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Sprinkle: Straw Bridges (for Informal Learning) (en español)

Quick Look

Grade Level: Middle school

Time Required: 1 hour

Expendable Cost/Group: US $1.00

Group Size: 3

Introduction
Bolded words are vocabulary and concepts to highlight with students during the activity.

The most common type of bridge is a beam bridge—a structure made of horizontal, rigid beams whose ends rest on two columns. The load of the bridge is supported by the columns. Load refers to the bridge’s own weight that needs to be supported, as well as any weight that is added to the bridge such as cars, trucks and people. A truss bridge is a type of beam bridge that has triangular units to distribute the load and support the bridge.

Line drawings show side views of three truss bridge designs composed of different sizes and shapes of triangles located either above or below a main bridge beam spanning two columns. The three designs are known as a Howe-Kingpost truss design (top), deck truss design (middle) and Warren truss design (bottom).
Example truss bridge designs.
copyright
Copyright © 2006 ITL Program, College of Engineering, University of Colorado Boulder

Supplies

Each group needs:

  • 20 plastic drinking straws
  • masking tape
  • scissors
  • 1 sheet of paper
  • pencil or marker

For the class to share:

  • measuring stick or ruler
  • various hardcover and softcover books to use as weights
  • 2 desks or tables of equal height from the floor, to serve as the testing station
  • scale, one that can weigh a minimum of 3 lbs. (~1.4 kg)

Procedure

Overview

Your engineering challenge is to design and build a bridge that can span a gap while holding as much weight as possible. Your materials are straws and tape, and your bridge must meet the design constraints.

Bridge Design and Construction

  1. Organize the students into groups of three.
  2. Hand out paper and a writing utensil to each group.
  3. Show students some images of truss bridges (or show the images provided in this write-up).
  4. Inform students of the following design constraints:
    • bridge must be at least 10 inches (25 cm) in length and able to span an 8-inch (20-cm) gap
    • bridge must be able to securely hold gradually added weight placed in its center until it fails (begins to bend)
    • bridge must incorporate a truss design
    • bridge must be made of 20 (or fewer) straws
    • groups may not receive any replacement straws
    • bridge may not be taped to the desk
  1. Direct groups to brainstorm design ideas for their bridges and record and sketch their ideas on the paper (5-7 minutes). Advise students to carefully plan out their bridges before cutting the straws since they will not receive any replacement materials.
  2. After every group member has presented his/her ideas, have groups each decide on one bridge to build.
  3. Give teams 30 minutes to create their bridges.
  4. Tip: It may be difficult for small hands to figure out the taping of straws that are at add odd angles to each other. Circulate the room to assist students with any construction challenges they encounter.

Bridge Testing

  1. Make a testing station by placing two desks (or tables) approximately 8 inches (20 cm) apart.
  2. One-by-one, help each group test its bridge:
    • Place the bridge across the gap between the two desks, positioning it with equal lengths of bridge ends resting on each desk (see Figure 1).
    • Slowly place one book on top of the bridge.
    • Continue to add books until the bridge fails or noticeably begins to bend (do not add so much weight that the bridges are ruined).

A photograph shows a truss-style straw bridge (Howe-Kingpost design) made of plastic drinking straws and clear tape spanning the gap between two wooden blocks, ready for testing.
Figure 1. Bridge placement for testing.
copyright
Copyright © 2006 ITL Program, College of Engineering, University of Colorado Boulder

    • Direct students to weigh the books that were successfully supported before the bridge collapsed. This means removing from the stack of books the last book, the one that caused the bridge to collapse.
    • Write the group’s total weight on the classroom board.
  1. If time permits, have students make improvements to their bridges and retest.
  2. As a class, examine the class data, and ask and discuss the Thought Questions.

Wrap Up - Thought Questions

  • Which bridge design held the most weight?
  • Which part of your bridge gave out first? Why?
  • How would you improve your bridge design?
  • How would you change your design if you had to span a 20-inch (51-cm) gap using the same materials?

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Copyright

© 2016 by Regents of the University of Colorado

Last modified: June 10, 2020

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