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Bridge Lesson on Forces, Arch Bridges, & Truss Bridges

Updated on November 6, 2015
Arch Bridge Photo source:
Arch Bridge Photo source:

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! 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, after school program, camp, or co-op!

Student's drawing of a bridge
Student's drawing of a bridge

Types of Bridges

1. Pray. Read & discuss Ephesians 2:20.

2. Have children draw a quick sketch of what comes to mind when they think of a bridge.

YOU WILL NEED: writing utensils & paper

3. Discuss bridges: Ask children to think about the last time they remember crossing a bridge. Where were they? How were they travelling (on foot, by car, bicycle, bus, or train)? What were their impressions? Did they notice the bridge? Ask what structures they consider to be bridges. Usually when people think of bridges, they think of structures spanning water.

Ask the children to think about other obstacles bridges can span.

4. Introduce 5 major types of bridges (beam, arch, suspension, cable-stayed, & cantilever) by showing pictures from books.

YOU WILL NEED: a book with pictures of the 5 major types of bridges

Our Favorite Books on Bridge Types

Also look for Cross a Bridge by Ryan Ann Hunter, Bridge Building: Bridge Designs and How They Work (High Five Reading) by Diana Briscoe, Bridges Go from Here to There by Forrest Wilson, Rosita's Bridge by Mary McMillan Fisher, Sven's Bridge by Anita Lobel, From Cement to Bridge (Start to Finish) by Robin Nelson, and Bridges by Etta Kaner.

Bridges Are to Cross (Picture Puffins)
Bridges Are to Cross (Picture Puffins)

This was our favorite picture book that gives an overall introduction to bridges.

SeeMore Readers Bridges (Level 2) L
SeeMore Readers Bridges (Level 2) L

This has photographs rather than illustrations, but it still held my children's interest. It has nice photographs to clearly show the various types of bridges.


5. Have 5 children come to the front. Each child will hold a picture of one of the main types of bridges. Have children bring up their drawings they did of what they first thought of when they thought of a bridge. Have each child hand their drawing to the person who is holding the picture of the type of bridge that they drew. (Everyone can help if the child needs assistance in categorizing their bridge type.) Which bridge type did most people think of when they thought of a bridge?

(Introduction ideas came from

Tension & Compression
Tension & Compression

Tension & Compression

5. Explain that bridges, like all built structures, rely on unseen forces that hold them together which allows them to support additional weight, also called "loads." We usually don't think about forces acting on an object or structure that isn't moving. A force is a push or pull on an object. When an object is at rest (not moving), the forces acting on it are balanced.

6. Demonstration of forces: Place a chair in the middle of the floor. Ask the children whether there are any forces acting on this chair. Explain that even without anyone pushing on the chair, there are forces acting on it. The force of gravity is pulling down on the chair. It doesn't collapse because it supports its own weight. Have a child push the chair a short distance. Ask, "What force just acted on the chair?" (A push unbalanced the forces on the chair & made it move.) Now have 2 children face each other and push on either side of the chair so that it doesn't move. Ask, "Are any forces acting on the chair? If so, why doesn't it move?" (Although two forces are acting on the chair, they are balanced, causing it to remain in place.) This is the case with a bridge. Two forces, compression and tension, are working together in balance to keep the bridge up at all times.

(The chair activity idea came from

Bill Nye on Bridges & Forces - My children LOVE watching Bill Nye!

Our Favorite Book on Forces

Forces Make Things Move (Let's-Read-and-Find-Out Science 2)
Forces Make Things Move (Let's-Read-and-Find-Out Science 2)

Since we are talking about forces on bridges, it is always a good idea to refresh (or introduce) the concept of forces to the children. This picture book does a great job of explaining forces so that even very young children can understand the concept, but older children can glean what they need to know as well.

