Bridges must be strong and bear a great deal of weight, or force, to keep us safe as we travel across them. Basic geometric shapes are used in bridge design - namely, squares, triangles, rectangles and half circles. If we use two different geometric shapes, a square and a triangle, to create the structure for a bridge's supports, which one would be able to bear more weight? This science project demonstrates a basic concept of engineering to help us understand why certain shapes are used, and which shapes withstand the most force.

Hypothesis

If a bridge is built with triangle-shaped supports, then it will be stronger and hold more weight than a bridge built with square-shaped supports.

Objective

In this science project, you will build two model bridge decks, one with the main supports made of triangle-shaped supports and a second made of square-shaped supports. You will then test each bridge to see which bridge supports more weight.

Background

Bridges are purposefully designed, or engineered, using geometric shapes that distribute force, thus making the structure strong enough to bear weight. Force is the "push" from the weight being placed on it. The bridge must bear not only the weight of the materials used to make the bridge, but also the weight of of any traffic travelling across it. One shape that distributes force effectively is a triangle.

But why is a triangle stronger than a square? This is because of how a triangle distributes or carries a force. When a load is placed at any vertex (corner) of a triangle, the force is evenly distributed and the shape remains stable. The force is carried equally to the base on both of its sides.

The square, on the other hand, has a weak center. The top of the square carries the force of the entire load. This is also why a roof of a house is so often trianglular in shape: a square roof will collapse more easily, but a triangular shaped roof will be very strong and stable.

In bridge design, triangles are used in a series or pattern to achieve structural strength. This set of connecting triangles used is called a truss. Connecting multiple triangles together as a truss forms a stronger structure than using just a single triangle.

Scientific Terms

triangle,
square,
vertex,
base,
truss,
force

Materials

1 box of flat toothpicks

craft glue, wood glue or Elmer's glue

cardboard (about 12" x 4")

waxed paper

paper towels

small bucket

rock material: gravel, pebbles or sand

scoop or plastic cup

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.

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.

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.

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.

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.

Discussion

1. Compare how many scoops each bridge holds before collapsing. How many more scoops can the triangle truss bridge hold compared to the square truss bridge? Or is it the other way around?

2. What did you notice happening to the triangle-shaped trusses before they collapsed?

3. What did you notice happening to the square-shaped trusses before they collapsed?

Questions & Answers

What geometric shape is commonly used in bridge design?

Triangles are commonly used in bridge designs

Why is a triangle stronger than a square?

Triangles are stronger than squares because a force is evenly distributed from the vertex to the base of each side of the triangle, allowing them to remain stable. A square has a weak centre which collapses more easily.

Make it Your Own

In this science project, you were given specific directions for bridge designs to compare whether a square support system or a triangular support system would support more weight. Here are some other things to consider:

Build two bridges with the same design, but use different types of building materials, such as popsicle sticks, straws or Q-tips. Is one building material stronger than another?

Research bridge designs. Build two types of model bridges from toothpicks (or another similar material). Which design proves the strongest?