Bridge Over Cardboard Water

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Bridge Over Cardboard Water

A group of triangles can be joined together to form a truss. There are many different truss designs, including the Pratt, Whipple, and Fink (sounds like a law firm). Although truss bridges are not as good for spanning great distances as other bridge designs, they are extremely strong. Steel trusses, for example, are often used in railroad bridges. To build your own truss bridge, you need a steady hand and a bit of patience. But if you stick with it, you will have one terrific truss bridge.

For a printable version of this project, click here.

Materials

• toothpicks (flat kind)
• glue gun (low heat kind)
• cardboard
• marker pen

Note: You can use popsicle sticks if you find toothpicks too fiddly. If you don't have a glue gun, white glue works--it just takes much longer to dry and requires a lot more patience.

Instructions

1. Cut a piece of cardboard so it is 35 cm long and 15 cm wide.
2. Draw a 15 cm “river” in the middle of the cardboard.
3. Cut a 4 cm by 4 cm hole in the middle of the river.
4. Draw two 5 cm squares on the cardboard, as shown.
5. Design and build a truss bridge using the toothpicks and glue gun. (Get an adult to help you with the glue gun if necessary.) The actual design of the bridge is up to you, but your finished bridge must meet the following specifications:
• The bridge can only touch the cardboard inside the squares.
• The bridge must be at least 4 cm above the river (to allow boats to pass under).
• The bridge must use triangles (or it's not a truss bridge).

Testing

Bridge Over Cardboard Water Testing
Once bridge construction ends, testing begins. Attach a string to a yogurt container so it hangs without tipping (see photo). Poke the string up through the hole in the middle of the cardboard. Use a pencil to hang the container from the bridge. Now gently add weight to the container. Try using marbles, rocks, ball bearings, or coins.
How much weight will your bridge take before it breaks under the load? Once your bridge has broken, use a kitchen scale to weigh the contents of the yogurt container and the pencil. (If you don't want to break your bridge, stop adding weight as soon as there's any bending or creaking.)