Beam Bridges
Beam Bridges

Beam Bridges

7. Give each group of children a slightly damp (i.e. not rock hard) sponge and a marker. One child should draw a few lines, about 1" apart, on the sponge. (Don't measure or use a ruler. Exact spacing isn't important.) Have the children bend the sponge into a U-shape as if a load were on it. See how the lines get closer together on the inside of the U? That's because a force called compression makes things shorter. On the outside of the U, the lines get farther apart. A force called tension makes things longer.

YOU WILL NEED:: (per group) kitchen sponge & marker

8. How do these forces of compression and tension work on a bridge? The sponge is like a simple beam bridge, just like if you put a log across a stream in order to cross it. Position 2 books of approximately equal height (3-4 inches) 1-2 inches apart so that the flat sponge can "span" them. Put the sponge across the distance between the 2 books. Have one child use a pen, pencil, or finger to place a downward force on the top of the sponge - just enough to cause the sponge to bend but not completely collapse. What happens to the parallel lines drawn on the top and bottom? (The lines on the top move closer together. The lines on the bottom move farther apart.) Ask where the sponge is in compression or where the compressive forces are located. (The inside of the U-shape/the top.) Where is the sponge in tension, or where are the tensile forces located? (The outside of the U-shape.) How could you balance out the forces of tension and compression acting on the bridge to make it stronger? (Some ideas include using a stiffer material for the beam, or adding supports, such as knitting needles or pencils, to the sponge.)

(The sponge activity idea came from

Building Big
Building Big

My older children really enjoy the books by David Macaulay. This is his book on architecture, and it does include a section on bridges.


9. Compression is the act of being pushed or pressed together. Everyone should place their hands with their palms together & elbows bent. Tell them to press their palms together. This pushing force is called compression. Explain that tension is the act of being stretched or pulled. Have the children place their hands in front of them, and clasp curled fingertips together. Tell them to pull on their hands. This pulling force is called tension.

10. A suspension bridge must be balanced to stand up. It uses tension in the cables to create an overall force of compression in the towers. First, demonstrate the forces of tension and compression. Ask children to stand, each having one partner. To demonstrate tension, have each team member grasp the other's forearms. Both children lean back. Their arms should stretch out between them. Go around to several pairs and lean gently on top of their arms to test their "structure". Explain that when you lean on them you are pushing down and causing their arms to stretch, or be put into tension. Have the children remain standing.

Demonstrating compression
Demonstrating compression

11. To demonstrate compression, have partners press the palms of their hands together and lean toward one another. The children will be making an arch with their bodies. Go around to each pair and push on top of the arches. Explain that when you are pushing down you are causing them to push together, or to be put into compression.


12. Live Load and Dead Load: There are more forces that act on a bridge. Explain that bridges must be able to support two types of forces, called loads, or they will collapse. Show the toy bridge. Ask what weight it needs to support. Dead load is the weight of the bridge itself, such as its columns, beams, nuts, bolts, trusses, cables, etc. Now place the toy car, person, cow, or other item/items on the bridge, and tell the children that these are live loads. Ask them to tell you what a live load is. Live load is the weight or force of temporary external elements acting on the bridge, such as people, vehicles, etc. Ask students to come up with more examples of live load. If they need hints, blow on it (wind) and shake it (earthquake). Explain that environmental factors, such as temperature, earthquakes, and wind also constitute live loads acting on bridges. Hot and cold temperatures cause parts of the bridge to change shape and put pressure on other parts of the bridge; earthquakes push and pull horizontally on a structure as the ground beneath jerks back and forth; and wind blows on a structure pushing it horizontally.

YOU WILL NEED: toy bridge (like a block in the shape of an arch) & examples of live loads (toy car, toy cow, toy person)

(The above compression, tension, and load activity ideas came from

Arch Bridges
Arch Bridges

Arch Bridges

13. The difference between bridge designs is how a bridge carries the weight (load). Let's start with the most simple of bridges, the beam bridge. Stand a pair of books upright like bookends, each about 6 inches apart. Have children work in groups or pairs with one child holding up the 2 books and one child adding the "load" to the bridge. Place a piece of paper across the first pair. Gently place paperclips one by one in the middle of your beam bridge. At what point can it no longer hold its load and it begins to sag?

