Working as engineering teams, students design and create model beam bridges using plastic drinking straws and tape as their construction materials. Their goal is to build the strongest bridge with a truss pattern of their own design, while meeting the design criteria and constraints. They experiment with different geometric shapes and determine how shapes affect the strength of materials. Let the competition begin!
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- American Assoc Advancement of Science Proj 2061: Science
- 1B (9-12) #7 New ideas in science are limited by the context in which they are conceived; are often rejected by the scientific establishment; sometimes spring from unexpected findings; and usually grow slowly, through contributions from many investigators. (Grades 9 - 12)  ...show
- Colorado: Math
- Colorado: Science
- a. Predict and evaluate the movement of an object by examining the forces applied to it (Grade 8)  ...show
- International Technology and Engineering Educators Association: Technology
- Next Generation Science Standards: Science
- Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions. (Grades 6 - 8)  ...show
- Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem. (Grades 6 - 8)  ...show
- Describe and design model truss bridges.
- Identify effective geometric shapes used in bridge design.
- Identify several factors that engineers consider when design bridges.
Each group needs:
- 20 plastic drinking straws (not the bendy type)
- scotch tape
- measuring stick or ruler (or one for the class to share)
For the entire class to share:
- small paper cup
- 200-300 pennies (to use as weight)
- wooden support structure (or use two desks)
- balance (for weighing, or count the pennies instead of weighing)
To make the wooden support structure (see Figure 5; optional; may use two desks instead):
- two 7-inch (18-cm) pieces of 2 x 4 wood (for bridge abutments; use scrap 2 x 4s)
- 7 x 13-inch (18 x 33-cm) piece of .25-inch (.6-cm) thick wood (for water base between abutments)
- hammer and nails
- (optional) blue paint for the base of the support structure, to represent water under the bridge
- inside length "span" = 10 inches (25 cm)
- total length (span plus two abutments) = 13 inches (33 cm)
- abutment height = 3.5 inches (9 cm)
- abutment width = 7 inches (18 cm)
|abutment:||A mass, as of masonry, receiving the arch, beam, truss, etc., at each end of a bridge.|
|beam:||A long, rigid, horizontal support member of a structure.|
|beam bridge:||A bridge that consists of beams supported by columns (piers, towers).|
|column:||A long, rigid, vertical (upright) support member of a structure.|
|compression:||A pushing force that tends to shorten objects.|
|deck:||The "top" of the bridge on which we drive or walk.|
|design:||To form or conceive in the mind. To make drawings, sketches or plans for a work. To design a new product. To design an improved process.|
|engineer:||A person who applies her/his understanding of science and mathematics to creating things for the benefit of humanity and our world.|
|model:||(noun) A representation of something, sometimes on a smaller scale. (verb) To make or construct something to help visualize or learn about something else.|
|span:||The length of a bridge between two piers.|
|tension:||A pulling or stretching force that tends to lengthen objects.|
|truss:||A structural frame based on the geometric rigidity of the triangle and composed of straight members.|
Before the Activity
- For bridge testing, make a wooden support structure (see Figure 5; optional), or place two desks ~10 inches (25 cm) apart.
- Gather materials and make example square and triangle shapes with tape and straws as shown in Figures 6 and 7.
- Divide the class into groups of two students each.
With the Students
- Discuss truss bridges with students. Ask students to vote by a show of hands to the following question, "Which shape is more stable, triangles or squares?" Tally their responses and write the totals on the board. Explain with visual demonstrations that squares are less stable than triangles. Do this by showing example straw shapes similar to those in Figures 6 and 7. Stand the shapes up on a desk and push down on the top of them. With very little force applied, the open square shape twists, while the square shape composed of inner triangles withstands much more force.
- To each team, pass out 20 straws, scotch tape, scissors and a ruler. Remember, you are teams of engineers making model bridges using straws and tape as your construction materials. Think carefully about what your design will look like. The "design objective" is to make a bridge that spans the river and supports the most weight. Your bridge design must span a distance of 10 inches (25 cm), which means that the bridge must measure longer than that so it can rest on the abutments on each side of the river. Your bridge must have a place to securely hold a small cup in the center of the span. When we test your bridge, the cup will be filled with pennies until the bridge collapses. That amount of pennies and its cup will be weighed. Other "design constraints" to consider are that no part of the bridge may touch the "water" (or bottom of the wooden support structure) and the bridge cannot be taped to the wooden support structure. Also, the materials are limited. While you can cut your straws to any length you want, you will not be given any additional (or replacement) straws even if you accidentally cut them to lengths you don't want. So, think, sketch and measure before you cut. Another point to make: A bundle of straws taped together does not satisfy the "spirit" of this bridge-building activity. However, it is not necessary to have bridges look as if small cars could go over them. If necessary, show students example truss designs (see Figures 1-4) as examples of the approach to take (not to copy).
- Give the student teams time to create their bridges. Give students time to brainstorm ideas, draw sketches, and make plans and calculations before doing any cutting and taping with their limited number of straws.
- Before strength testing the bridges, ask each team: Predict how much weight you think will make your bridge collapse. Record their predictions on the board. Place each bridge on the wooden support structure (see Figure 8). Position a small paper cup on the bridge at the center of the span; do not place the cup at any other location. Gradually fill the cup with pennies until the bridge collapses or the cup falls off (see Figure 9). Weigh the cup and the pennies on the balance. Make a note of this weight, and record it on the board next to its prediction. Repeat to test all bridges. Note, it may be helpful to add a lot of pennies quickly at first until it appears that the bridge is beginning to fail. At that point, add fewer pennies at a time, more carefully and slowly. The winning bridge design is the one that supports the most weight, while meeting the design criteria and constraints.
- Conclude by leading a class discussion of the bridge strength testing results. How would they improve their bridge design? Have students from each engineering team describe what they would do to make their bridges stronger.
- Remind students of scissor safety rules.
- Which shape is more stable: triangles or squares? (Explain with visual demonstrations that squares are less stable than triangles. Stand some example tape and straw shapes [Figures 6 and 7] on a desk and push down on the top of them. With very little force applied, the empty square shape twists, while the square shape composed of inner triangles withstands much more force.)
Activity Embedded Assessment
- For lower grades, as students design and build straw bridges to span 10 inches (25 cm), permit them to place intermediate supports in the "water."
- For higher grades, have students design and build a straw bridge to span a distance of 20 inches (50 cm) using the same amount of material and no intermediate supports in the "water."
Dictionary.com. Lexico Publishing Group, LLC. Accessed March 21, 2007. (Source of some vocabulary definitions, with some adaptation) http://www.dictionary.com
Jonathan S. Goode, Joe Friedrichsen, Natalie Mach, Chris Valenti, Denali Lander, Denise W. Carlson, Malinda Schaefer Zarske
© 2006 by Regents of the University of Colorado.
Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
Last modified: October 9, 2015