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D. Structures and Mechanisms · Strong and Stable Structures

Strength: Making Structures Stronger

D2.3 A1.2 A1.4About 40 minutes
Learning goals
  • I can explain that strength is a structure's ability to support a load.
  • I can describe ways to make a structure stronger, like using triangles, arches, columns and extra supports.
  • I can make a material stronger by folding, rolling or layering it.
  • I can do a fair test to compare the strength of paper bridges.

Words to know

strength
How much load a structure can hold up before it bends too much or breaks.
strong
Able to hold a big load without breaking or bending too much.
column
A tall post that holds up a load from below, like a pillar.
arch
A curved structure that carries a load around its curve and down to its ends.
brace
An extra part added to make a structure stronger or stiffer, often a diagonal piece that makes triangles.
laminate
To glue thin layers together to make a thicker, stronger material, like plywood.
fair test
A test where you change only one thing and keep everything else the same.

Can paper hold up a book?

A flat sheet of paper can't even hold up a crayon. It just flops. But roll that same paper into a tube and stand it up, and it can hold up a book!

Same paper. Different shape. Much more strength. Today you will learn the secrets engineers use to make structures strong.

What is strength?

The strength of a structure is its ability to support a load. A strong structure can hold a heavy load without bending too much or breaking.

A strong bookshelf holds lots of books without sagging. A strong bridge holds heavy trucks. A strong roof holds deep, wet snow.

Secret 1: Use strong shapes

  • Triangles are the strongest shape for a frame. Push on the corner of a square frame and it leans over into a slanted shape. Push on a triangle and it keeps its shape. That is why you see triangles in bridges, roofs, cranes and electric towers.
  • Arches are curves that carry a load around the curve and down into the ground at both ends. Old stone bridges and doorways use arches.
  • Domes and other curved shells, like an egg, spread a push over the whole shape.
A steel truss bridge with many triangle shapes crosses a calm river between two stone supports.
Look for the triangles in the bridge's frame. Triangles keep their shape, which makes the bridge strong enough to hold heavy loads. AI-generated image
square framepushed: it leanstriangle: stays
Push the corner of a square frame and it leans over. A triangle keeps its shape.

Fix a square with a triangle

You can make a wobbly square strong by adding a diagonal brace from one corner to the opposite corner. Now the square is made of two triangles! Look at gates, bridges and shelves for diagonal braces.

Secret 2: Make materials stronger

You can change a material to make it stronger.

  • Fold it. Fold paper back and forth like a fan (accordion folds). It becomes much stiffer.
  • Roll it. Roll paper into a tube. A tube is a strong column when it stands up.
  • Corrugate it. Cardboard boxes have a wavy layer glued between two flat sheets. That wavy layer makes cardboard strong and stiff.
  • Layer it (laminate it). Glue thin layers together. Plywood is made of thin sheets of wood glued in layers, and it is stronger than one thin sheet.
  • Add a stronger material. Concrete is strong when squeezed, but it cracks when it is stretched. Builders put steel rods inside concrete to make it much stronger.

Secret 3: Add supports and join parts well

  • Add more columns or legs under the load.
  • Make parts thicker.
  • Make the span (the gap with nothing underneath) shorter.
  • Join parts firmly. A structure is only as strong as its weakest joint.
Worked example

Let's test it: which paper bridge is strongest?

Make three bridges from the same size of paper across a gap between two stacks of books: (1) a flat sheet, (2) a sheet folded back and forth like a fan (accordion folds, with the folds running from one stack of books to the other), (3) a sheet with its two long edges folded up to make walls (a U shape). Which holds the most coins?

  1. Predict: I think the folded bridge will be strongest because folding makes paper stiff.
  2. Keep it fair: same kind of paper, same gap between the books, coins placed in the middle each time, one coin at a time. Only the shape of the paper changes.
  3. Test: add coins one at a time until the bridge sags to touch the table or the coins fall. Count the coins.
  4. Record the results in a table (see one family's results below).
  5. Conclude: the flat sheet held almost nothing. Folding the paper made it much stronger.

