Building Lab, part 2: Shapes and Building

These investigations are designed so that the students can answer the question which shape is more stable, a triangle or a square.

Building with Straws

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Begin by building shapes using straws and paper clips. To connect two straws, slip the wide end of a paper clip into the end of one straw. Hook a second paper clip to the first. Now insert the wide end of the second clip into a second straw.

Compare the stability of the shapes. Stand each shape up and press down on the top corner. What happens? How much does each one bend and twist? How hard can you press down on each shape before it collapses?

When compression force is applied to the joints, a triangle changes shape less than a square, because the compression in the two sides is balanced by the tension in the crosspiece at the bottom, which pulls the sides together, making it a more stable shape  When compression is applied to a square, the joints rotate easily, and the shape changes.

Other Investigations

Can you reinforce the less stable shape by adding straws and paper clips?

Build the most stable structure you can using straws and paper clips. How much weight can your structure support?

Building with Toothpicks and Gum Drops

Activities and photos from 6/23/08.

Triangles make structures more stable because they formed a truss, which is a skeleton-like structure composed of struts, some of which are in compression and some in tension, which are joined to form a series of triangles.

Activities and photos from 6/23/08.

Have your students begin by building a bridge with trusses by constructed a rectangular box of toothpicks and gumdrops. Next, test its stability by pressing down on it and wiggling it and your students should find that it is not so stable.

Activities and photos from 6/23/08.

Next challenge them to add more materials to strengthen the box, by adding cross-pieces and triangular braces. Then have them extend their trusses to see how wide a gap they can make that is still stable.

Shape Walk

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Bollman Truss Bridge, spanning Little Patuxent River near Savage Mill in Savage, MD.

Take a walk with a camera and scavenger hunt looking for examples of shapes used in structures. Scaffolding cross-braces and trusses under bridges and railroad overpasses are good places to see triangles.