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Bridge Lesson on Suspension, Cantilever, & Cable-Stayed Bridges and Bridge Building Contest

Updated on March 10, 2015
Golden Gate Bridge
Golden Gate Bridge | Source

This is part 2 of a 2 part hands-on unit study on bridges. Create and act out cantilever, suspension, & cable-stayed bridges, compete in a bridge-building contest, and more! These lessons are geared toward 4th-5th grade level children and their siblings. They were created for a weekly homeschool co-op. We meet each week for 2 1/2 hours and have 33 children between the ages of 1-13. Use these fun lessons with your classroom, family, camp, after school program, or co-op!

Photo Credit: http://www.merriam-webster.com/art/dict/bridge.htm
Photo Credit: http://www.merriam-webster.com/art/dict/bridge.htm

1. Pray. Read & discuss John 14:6 (How is Jesus like a bridge?)

2. Review what we learned last week. Review compression and tension. Review the main types of bridges. Briefly discuss the definitions of distance (total length of a bridge) and span (the length between two supports of a bridge). Discuss which types of bridges have longer spans than others. Suspension and cable-stayed bridges span the longest distance, and beam bridges span the shortest distance. Truss and arch bridges span medium distances. Traditional arch bridges that are made from brick or stone span much shorter distances than newer versions made from metal.

YOU WILL NEED: pictures of types of bridges

Balancing a ruler to demonstrate how a cantilever bridge works - Photo taken by Michelle Harrison, who participates in our class: https://www.facebook.com/MichelleHarrisonPhotography
Balancing a ruler to demonstrate how a cantilever bridge works - Photo taken by Michelle Harrison, who participates in our class: https://www.facebook.com/MichelleHarrisonPhotography

Cantilever Bridges

3. Where does a ruler need to be for it to balance on your finger? Have children try it out. If you placed your finger in the middle, at that point the 2 sides of the ruler weigh the same. They serve as counterweights to each other, just like 2 children of the same weight on a balances see saw. Try balancing the ruler near one of its ends. What happens? The longer, heavier side keeps tilting down. Try again, this time putting a finger on top of the short end. It balances! The finger is the support, the second is the counterweight, and the ruler is the cantilever. Because it's made of a stiff material, it sticks straight out. A cantilever bridge works the same way. It takes advantage of the stiffness of a beam's material and the ability to balance a beam on one support.

YOU WILL NEED: 1 ruler per child

(The bridge activity idea came from Bridges by Carol A. Johmann).

The Forth Bridge over the Firth of Forth in Scotland

Photo Credit

Cantilever model - Photo taken by Michelle Harrison, who participates in our class
Cantilever model - Photo taken by Michelle Harrison, who participates in our class

4. Work in pairs to create a model of a cantilever bridge. Show a picture of Scotland's Forth Bridge. Pass out 3 long strips of stiff cardboard all the same size, and 2 short strips with the same width as the long pieces. Place 3 small disposable cups upside down on the table. Balance the long strips on the cups, and lay the short strips in between the cantilever arms. Does your bridge need abutments?

YOU WILL NEED: 3 long strips of stiff cardboard all the same size (we used strips from cereal boxes), 2 short strips with the same width as the long pieces, & 3 small disposable cups

(The bridge activity idea came from Bridges by Carol A. Johmann).

Cantilever model - Photo taken by Michelle Harrison, who participates in our class
Cantilever model - Photo taken by Michelle Harrison, who participates in our class
Cantilever Demonstration - Photo taken by Michelle Harrison, who participates in our class
Cantilever Demonstration - Photo taken by Michelle Harrison, who participates in our class

5. In the late 1800's, people were suspicious that such an odd-looking design would actually be safe. To convince people that this design really would be safe, Benjamin Baker, a designer of Scotland's Forth Bridge went around giving a living demo. He had 2 men on chairs support a third man in between them on a seat. They used short poles to hold up the seat and to reach the ropes attached to the piles of bricks. The men's arms were in tension, as were the ropes, and the chair legs and the poles were being compressed. Demonstrate this. Have 2 of your stronger children sit in chairs. Each child should hold a broom/mop handle in each hand. The outside broom handle will be attached to a 5 gallon bucket full of water (or other heavy object) using rope. The inside broom handle will be attached to a large bed sheet. The large bed sheet will act a hammock for a younger/lighter child to sit in.

