Hands-on Activity Balsa Towers

Learning Objectives

After this activity, students should be able to:

• Draw structurally sound 2D designs on paper.
• Construct 3D structures from 2D designs.

Materials List

• markers
• large sheets of paper, such as butcher paper
• quick drying epoxy glue (90-second or 5-minute)
• 1/4 x 1/4 inch balsa wood strips
• 1/8 inch balsa wood sheets
• (optional) dremel tool
• measuring rulers
• utility knives (for students, if possible, otherwise one for the teacher)
• newspaper, to protect table tops from glue
• scrapwood, to cut on (and protect the table tops)
• goggles, one per person
• scale, to weigh towers
• flat board, to set on top of a tower and on which to place weight for testing
• weights or many identical books, to use as mass/weight to test tower strength
• Structural Strength Testing Handout, one per student

Source for balsa wood and glue: http://www.specializedbalsa.com/

Worksheets and Attachments

Introduction/Motivation

Your engineering design challenge today is to build a structurally sound tower with a favorable strength-to-weight ratio using only the materials provided. Working in teams, you will experiment with various designs and come up with what you believe is the best one.

Who can tell me what we mean by "strength-to-weight ratio"? (Listen to student explanations. Correct and amend as necessary.) That's right, it is the ratio of the amount of weight a structure can hold to the mass of the structure itself.

Which team will succeed in building a tower with the highest strength-to-weight ratio? Let's get started!

Procedure

1. Gather materials and make copies of the Structural Strength Testing Handout, one per student.
2. Divide the class into groups of three or four students each. Hand out the large-sized paper and writing implements.
3. Direct the teams to brainstorm and imagine possible solutions and then sketch their tower ideas and designs on the large-sized paper. One possible tower-building technique is to build each side (either 3 or 4) and then attach each side together. Or, take a ground-up approach and build all of the sides of the tower at the same time. Expect students to discover what shapes are the strongest in the design of a physical structure.
4. Distribute the building materials.
5. Explain safety techniques that pertain to the utility knives, epoxy glue and dremmel tool. See the Safety Issues section.
6. Demonstrate for students on how to safely cut and glue together two pieces of balsa wood. Note that epoxy glue has two components: resin, and hardener. To use it, apply a small amount of the resin to the area to be glued, and then apply the hardener, which makes it dry practically instantly.
7. Give the teams time to build the towers on their own.
8. If some groups finish early, suggest that they decorate their towers, keeping in mind the strength-to-weight ratio objective.
9. Hand out the worksheets for students to record their testing data and the data from other groups.
10. Test each tower to see how much it weighs, and how heavy a load it can support. In order to test a tower's strength, place a flat board on the top of the tower. Then, carefully apply masses (such as a book at a time) to simulate a load. Remind students to record the results (tower weight and load weight at failure) for every team's tower test.
11. Have students calculate strength-to-weight ratios and graph the class results on the worksheets.
12. Lead a class discussion: Compare results. Which team design was the most successful? Why?
13. After the initial testing, expect that students have learned a lot about what worked and what did not work. Point out that the engineering design process is "iterative," meaning it is a cycle that is repeated over and over so that improvements can be made from what is learned in testing, until a successful design is achieved. Do they have ideas to improve the strength-to-weight ratio of their towers? Give groups time to redesign and reinforce their towers, and test again.
14. Compare designs and have teams share their designs to the class. 

Vocabulary/Definitions

buckling: When a column fails by bending at some point in the height of the column, usually towards the midpoint and caused by a vertical force.

civil engineering: The field of engineering pertaining to non-moving structures such as roads, sewers, towers, buildings and bridges.

deflection : The amount a structure bends or moves from its "at rest" position.

lateral force: A force that impacts a structure horizontally, such as winds and earthquakes.

strength-to-weight ratio: A ratio of the amount of weight a structure can hold to the mass of the structure itself.

Assessment

• Did all group members participate in the design, construction, and testing of the tower?
• How well did the towers perform, compared to expectations?
• What would students do differently next time (did they learn from their mistakes)?

Investigating Questions

• Which shapes/structures seem to be the strongest while using the least material?
• If you were going to tell someone how to build a strong and light tower, what instructions and advice would you give?

Safety Issues

• Several safety issues must be taken into account when building the towers. Require students to wear safety goggles when cutting with utility knives, using epoxy glue and using the dremel tool. Also, since utility knives are very sharp, supervise their use at all times and direct students to always cut down and away from themselves and other people. Epoxy glue is very strong and dries very fast so students should be careful not to get any on their skin.
• If not enough adults are available to adequately supervise students using utility knives, use 1/8 inch square balsa wood strips because they can be cut with scissors.

Troubleshooting Tips

If a team's tower is weak or unstable, have students examine each region of the tower and think about how they can reinforce it.

If epoxy glue is not practical or students have trouble with it, super glue works as an alternative.

Activity Extensions

Lead a class brainstorming session in which you ask students what they would tell someone who wanted to build a strong tower and had no idea how.

Copyright

Contributors

Supporting Program

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.