Bridge over cardboard water

Bridge over cardboard water

A group of triangles can be joined together to form a building truss. Trusses are used to build the roofs on houses and for many other structures, including bridges. 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.

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

Cut a piece of cardboard so it is 35 cm long and 15 cm wide.

Draw a 15 cm "river" in the middle of the cardboard.

Cut a 4 cm by 4 cm hole in the middle of the river.

Draw two 5 cm squares on the cardboard, as shown.

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

Once bridge construction ends, testing begins. Attach a string to a yoghurt 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 does your bridge take before it breaks under the load? Once your bridge has broken, use a kitchen scale to weigh the contents of the yoghurt 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.)

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