YOU WILL NEED: 4 brooms/mops with wooden handles, 1 large bed sheet, 2 chairs, 2 ropes, & 2 5 gallon buckets full of water

(The bridge activity idea came from Konos Volume III and Bridges by Carol A. Johmann.)

This image may give you a better idea of what to do. It also includes the original photo of Benjamin Baker's demonstration.

Photo Credit

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

Suspension Bridges

6. Show pictures of the Brooklyn and Golden Gate Bridges from a book.

YOU WILL NEED: pictures of the Brooklyn and Golden Gate Bridges from a book

7. Have each group make a simple suspension bridge.

a. Tie one 2-foot long piece of string around the middle of a book while it is laying flat on the table. Do the same with a second 2-foot long piece around a different book. Stand these 2 books on end with the string at the top. Take a third piece of 2-foot string and tie each end to the string on the tops of the books. Position the books about 18 inches apart. Pull a little on the string. What happens? (The books fall inward easily.)

YOU WILL NEED: 4 large books of the same size, tape, 2 2-foot long pieces of string, 1 4-foot piece of string, 6+ matchbox cars or other items that can act as weights that can hang off a string

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

b. Remove the strings from the books. Take a 4-foot long piece of string & place a stack of books on top of one end of the string. Place another stack of books on the other end of the string. Using the original 2 books, place them under the string standing on end. Try to position the distance between the 2 books the same as before, about 18 inches. Use tape to hang items from the string. How many items can this string support? Is the string (cable) in tension or compression? (The string is in tension; it can only support a tensile force.) Are the books (towers) in tension or compression? (The books are in compression.) Do the stacks of books (anchors) push or pull on the string (cable)? (The stacks of books pull on the string because the string is pulling on them.) Point out how the anchorages (stacks of books) help to stabilize the bridge.

(The simple suspension bridge activity idea came from www.teachengineering.org).

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

Favorite Resources on the Brooklyn Bridge

You Wouldn't Want to Work on the Brooklyn Bridge!: An Enormous Project That Seemed Impossible
You Wouldn't Want to Work on the Brooklyn Bridge!: An Enormous Project That Seemed Impossible

Also look for the longer picture book "Brooklyn Bridge" by Lynn Curlee.

 
Twenty-One Elephants and Still Standing
Twenty-One Elephants and Still Standing

Also look for the equally delightful picture book related to the same event: "Twenty-One Elephants" by Phil Bildner.

 

8. The way a suspension bridge works is a lot like playing tug-of-war when there is no winner. If both sides are weighted equally, the rope and all the players balance and stay put. That's what a good bridge does. But if one side is stronger, down comes the other! (And the bridge comes tumbling down.) Divide children into 4 groups and give a rope to each set of 2 groups. Have children play tug-of-war but try to keep everyone standing up. As you pull, feel the tension (stretch) in your arms and shoulders. That's what it's like to be a cable on a suspension bridge.

YOU WILL NEED: 2 ropes

(The bridge activity idea came from Bridges by Carol A. Johmann).

Favorite Resources on the Golden Gate Bridge

9. a. Build a human suspension bridge. Two pairs of taller children will stand across from each other and hold the "cable" ropes on their shoulders. These children are the towers. Four children act as anchors. Each one sits on the floor directly behind each tower and holds the ends of the cables. Six to eight children can act as suspenders. Put three or four children in a straight line between each opposing tower. They can kneel or sit while pulling the cables down toward the floor. The floor serves as the roadway. Younger children can act as cars and can run through between the two lines. To see a clear illustration of this, go to teachers.egfi-k12.org.

b. Ask the children who are acting as towers to describe what forces are at work in their "bridge". Have them describe how each force works upon them. They should feel the rope pulling down on their shoulders. What happens to the bridge if there are no anchors? If there are no suspenders?

c. You can also discuss the pros and cons of a suspension bridge. For instance, these bridges are typically found in large cities with lots of boat traffic. They can be built high above sea, or land, with a large span between their towers, leaving the waterway clear for boats. However, they are very costly in materials and time.

