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Procedure

1. Lay down two (2) sheets of waxed paper about 18 inches long.

2. Begin by making 18 triangles. You will need 54 toothpicks. Take three (3) toothpicks at a time (do not use any toothpicks that are splintered or broken) and place them on the waxed paper. Put a drop of glue on both ends of one (1) toothpick.

*Some glue will drip off the toothpick onto the waxed paper: that's OK, as the glue will only stick to the toothpick and not the waxed paper.

Place two more toothpicks in the glue at the tip of the first toothpick. For each vertex, place the skinny end of one toothpick on top of the fat end. Glue the third toothpick at the vertex to complete the triangle.

making a truss bridge with toothpicks

3. Carefully adjust the triangle so that no ends are overlapping and all toothpicks lay flat. Using a fourth toothpick as a tool to adjust the triangle is helpful. Wipe off any glue that gets on your fingers with a wet paper towel.

make triangles with toothpicks

4. Next, make 10 squares. You will need 40 toothpicks. Take four (4) toothpicks at a time. Put a drop of glue on both the ends of one (1) toothpick. Place two more toothpicks in the glue at the tip of the first toothpick. For each vertex, place the skinny end of one toothpick on top of the fat end. Glue the third and fourth toothpick at the vertex to complete the square.

5. Adjust all toothpicks so they lay flat and have no overlapping edges at the vertex. Make sure all vertexes are right angles.

truss bridge science project

6. Allow all triangles and squares to dry 24 hours.

7. Now, build two (2) trusses by placing the triangles in an alternating pattern (shown in the picture below) then gluing together the single triangles. Let dry at least 4 hours.

complete truss bridge made with toothpicks

8. Next, build two (2) square shaped trusses by gluing together the single squares. Let dry 4 hours.

9. Stand up the two triangle trusses (an extra hand from your parents would prove useful) and glue a connecting toothpick at the vertex of the middle triangle of each truss to form a "roof" for your bridge. Continue to glue connecting toothpicks at each triangle's vertex.

10. Repeat the process in step 9 with the two square trusses.

11. Allow both finished bridge models to dry for at least 12 hours.

12. Finally, test how much force each bridge model can hold.

To do this, place a 12" x 3" piece of cardboard on top of one bridge. This simulates the bridge deck. Place the bucket on top of the center of the cardboard. Pour the rock material (gravel, pebbles or sand) into the bucket one (1) scoop at a time. Wait briefly before adding each scoop to observe any shifting, buckling or collapsing of the trusses.

13. Record how many scoops of rock material it took to collapse each bridge.

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    Complexity level:
    5
    Project cost ($):
    Time required:
    The glue needs to thoroughly dry (2-3 days) prior to testing the bridge's integrity and ability to bear a load.
    Material availability:
    Safety concerns: