: Loads and Forces, Buckling
Materials
• Uncooked spaghetti

Push on the ends of a piece of uncooked spaghetti. The sideways bending is called buckling. The compression force that you apply causes complex internal forces that bend the spaghetti sideways. If you push hard enough, it will snap. The snapping starts on the edge where the tension force within the spaghetti is great enough to pull it apart.

Brittle materials like spaghetti, stone or glass break rather easily this way. Stone columns must be made so they won't buckle—because once they start to buckle, they will collapse.

Experiment 2: Beams
Matrials
• Flat rubber eraser
• Pencil, pen or marker

Make a clapper bridge out of a flat rubber eraser. First mark the eraser with some parallel lines. Push on it. It is hard to break, but you will see something else too. The lines spread apart at the bottom edge. When you increase the load on the bridge, you are producing tension on the bottom edge. (You can also see compression on the top edge.) Long, heavy stone beams don't work because stone cannot take much tension without cracking.

Experiment 3: Arches
Materials
• Cardboard
• 10 books
• 2 small books

Try making a thin, flexible arch of cardboard. Bend it carefully—a crease will make a weak spot and spoil the arch. Support the ends first with two small books. Push down to see how the arch pushes out on these abutments. Then using a pile of books push down to see how the arch pushes out on these abutments. Next use a pile of books at each end. When you push down, you will see the arch buckle at the sides.

Experiment 4: Triangles and Trusses
Materials
• Coffee stirrers
• Pipe cleaners
• Drinking straws
• Paper clips
• Strips of wood
• Small finishing nails
• Toothpicks
• Miniature marshamallows

Here's the basic connection with coffee stirrers: Using four stirrers and four joiners, make a square frame. Notice that you can change its shape easily into a new form (a parallelogram).

Now attach one more stirrer with two joiners to make the parallelogram into two triangles. A triangle tends to retain its shape when you push or pull on it. When you push on one corner, for instance, the other two corners try to spread apart, but the opposite sides holds them together. When you load this truss frame by pushing on it, it will keep its shape much better. Now build a structure using triangles as the basic building unit.

Experiment 5: Suspension Bridge
Materials
• String
• 2 books
• Board

Tie a string between two books. Push down on the string. The "cable" pulls down and inward, toppling the books.

Now set up the books on a board. Pass the string over the books to thumbtack-anchors. Push down slowly, harder and harder. You'll be able to put a lot of load on the string-cable; you may even be able to push hard enough to lift the thumbtacks out of the board.