YOU WILL NEED: (per group) a pair of books, paper, paperclips, & heavier items such as pennies

14. How can you make it stronger? Let children try out some theories.

Arch Bridges
Arch Bridges

15. The Romans came up with one solution to this: the arch. Have the children try to for an arch using just the sheet of paper. It will collapse because its ends move outward. Curve a piece of paper upward and tuck the ends inside the book covers to form an arch. The books are acting as abutments. What kind of force do the abutments (as represented by the textbooks) impose on the arch, pushing (compression) or pulling (tension)? (Answer: The abutments push back on the arch since the arch is pushing on the abutments.) Point out how the stacks of books act as abutments keeping the ends of the arch from spreading apart. Gently add paper clips. Does this shape hold more of a load?

Arch Bridges
Arch Bridges

16. Tape 2-3 sheets of paper together end to end to make 1 long piece. It needs to be as high as the books when it's propped up between them in an arch. Rest a piece of paper on top. Can this bridge hold more paper clips than the first 2? Why? (The arch helps to distribute weight.) (Note: Not all groups had success with this.)


17. Accordion-fold a piece of paper lengthwise. Place it across the top of the books. Fold another piece of paper in the same way. Lay 1 sheet of paper under and 1 sheet over the accordion-folded paper. Try not to squish the folds. Do you think this will hold more than the arch bridge? How many paper clips can this bridge hold before it begins to sag? The triangles in this bridge make it stronger. We'll come back again, so remember that!

(The bridge activity idea came from

18. (If you are not limited by time) Divide children into groups of 6. Have 3 children stand in 1 line facing the other 3 children who are also standing in a straight line, 1 behind the other, facing the other team. The middle 2 children will place their hands on each other's shoulders. The outside children should put their hands on the waist of the child in front of them. Once they're set, start pushing. Push equally so everyone stays up. As soon as you're pushed, feel the compression (pushing) force that squeezes your body. That's what it's like to be a stone in an arch bridge. Let each child have a turn in the middle. The place in the middle is called the keystone.

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

Rainbow Bridge Photo Credit:
Rainbow Bridge Photo Credit:
Play-dough arch bridge
Play-dough arch bridge

19. Show a picture of Rainbow Bridge. Have children use play-dough to form an arch bridge. Place it on its side. Cut it into 5-7 wedge-shaped pieces. Try to put it together again so that it can stand up. Briefly discuss the role and importance of a keystone.

YOU WILL NEED: (per child) 1/2 a container of play-dough & plastic knife

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

Triangles & Truss Bridges

Making Squares and Cubes

20. a. Start with 4 toothpicks and 4 gumdrops or grapes. Poke the toothpicks into the gumdrops/grapes to make a square with a gumdrop/grape at each corner. Poke another toothpick into the top of each gumdrop/grape. Put a gumdrop/grape on the top of each toothpick. Connect the gumdrops/grapes with toothpicks to make a cube. (A cube has a square on each side. It takes 8 gumdrops/grapes & 12 toothpicks.) . Use more toothpicks and gumdrops/grapes to keep building squares onto the sides of the cube. When your structure is about 6 inches tall or wide, try wiggling it from side to side. Does it feel solid, or does it feel kind of shaky?

YOU WILL NEED: (per child) at least 100 gumdrops, gummy fruit, or grapes & 100+ toothpicks

Making Triangles & Pyramids

b. Start with 3 gumdrops/grapes and 3 toothpicks. Poke the toothpicks into the gumdrops/grapes to make a triangle with a gumdrop/grape at each point. Poke another toothpick into the top of each gumdrop/grape. Bend those 3 toothpicks in toward the center. Poke all 3 toothpicks into one gumdrop/grape to make a 3-sided pyramid. (A 3-sided pyramid has a triangle on each side. It takes 4 gumdrops/grapes and 6 toothpicks.) Use more toothpicks and gumdrops/grapes to keep building triangles onto the sides of your pyramid. When your structure is about 6 inches tall or wide, try wiggling it from side to side. Does it feel solid, or does it feel kind of shaky?

