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Stable And Unstable StructuresStable-And-Unstable-Structures

  • Subject: Technology
  • |
  • Grade(s): 6-8
  • |
  • Duration: One to two class periods

Lesson Plan Sections


Students will
1. examine the structural flaws that caused three bridges to collapse,
2. determine what factors need to be considered in building a stable structure, and
3. compare and contrast the pros and cons of various bridge building materials.


The class will need the following:
Computers with Internet access (optional but very helpful)
Each student will need the following:
Pencils and pens
Classroom Activity Sheet: Designing Bridges (see printable version)
Structures Fact Sheet (Distribute this to students if they don't have access to the Internet and need the information in order to complete the activity.) (see printable version)
Take-Home Sheet: Top 10 Construction Achievements of the 20th Century (see printable version)


1. Begin the discussion by showing the class two photographs of the Tacoma-Narrows Bridge, in the state of Washington, in the process of collapsing. You can find these images at the following Web sites:
Photo 1
Photo 2
2. Ask students to brainstorm about the causes that forced this bridge to wobble and then fall apart. (You may want to ask them if certain weather conditions may have contributed to the bridge's collapse.) Write their suggestions on a piece of newsprint. After discussing students' ideas, explain that the cause of the collapse was winds of more than 40 miles per hour.
3. Discuss with the class two other bridges that have collapsed:
  • Silver Bridge, Point Pleasant, West Virginia, 1967. In the worst bridge disaster in the history of the United States, 37 trucks and cars fell into the water when this bridge collapsed. The damage was caused by a broken I-bar, a small metal beam that connects the bridge's different parts. As engineers found out later, one I-bar had a tiny crack at the time of construction; over time the wear and tear of weather and traffic broke the I-bar apart. Once one side of the bridge fell, the other side couldn't handle the weight, so it collapsed too.
  • Melbourne Bridge, Melbourne, Australia, 1968. When constructing the bridge, engineers and architects made a simple mathematical error that resulted in an instability in the bridge's steel girder box. When some of the steel expanded from heat, the bridge fell 120 feet to the ground.
4. Using the information about bridge collapses as a starting point for discussion, ask students what variables must be considered when building a bridge. Point out that these include environmental factors, such as wind and temperature, building materials, and shapes used to support the structure. Also, discuss with the class the natural forces with which structures must contend, such as the weight of a building pressing down on the lower columns (compression) and natural stretching of materials (tension). Explain how these factors also must be taken into consideration when designing a bridge.
5. Divide the class into small groups of three or four students. Tell each group to design a plan, or blueprint, for a bridge to cross a gap in your city or town; the bridge could cross a river or join two sections of land. Their goal is to propose the strongest, safest bridge they can with the least expensive materials. As students work, have them answer the questions listed below and record their findings on the Classroom Activity Sheet: Designing a Bridge.
  • What natural forces might affect your bridge? How can you compensate for them?
  • What materials are most suitable for your bridge? Keep in mind wear and tear on the bridge, temperature, wind speed, and expense when making your decision.
  • How much weight can triangles, rectangles, and arches support? Which is most suitable for your bridge? Why?
To guide students in their research, you may want to distribute copies of the Structures Fact Sheet, which provides information on common forces (such as tension and compression), properties of different materials (such as steel and concrete), and how shapes (such as rectangles and arches) affect designs. To conduct the necessary research, have students visit the Web sites listed below.

Forces Lab: squeezing, stretching, bending, sliding

Materials Lab: wood, plastic, aluminum, brick, concrete, reinforced concrete, cast iron, steel

Shapes Lab: rectangles, arches, triangles

6. Have the groups write down their recommendations and draw their blueprints on the Classroom Activity Sheet: Designing a Bridge. Suggest that each student make a copy of his or her group's recommendations. Then have the groups present their designs to the class. Give other students a chance to comment on the strengths and weaknesses of each design.
7. Assign the Take-Home Sheet: Top 10 Construction Achievements of the 20th Century for homework. Students will research one of the structures honored by the architectural community in 1999. They will record important facts about the structure and find out how it is reinforced to protect against destructive forces such as high winds, floods, and earthquakes. After students complete the assignment, have them share their findings with the class.

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Students in high school could take this assignment one step further by creating 3-D models of their proposed bridges. Alternatively, they could simulate a famous bridge disaster by first building a small, inexpensive model of a bridge that collapsed, then replicating the forces that caused its collapse. For example, high winds can be simulated using a fan, and an earthquake can be simulated by shaking the table supporting the model. The following Web site on bridge disasters might be helpful in completing this project:

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Discussion Questions

1. What are some of the forces that could cause a bridge to collapse?
2. What are the differences between concrete and reinforced concrete?
3. Which of the following shapes would be best able to handle pressure from the top: horizontal rectangle, arch, or triangle? Why? Which would be the weakest?
4. If you were developing a list of safety measure for bridges, what items would you include on your list? Why?
5. In addition to the bridges discussed in this lesson, can you name some other structures, such as dams, tunnels, or buildings, that have collapsed? Why did these structures collapse?
6. In 1979, the Kemper Arena in Kansas City, Mo., Was hit with severe thunderstorms. Because of its poor drainage systems, the roof filled with water. Why do you think this caused the building to cave in?

