Tacoma Narrows Bridge lesson plans - Science - Many kinds of bridges

Many kinds of bridges

Bridges of all types are amazing structures, and have been used from the time of the earliest humans. They vary in complexity from simple logs straddling streams to beautiful, highly technical modern structures such as suspension bridges. The simplest beam bridges span short distances, while fragile-looking suspension bridges span the longest crossings. There are many factors to take into account when building a bridge: the distance needing to be spanned, geographic factors, cost, aesthetics, available materials and technical expertise. All of these factors determine how bridges are built.

Lesson objectives

As a result of this lesson, students will be able to:

  1. Describe four common bridge types, including suspension, arch, beam and truss.
  2. Understand some of the basic design considerations engineers take into account when building bridges;
  3. Understand basic bridge building vocabulary;
  4. Understand the function of bridges in modern society.


Two days or class periods, plus a week at home to build their own bridge.

Materials needed:

PowerPoint slides of bridge types, list of bridge terms and their definitions, plastic straws, pins and plastic containers. Legos or other small blocks, strips of poster board 18 inches long and 2 inches wide, and packages of individually wrapped stackable candies, such as Starbursts are also required.

Lesson steps

Day one:

1. Explain to students that there are several important elements to be considered before a bridge is built. These include the proposed location, the strength needed, the costs involved, and how the bridge will look.

2. Explain that bridges need to withstand strong natural forces such as wind, hurricanes, and earthquakes, as well as supporting their own weight and the weight of cars and trucks driving across them.

3. All bridges move, and therefore have to be able to bend and flex with the forces of compression and tension acting upon them. (Show students what those words mean by using your hands, a braided rope, or paper creased in a simple accordion fold.)

4. Show students slides of the beam, or girder bridge. This bridge can be as basic as a log across a creek. Its simple design makes this type of bridge easy to build and relatively inexpensive. Ask students to build their own beam bridges, using the blocks, candy and strips of poster board provided. Let them experiment with how many candies each bridge will hold, depending on where the supports are placed. This activity will help them to understand that the longer the span, the weaker the bridge, and if the span is too great, the bridge will collapse of its own weight.

5. Next show students slides of arch bridges. Explain that the arch is one of the earliest architectural elements developed, and was used extensively by the Romans in their buildings, bridges and aqueducts. Ask a couple of volunteers to demonstrate the arch bridge by pressing their palms together high over their heads and leaning forward. Ask students where the strength of this bridge lies. Ask what would happen if the volunteers were standing on a slippery floor. Then ask how could they counteract that movement in their bridge design. Lead students to understand that it's the strength of the abutments that determine the strength of the bridge.

Day two:

1. On the second day, put the students in groups of five, and give each group about 50 plastic straws, 50 pins and a small plastic container, and ask them to create a structure that can suspend the container 30 cm high. Give them about 15 minutes. Then, have them test their structures by carefully filling up the containers with candy until they collapse. Ask students to keep track of how many candies their structures were able to hold. When all groups are finished, come together as a group and discuss students' findings.

2. Ask students to build two each of three different shapes: triangles, squares, and circles. Have them experiment with these shapes by placing the containers on top and filling them with candy. The weight of the candy will test the strength of each shape. Again, come together and discuss students' findings. These experiments are designed to show the structural superiority of the triangle.

3. Show students slides of geodesic domes as well as truss bridges. Ask them why the domes don't need interior walls. Then ask them why truss bridges are most commonly used for trains and drawbridges. Ask them what the drawbacks might be of using truss bridges. They should comment on the bridge's strength because of its shape, but also note that they are expensive to build, heavy and not that aesthetically pleasing to the eye.

4. Finally, show the students slides of suspension bridges. Point out the cables, and explain how the suspension bridge works. Ask students what the advantages and disadvantages of suspension bridges are. They should note the beauty of the bridge and its ability to span long distances, but also see the complexity of the design, and the expense of the materials used.

5. After students have mastered bridge vocabulary and concepts, assign them the task of building their own bridge. They will become the bridge engineers, and apply their own expertise to solve a real transportation problem. Ask them to use their imaginations to create a geographical situation that requires building a bridge. They should choose the bridge that would work best in the situation they create, while taking into account the location, the strength required and aesthetic considerations. They will then build the bridge using materials of their choice.

6. When the bridges are completed, ask students to bring them to school and present them to the class. They should explain the transportation problem, the solution, and why this particular bridge type was chosen. They can discuss the advantages and disadvantages of their bridge, and describe its attributes, including strength and aesthetic appeal. Ask students to use as much bridge building terminology as possible in their presentations.

Related links on this site:

Commonly used engineering terms for building bridges


Before students begin working on their bridges, bring them together in a large group and ask them to help create a grading rubric. Ask them what attributes a top-quality bridge project might have, and list those attributes on an overhead projector or white board.

Possibilities might include:

Evaluate each attribute on an appropriate scale based on your own school's grading system, for example giving points or letter grades. Include student evaluations also, if desired.