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Lesson: Polygons and Popsicle Trusses

Quick Look

Grade Level: 9 (8-10)

Time Required: 2 hours

(can be split into two 60-minute sessions)

Lesson Dependency:

Subject Areas: Geometry

A photograph shows a truss structure made from Popsicle sticks glued together at their corners so it is a very open and angular, forming a long yet narrow bridge shape. Two small round stickers are attached at the corners of three types of polygons to identify target angles, which are measured before/after compression load testing for deformation.
Students analyze the different polygon performances in truss structures they design, build and test.

Summary

Students learn about the role engineers play in designing and building truss structures. Simulating a real-world civil engineering challenge, student teams are tasked to create strong and unique truss structures for a local bridge. They design to address project constraints, including the requirement to incorporate three different polygon shapes, and follow the steps of the engineering design process. They use hot glue and Popsicle sticks to create their small-size bridge prototypes. After compressive load tests, they evaluate their results and redesign for improvement. They collect, graph and analyze before/after measurements of interior angles to investigate shape deformation. A PowerPoint® presentation, design worksheet and data collection sheet are provided. This activity is the final step in a series on polygons and trusses.
This engineering curriculum aligns to Next Generation Science Standards (NGSS).

Engineering Connection

To design strong, stable and safe bridges, engineers consider many factors. Foremost, they want their truss designs to adequately support the bridge and its expected load. They ask: Does it make sense to build? Is it strong? Is it safe? What will it look like? What does it cost? In this activity, students act as civil engineers following the steps of the engineering design process to design and create miniature truss bridges that they subject to compressive load testing—which presents an opportunity to experiment with their own creative compositions of geometrical shapes—just as real-world engineers do.

Learning Objectives

After this activity, students should be able to:

  • Explain why a triangle is the strongest shape and integrate this knowledge into the design of a truss bridge structure that they subject to loading tests.
  • Explain how the deformation of a shape affects the sum of its interior angles.
  • Practice following the steps of the engineering design process when designing truss/bridge structures.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Analyze complex real-world problems by specifying criteria and constraints for successful solutions. (Grades 9 - 12) More Details

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  • Models (e.g., physical, mathematical, computer models) can be used to simulate systems and interactions—including energy, matter, and information flows—within and between systems at different scales. (Grades 9 - 12) More Details

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  • Apply geometric methods to solve design problems (e.g., designing an object or structure to satisfy physical constraints or minimize cost; working with typographic grid systems based on ratios). (Grades 9 - 12) More Details

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  • Use geometric shapes, their measures, and their properties to describe objects (e.g., modeling a tree trunk or a human torso as a cylinder). (Grades 9 - 12) More Details

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  • Infrastructure is the underlying base or basic framework of a system. (Grades 9 - 12) More Details

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  • The design process includes defining a problem, brainstorming, researching and generating ideas, identifying criteria and specifying constraints, exploring possibilities, selecting an approach, developing a design proposal, making a model or prototype, testing and evaluating the design using specifications, refining the design, creating or making it, and communicating processes and results. (Grades 9 - 12) More Details

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  • Established design principles are used to evaluate existing designs, to collect data, and to guide the design process. (Grades 9 - 12) More Details

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  • A prototype is a working model used to test a design concept by making actual observations and necessary adjustments. (Grades 9 - 12) More Details

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  • Apply geometric methods to solve design problems. (Grades 9 - 12) More Details

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  • Use geometric shapes, their measures, and their properties to describe objects. (Grades 9 - 12) More Details

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Worksheets and Attachments

Visit [www.teachengineering.org/lessons/view/cub_polygons_angles_trusses_lesson01_activity2] to print or download.

Pre-Req Knowledge

Students should have an understanding of interior and exterior angles and be able to locate them inside and outside of triangles; refer to the associated lesson, Polygons, Angles and Trusses, Oh My!.

Introduction/Motivation

(Be ready to show the slide presentation, starting with slide 15)

(Slide 15) Have you ever walked across a simple footbridge made of boards or a rope bridge and noticed how the bridge changes shape (bends) as you walk across the center? It seems to change shape more over the center of the bridge than at the edges. This bending of the bridge is called deformation.

(Slide 16) Deformation refers to something that changes shape when pressure is applied. Engineers call the pressure—the force—that you apply when you walk on a footbridge a load. Have you ever noticed a bridge moving up and down as cars cross over it? The cars also create a load on the bridge that causes deformation. Engineers consider many factors in bridge design, including an estimate of the maximum load a bridge can support and how much deformation the bridge material can withstand before breaking.