c. Why are triangles so strong? As you've probably already discovered, squares collapse easily under compression. Four toothpicks joined in a square tend to collapse by giving way at their joints, their weakest points. A square can fold into a diamond.

d. But if you make a toothpick triangle, the situation changes. The only way to change the angles of the triangle is by shortening one of the sides. So to make the triangle collapse you would have to push hard enough to break one of the toothpicks. Can you identify which parts of the triangle are in tension and which are in compression? (If pressure is applied to any of the corners, the two sides radiating from that point will be in compression, while the side opposite that point will be in tension. If pressure is applied to any of the sides, that side will be in tension, while the other two sides will be in compression.) Can you add a toothpick to the square and rectangle so they don't move? (Take notice of the shapes they have made inside the square and rectangle; they are triangles.)

Building Truss Bridges

e. Use your gumdrops and toothpicks to build a truss bridge, which combines triangles and squares. Place two gumdrops/grapes with their bases laying flat on your work surface. Insert the pointed ends of a round toothpick into the center of each gumdrop's/grape's side to create a barbell shape. Insert another toothpick to each gumdrop/grape at a 45-degree angle. Insert both bare ends of the toothpicks into the same gumdrop/grape to form a complete triangle. Insert a toothpick into one of the base gumdrops/grapes of the first triangle at a 45-degree angle, and another toothpick into the top gumdrop/grape. Insert both of the new toothpicks' bare ends into a new gumdrop/grape; this will create a second inverted triangle. Repeat this process until you have three upright and two inverted triangles that form a trapezoidal shape. Repeat the triangle-making process to create another trapezoidal shape; this will run parallel to the first and give you the second side of your bridge. Connect the two trapezoidal sides of the bridge using a total of seven toothpicks to run across from gumdrop to gumdrop. Look down from an overhead vantage point to check that you have three squares at the base and two squares at the top.

Truss Bridge Photo Credit:
Truss Bridge Photo Credit:

Truss Bridges & Trains

f. As trains became a popular way to transport goods in the late 1800's, bridge engineers faced a new challenge: how to design a bridge that could support the massive weight of trains as well as their vibrations. Bridge builders conceived of a truss bridge, which is essentially a beam bridge supported by a series of triangular shapes called trusses. These shapes are rigid-they do not move when pushed-and they make a bridge extremely strong. Truss bridges get their strength from triangles. Triangles can be arranged into many truss patterns, which can be located on the sides of the roadway, or beneath the base of the bridge. Refer to pictures of truss bridges for examples of different truss patterns.

YOU WILL NEED: pictures of truss bridges

(The gumdrop truss bridge activity idea came from

Bridges & Art

21. Pass out pictures of various famous bridges. Let children sketch a bridge using a pencil and then color it in using watercolor.

YOU WILL NEED: (per child) paper, writing utensil, water colors, cups/bowls for water, & books with pictures of famous bridges

(The bridge art idea came from Konos Volume III).

Favorite Books Featuring Famous Bridges - Books specifically on the Brooklyn Bridge & Golden Gate Bridge can be found in my next lesson in this unit.

Also look for "Bridges: Three Thousand Years of Defying Nature" by David Brown and "Bridges: A History of the World's Most Famous and Important Spans" by Judith Dupre.

Bridging the World
Bridging the World

This has really nice photographs of a wide variety from around the world, and it of course includes the most famous ones.

Bridges of the World Coloring Book
Bridges of the World Coloring Book

If your child loves to color, this is a great option. It also makes a nice picture book to read!