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Students should be able to work well together, complete the research accurately and thoroughly, develop a reasonable blueprint for a bridge, and clearly present their findings to the class. Use the following three-point rubric to evaluate students' work during this lesson:
  • Three points: Students worked well together in their groups, developed an exemplary plan for their bridge, drew an accurate blueprint that was labeled clearly and accurately, and made an interesting presentation to the class.
  • Two points: Students worked somewhat well together in their groups, developed a reasonable plan for their bridge, drew a somewhat accurate blueprint that was labeled clearly and accurately, and were prepared for their presentation to the class.
  • One point: Students had some difficulties working together in their groups, developed a partial plan for their bridge, drew a partial blueprint, and made a brief presentation to the class.

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To the Rescue!
When a structure collapses, federal, state, and local organizations rush to the aid of any victims. Brainstorm with the class which organizations help during a disaster. Ask them about the role of these organizations. Have students each become a "Disaster Action Kid," visit the Web site for kids set up by the Federal Emergency Management Agency:

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Suggested Readings

Catastrophe! Great Engineering Failure - and Success
Fred Bortz. W.H. Freeman and Company, 1995.
Good engineers should try to anticipate anything that could go wrong with whatever it is they are designing. Most do, and some unfortunately do not. This book examines six cases where a mistake in design led to disaster. The fascinating details of each incident is described, illustrated in photographs and drawings, and analyzed so the reader can understand what went wrong and why.

Collapse: When Buildings Fall Down
Phillip Wearne. TV Books, 2000.
The author tells the stories of how eleven of the worst structural engineering disasters of the last fifty years occurred. The stories of these disasters are also the stories of the forensic engineers who sifted through layers of debris, studied architectural drawings, and staged reconstructions in order to search for the true cause of each catastrophe.

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Great Engineering Failures: Tacoma Narrows Bridge
The collapse of the Tacoma Narrows Bridge is featured on this web page. The site will lead you to other great bridge disasters.

A multimedia demonstration showing that not only can "resonance" destroy a bridge, it can also reduce to rubble your family's favorite glassware and even disintegrate a kidney stone.

An elementary classroom for the budding civil engineer tells all about structures and what a civil engineer does for a living.

CONTEST: Welcome to the Bridge Building Home Page
The Illinois Institute of Technology invites you to participate in their annual basswood bridge building contest.

Engineering Disasters: Learning from Failure
The engineering department at the State University of New York shares its list of websites on engineering disasters, from falling bridges to Chernobyl.

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Click on any of the vocabulary words below to hear them pronounced and used in a sentence.

speaker    box girder bridge
Definition: A type of bridge made of steel or concrete that is constructed from supporting beams that look like a long box.
Context: Box girder bridges, such as the Melbourne Bridge in Australia, are popular because they are light, strong, and economical.

speaker    collapse
Definition: To break down or fall in.
Context: When the Silver Bridge collapsed in 1967, 37 cars and trucks fell into the river.

speaker    compression
Definition: A pressing force that squeezes a material so that it becomes more compact.
Context: The weight of the building on its lower columns caused a great deal of compression.

speaker    I-beam
Definition: A steel joist or girder with short flanges and a cross section formed like the letter "I."
Context: A steel joist or girder with short flanges and a cross section formed like the letter "I."

speaker    tension
Definition: A stretching force that pulls on a material.
Context: The vertical cables of suspension bridges must remain in tension at all times because of the continuous weight of the roadway and cars.

speaker    torsion
Definition: the twisting and wrenching of a structure by the exertion of forces.
Context: In 1940, strong winds created too much torsion, causing the Tacoma-Narrows bridge to collapse.

speaker    unstable
Definition: Characteristic of a structure that collapses or deforms under a realistic load.
Context: The bridge was unstable and collapsed during the earthquake.

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This lesson plan may be used to address the academic standards listed below. These standards are drawn from Content Knowledge: A Compendium of Standards and Benchmarks for K-12 Education: 2nd Edition and have been provided courtesy of theMid-continent Research for Education and Learningin Aurora, Colorado.
Grade level: 6-8
Subject area: Technology
Understands the nature and uses of different forms of technology.
Knows that construction design is influenced by factors such as building laws and codes, style, convenience, cost, climate, and function.

Grade level: 6-8
Subject area: Technology
Understands the nature and uses of different forms of technology.
Knows that manufacturing processes use hand tools, human-operated machines, and automated machines to separate, form, combine, and condition natural and synthetic materials; these changes may be either physical or chemical.

Grade level: 6-8
Subject area: Technology
Understands the nature of technological design.
Knows that the design process relies on different strategies (i.e., creative brainstorming to establish many design solutions, evaluating the feasibility of various solutions to choose a design, and troubleshooting the selected design.

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Jordan D. Brown, a freelance author in New York City, enjoys writing books, magazines, and Web sites for kids and teachers.

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