YOU WILL NEED: 2 ropes, each at least 10 feet long

(The suspension bridge activity idea came from teachers.egfi-k12.org).

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

Cable-Stayed Bridges

10. One last type of bridge is a cable-stayed bridge. Suspension bridges may be the most impressive type of bridge with their long main span and beauty. These bridges have a roadway that hangs from steel cables supported by two high towers. The difference between suspension bridges and cable-stayed bridges is that suspension bridge cables are not directly connected to the towers. The cables of a suspension bridge are not connected to the bridge - the cables pass through a hole in the top of the towers. A suspension bridge has at least two main cables. These cables extend from one end of the bridge to the other. Suspender cables hang from these main cables. The other end of the suspender attaches to the roadway. Show pictures while you explain. If applicable, you can also show local bridges that are cable-stayed.

YOU WILL NEED: pictures of cable-stayed bridges from a book

Dames Point Bridge in Jacksonville, FL is the longest cable-stayed bridge in America.

Photo Credit

11. To feel the difference between a cable that ends at the tower top & one that goes over it & continues to the ground, grab your head with your right hand & gently pull. If you try to keep your head straight, your neck will feel compressed, but also pulled to the right. Next interlace the fingers of your hands, put them over your head & pull with both arms. Your head & neck will feel compressed, but your neck will not feel a pull to the right or left, since the force of your right arm is balanced by that of your left arm.

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

12. Feel what it is like to be a cable-stayed bridge. Either get a couple children to volunteer to be the cable-stayed bridges or if you're not limited by time, have each child try this. Have the children stand up and hold their arms out horizontally to each side. Their arms are a bridge and their head is a tower in the middle. What is holding up their arms? Their muscles. Using assistance from 1-2 other people, tie each end of a 5-foot piece of string around each elbow. Position the middle of the string on the top of their head. The string acts as a cable-stay and holds up the elbows. Using the 6-foot piece, repeat this process tying the ends around their wrists. Ask them, "Where do you feel a pushing or compression force?" (The ropes are in tension due to the weight of their arms (the bridge) while their head is in compression.) Notice how the load (weight of their arms) is transferred to the tower (their head). Notice the pattern made by the strings going over their heads.

YOU WILL NEED: (per child) two 5 foot & 6 foot pieces of string or yarn

(The cable-stayed bridge activity idea came from www.teachengineering.org).

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

Review

13. Flip through a book or calendar that shows famous bridges. As you name each of the bridges, have the children name each type of bridge they see.

YOU WILL NEED: a book or calendar that shows famous bridges

14. Review what we learned today.

More Great Books on Bridges

Photo taken by Michelle Harrison, who participates in our class
Photo taken by Michelle Harrison, who participates in our class

Bridge-Building Contest

15. Have children build the strongest bridge they can. They can work in pairs or groups if desired. Their bridges must be able to span a distance of 10 inches (so the bridges will need to at least be 12 inches). The bridges cannot touch the table/ground.

YOU WILL NEED: straws, spaghetti, popsicle sticks, string, cereal boxes, scissors, glue (tacky glue or glue guns would be best), tape, & books

16. Test out strength of bridges by spanning the bridges across 2 stacks of books and then piling books or other weights on the top.

17. Clean up.

Joke: What made the Galloping Gertie Bridge finally break down?

It couldn’t stand the suspense!

Great Bridge Video Clips

Ready for the other lesson?

Build grape and toothpick truss bridges, piece together a play-doh arch bridge, act out the forces involved in bridge building and suspension bridges, paint famous bridges, hold a bridge-building contest, and more in this fun 2 part unit study on bridges!

Our Favorite DVD's on Bridges

Konos Volume III
Konos Volume III

Konos Curriculum

Would you like to teach this way every day?

Konos Curriculum

I use Konos Curriculum as a springboard from which to plan my lessons. It's a wonderful Christian curriculum and was created by moms with active children!

Konos Home School Mentor

If you're new to homeschooling or in need of some fresh guidance, I highly recommend Konos' HomeSchoolMentor.com program! Watch videos on-line of what to do each day and how to teach it in this great hands-on format!

© 2012 iijuan12

Do you ever notice bridges? - Or just leave a note to let me know you dropped by! I love getting feedback from you!

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