A photograph shows the Milton-Madison Bridge, a truss bridge that crosses a river in Indiana. The metal truss bridge is painted blue and rests on concrete piers.
This space truss bridge design is positioned mostly above the bridge deck (roadway).
copyright
Copyright © 2015 (Nov/Dec) Public Roads, U.S. Department of Transportation http://www.fhwa.dot.gov/publications/publicroads/15novdec/05.cfm

Weight pushes straight down on a beam bridge, causing it to bend. A truss bridge is stronger than a beam bridge because the trusses create a framework that distributes the single point forces over a wider area. In terms of the bridge design, trusses may be positioned above or below the bridge deck (the road surface) and are designed in a variety of geometric patterns.

(Ask students the following questions and ask them to vote by thumbs up/down. Tally their votes on the board.) Which shape is more stable: triangles or squares? If an engineer designed a bridge truss only made of squares, would you feel safe standing on top of that bridge? What if the truss was composed of triangles? Under the same load, how would the deformation of the square be different that the deformation of the triangle? What makes up a strong structure? What types of shapes do you see in your everyday life that are sturdy and stable? These are all important considerations in truss design that you will be able to observe in the design and testing of your own bridge structures.

(Slide 17) Today, you are going to act as if you are engineers who are designing truss structures. Your engineering design challenge: Your engineering team has been tasked to create the strongest possible truss structure that will be used to design a bridge to cross one of our local rivers. Your objective is to design a truss that supports the weight of the vehicle traffic that will drive over the bridge. The truss must be sturdy and stable. You are also asked to make the truss design look abstract and unique; the clients do not want a truss structure simply made of plain triangles. Another design requirements is to use different regular polygons in your truss designs.

Vocabulary/Definitions

compression : A force that pushes inwards on a structural member.

engineering design process: A series of steps used by engineering teams to guide them as they develop new solutions, products and systems. The basic steps include: ask: identify the need and constraints; research the problem; imagine: develop possible solutions; plan: select a promising solution; create: build a prototype; test and evaluate prototype; improve: redesign as needed.

prototype: A first attempt or early model of a new product or creation. Used to test a design concept by making observations and experimental adjustments. May be revised many times.

structural load: Forces that apply to a structure, such as the weight of something applied to the top, sides and/or floors of a structure.

tension : A force that pulls outwards on a structural member.

truss: A structural form made from the joining of individual structural members that form triangles or other stable, rigid shapes. Due to its geometric rigidity, a truss distributes weight from a single point over a wider area.

weight: The force exerted on a body by gravity.

Assessment

Pre-Activity Embedded Assessment

Voting: As part of the Introduction/Motivation presentation, ask students to vote by thumbs up/down on some questions about the strength of different shapes. Tally their votes on the board. Then, at activity end, ask the same question to see if responses change. Ask students: Which shape is more stable, triangles or squares? If an engineer designed a bridge truss only made of squares, would you feel safe standing on top of that bridge? What if the truss was composed of triangles? How would the deformation of the square be different that the deformation of the triangle—under the same load? What makes up a strong structure? What types of shapes do you see in your everyday life that are sturdy and stable?

Activity Embedded Assessment

Worksheet: Have students use the Design a Truss Worksheet to guide the activity. It provides the design challenge problem statement, objectives, materials list, requirements and constraints, and procedure. Review students’ answers to the reflection questions to gauge their depth of comprehension.

Prediction: Make sure teams remember to predict (on their worksheets) how much weight (in books) their trusses might hold, as well as expected deflection amounts (in degrees) for their target angles.

Post-Activity Assessment

Reflection Discussion: Lead a concluding class discussion so students can share their results and reflections about the project. For data/graph takeaways, include a discussion about different truss designs. For example, compare and contrast how many books each truss held, the size of the measured target angles, and how they played a role in the design outcomes. Highlight the amount of load it took to see a change in the angle and/or the focus on truss design itself and how target angle degree impacted truss collapse. Ask students: Did you succeed in creating a truss that held a compressive load? Approximately how much load did your truss hold? How many Popsicle sticks did you end up using? Do you think that engineers ever change their original plans during the prototyping and testing phases of the engineering design process? Why might they? Which polygons seemed to provide the best results/best support for truss structures? (Triangles usually provide the best results.) Why was this the outcome? If you had to do it all over again, how would your planned design change yet again? Why?

Voting: Revisit the pre-activity tallies on the board. Ask the same question to see if responses changed.

Copyright

© 2016 by Regents of the University of Colorado

Contributors

Maia Vadeen; Malinda Zarske; Nathan Coyle; Ryan Sullivan; Andi Vicksman; Russell Anderson; Sabina Schill

Supporting Program

CU Teach Engineering (a STEM licensure pathway), Engineering Plus Degree Program, University of Colorado Boulder

Acknowledgements

This activity was developed by CU Teach Engineering, a pathway to STEM licensure through the Engineering Plus degree program in the College of Engineering and Applied Science at the University of Colorado Boulder.

Last modified: May 18, 2020

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