Lesson Plan on Bridges: Suspension, Cantilever, & Cable-Stayed Bridges and Bridge Building Contest
This is the second half of a two-part hands-on unit study on bridges. Activities include creating and acting out cantilever, suspension, & cable-stayed bridges, competing in a bridge-building contest, and more! These lessons are geared toward elementary and middle school level children. They were created for a weekly homeschool co-op that meets for 2 1/2 hours with 33 children between the ages of 1-13. Use these fun lessons with your classroom, family, after school program, camp, or co-op!
Review and Span Discussion
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 the main types of bridges
Cantilever Bridges Balanced like Rulers
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 per student:
- 1 ruler or paint stick
(The ruler activity came from Bridges by Carol A. Johmann.)
Cantilever Bridge Miniature Models
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 per pair of students:
- 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
(This idea came from Bridges by Carol A. Johmann.)
Cantilever Bridge Human Model
5. Create a human model of the Cantilever Bridge.
- 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 wooden broom/mop handle in each hand. The outside broom handle will be attached tightly to the handle of 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
- two 5 gallon buckets full of water or other heavy objects
(This idea came from Konos Volume III.)
Simple 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.
- 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.)
- How can we stabilize this so the books don't fall over? (Give students a couple minutes to test out theories.)
You will need per group (items will also be used with Activity 8):
- 2 large books of the same size
- two 2-feet long pieces of string
Stabilized Suspension Bridges
8. Using the same materials from Activity 8, create stabilized suspension bridges.
- 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.
You will need per group:
- items from Activity 7 (2 books and string)
- 2 large books of the same size
- one 4-feet piece of string
- 6+ matchbox cars or other items that can act as weights that can hang off a string
(The simple suspension bridge activity idea came from www.teachengineering.org).
Suspension Bridge Tug-of-War
9. 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 quick 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 (these will also be used for Activity 10):
- 2 ropes, each at least 10 feet long
(This activity idea came from Bridges by Carol A. Johmann).
Human Suspension Bridge
10. 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 (from Activity 9):
- 2 ropes, each at least 10 feet long
(The suspension bridge activity idea came from teachers.egfi-k12.org).
Suspension vs. Cable-Stayed Bridges
11. 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
- (optional) a simple sketch of a suspension bridge and cable-stabled bridge (like the ones pictured above
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.
Human Cable-Stayed Bridge
12. To feel the difference between a suspension bridge 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.
13. 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:
- two 5 foot & 6 foot pieces of string or yarn (either 2 sets for 2 volunteers or one set for each student)
(The cable-stayed bridge activity idea came from www.teachengineering.org).
14. 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, calendar, or PowerPoint that shows famous bridges
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:
- uncooked spaghetti noodles (optional)
- craft sticks
- string or yarn
- cereal boxes (optional)
- glue (tacky glue or glue guns would be best)
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.
You will need:
- lots of books (at least 30)
17. Clean up.
Joke: What made the Galloping Gertie Bridge finally break down?
It couldn’t stand the suspense!
Looking for great children's books?
If you can only read one book this week, choose this one. My children loved the amusing illustrations and handy hints. I love that it packs in plenty of information about how the Brooklyn Bridge was built. The author presents the information as if you were one of the workers on the bridge.
We also enjoyed reading:
- Twenty-One Elephants and Still Standing by April Jones Prince (about the Brooklyn Bridge)
- Twenty-One Elephants by Phil Bildner (about the same Brooklyn Bridge event as described in the above book)
- Where Is the Brooklyn Bridge? by Megan Stine (a longer picture book good for upper elementary and above)
- Pop's Bridge by Eve Bunting (our favorite book on the Golden Gate Bridge)
- The Great Bridge-Building Contest by Bo Zaunders
- The Little Red Lighthouse and the Great Gray Bridge by Hildegarde H. Swift (about the George Washington Bridge next to Jeffrey's Hook Lighthouse)
- Mackinac Bridge: The Story of the Five-Mile Poem (Tales of Young Americans) by Gloria Whelan
Our Favorite Youtube Videos
Also look for the movies "Building Big: Bridges", "Modern Marvels - Golden Gate Bridge" (History Channel), and "Modern Marvels - Brooklyn Bridge" (History Channel).
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!
- Bridge Lesson on Forces, Arch Bridges, & Truss Bridges - This is part 1 of a 2 part hands-on unit study on bridges. Build grape and toothpick truss bridges, piece together a play-doh arch bridge, act out the forces involved in bridge building, and more!
- Bridge Lesson on Suspension, Cantilever, & Cable-Stayed Bridges and Bridge Building Contest - 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!
- Bridge Unit Presentations and Field Trip Ideas - This is the culminating project following a 2 part hands-on unit study on Bridges. Also included are the field trips we went on during this unit and fun recipe ideas with a bridge theme.
Looking for all of my hands-on unit study lessons?
- Fun, FREE Hands-on Unit Studies - Over the years I have posted over 30 science and social-studies based unit studies, compromised of more than 140 lessons. For each lesson I have included activities (with photos), our favorite books and YouTube video clips, lapbook links, and other resources.
Would you like to teach this way every day?
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!
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 Shannon