22. Five minute review what we learned today. Ask questions such as: Name a type of bridge (beam, arch, suspension, cable-stayed, & cantilever) What is the definition of a force? (a push or pull on an object) Name a force that acts on bridges. (compression, tension, load) Use your hands to show me the force of compression. (Push them together.) Use your hands to show me the force of tension. (Pull them apart.) What is a dead load? (the weight of the bridge itself, such as its columns, beams, nuts, bolts, trusses, cables, etc.) What is a live load? (the weight or force of temporary external elements acting on the bridge, such as people, vehicles, etc.) Describe what a beam bridge might look like. (a log across a stream) Describe what an arch bridge might look like. (a bridge with arches under it) What shape can make a bridge stronger? (triangle) What is a keystone? (The stone in the crown of the arch that holds the other stones together.) Describe what a truss bridge might look like. (a beam bridge made of triangles and squares) What was your favorite activity from today?

(Photo Credit:

Joke: Why did one bridge hope the other bridge would fall down?

Because they were arch enemies!

Material List for the Lesson

Items for Families to Bring Per Child:
-a pencil, marker, watercolor paints with paintbrush, & small container for water for paints
-5 sheets of white computer paper (4 of the pages can be scrap paper but 1 should be clean for the watercolor picture.)
- kitchen sponge (1 for every 2 children)
-20 paperclips
-a handful of small items that are heavier than paperclips (such as pennies)
-scotch tape (1 per family)
-1/2 a container of play dough
-at least 100 grapes, gumdrops, or gummy fruit
-at least 100 toothpicks
-protective cover shirts (optional – we will only be using watercolor paints)
-at least 1 picture of a famous bridge that your child can sketch (from a library book, the Internet, etc.) (1 for every 3 children)

Items to Bring to Share with the Group:
-a book with pictures of the 5 major types of bridges
-large amount of books or other items that can be used for bridge span testing
-toy bridge (like a block in the shape of an arch) & examples of live loads (toy car, toy cow, toy person)
-a picture of Rainbow Bridge (from a book or the Internet), 1 plastic knife per child, and 1 piece of wax paper per child (to protect the table)
-paper towels or napkins

The World’s Biggest Arch Bridge

Need More Activity Ideas?

Bridges: Amazing Structures to Design, Build & Test (Kaleidoscope Kids)
Bridges: Amazing Structures to Design, Build & Test (Kaleidoscope Kids)

This could be a curriculum all by itself. It has great activity ideas and explanations. Also look for "Bridges and Tunnels: Investigate Feats of Engineering with 25 Projects" (Build It Yourself series) by Donna Latham, "Building Toothpick Bridges" (Math Projects: Grades 5-8) by Dale Seymour Publications Secondary, "Draw 50 Buildings and Other Structures: The Step-by-Step Way to Draw Castles and Cathedrals, Skyscrapers and Bridges, and So Much More" by Lee J. Ames, "Steven Caney's Ultimate Building Book" by Steven Caney, and "Build It!: Activities for Setting Up Super Structures" (Design It) by Keith Good.

Let's Try It Out with Towers and Bridges : Hands-On Early-Learning Activities
Let's Try It Out with Towers and Bridges : Hands-On Early-Learning Activities

This is a good option for toddlers and preschoolers. It includes easy explanations about bridges with a simple building activity after each explanation.


Ready for the next lesson?

Tress bridge made of toothpicks
Tress bridge made of toothpicks

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!

Looking for All of My Lessons?

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. I posted links to all of my unit studies and lessons at Fun, FREE Hands-on Unit Studies .

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' 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 have a favorite bridge? Do you even notice them? - Or just leave a note to let me know you dropped by! I love getting feedback from you!

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    • iijuan12 profile image

      iijuan12 5 years ago from Florida

      @digitaltree: Thank you so much!

    • digitaltree profile image

      digitaltree 5 years ago

      Nice Lens and lesson on building bridges.

    • iijuan12 profile image

      iijuan12 5 years ago from Florida

      @BikePro: Thank you for dropping by!

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      Graeme 5 years ago

      Thanks for the great article, and the Bill Nye video! Brings me back...

    • ceejaycmarshall profile image

      ceejaycmarshall 5 years ago

      I love this lens! Great unit to fit a bridge theme! Thank you.:)