Changing the shape of the paper (folding it) made the bridge much stronger, even though the material was the same.

BridgeCoins held (one family's test)
flat sheet1
accordion folds26
sides folded up (U shape)18
Your numbers will be different. Look for the pattern: the flat sheet is weakest.
Coins held by each paper bridge05101520251flat sheet26accordion folds18U shapeend view:
One family's paper bridge test as a bar graph. Folding the paper made it many times stronger than the flat sheet.
Worked example

Let's make a wobbly shelf stronger

A homemade shelf sags in the middle when it holds books. Give three ways to make it stronger.

  1. Add a support (a bracket or column) under the middle. That makes the span shorter.
  2. Use a thicker board, or glue two boards together (layering).
  3. Add a diagonal brace underneath to make a triangle.

Add a middle support, use a thicker or layered board, or add a diagonal brace.

Just for fun

Why do engineers love triangles?

Because squares are such pushovers!

Hands-on activity

Paper bridge challenge (a fair test)

You need: 3 sheets of the same paper plus 4 more for the bonus, tape, two equal stacks of books, a ruler, and about 40 coins (or small washers or LEGO bricks all the same size).

  1. Set the stacks of books 20 cm apart.
  2. Bridge 1: lay a flat sheet across the gap. Add coins one at a time to the middle. Count how many it holds before it sags to the table or the coins slide off.
  3. Bridge 2: fold a sheet back and forth like a fan (folds about 2 cm wide). Lay it across the same gap so the folds run from one stack of books to the other. Test the same way.
  4. Bridge 3: fold up the two long edges of a sheet about 2 cm to make low walls (a U shape). Lay it across the same gap, walls up. Test.
  5. Record your results in a table. Which shape was strongest?
  6. Bonus: roll 4 paper tubes, tape each one, and stand them up like columns. Gently place a light book on top. Add books one at a time. How many can they hold?

What to look for: the flat sheet is the weakest; the folded shapes are much stronger, and paper tubes standing up make surprisingly strong columns. Safety: coins and small pieces are a choking hazard, so keep them away from little children. Add books gently and keep your fingers out from under them.

Your turn

1

What is the strength of a structure?

2

Which shape keeps its form best when you push on a corner: a square or a triangle?

3

Name two ways to make a sheet of paper stronger.

4

How can you make a square frame strong?

5

Why do builders put steel rods in concrete?

6

In the paper bridge test, what is the one thing you change?

7

Name a structure that uses an arch.

More practice: warm-up

1

Is a flat sheet of paper or a rolled tube of paper stronger as a column?

2

What do we call a tall post that holds up a load from below?

3

What do we call a curved structure that carries a load to its two ends?

4

Is plywood one thick piece of wood or thin layers glued together?

5

True or false: changing only the shape of a material can make it stronger.

More practice: on your own

1

Why are there so many triangles in a crane or an electric tower?

2

Why is a cardboard box stronger than a box made from one sheet of thin paper?

3

A plank bridge over a creek bends a lot. Give two ways to make it stronger.

4

In the paper bridge test, why must you put the coins in the middle every time?

5

A chair wobbles and leans when you push it sideways. What could you add to make it stronger?

Challenge

1

Sam's results: flat paper bridge held 2 coins, folded bridge held 24 coins. Sam says, "Folded paper is 12 times as strong." How did Sam get 12? Is that a fair way to compare?

2

You have only 4 sheets of paper and tape. Design a way to hold up a dictionary. Explain why it works.

3

Why might an engineer use triangles instead of a solid wall to build a tall tower?

Check your understanding

Quiz

1.

The strength of a structure is its ability to...

2.

Which shape keeps its form best in a frame?

3.

Which makes a sheet of paper stronger?

4.

Plywood is strong because it is made of...

5.

To make a sagging shelf stronger, you could...

6.

In a fair test of paper bridges, you should...

For parents and teachers

Big idea: Strength is a structure's ability to support a load (D2.3). Structures get stronger with strong shapes (triangles, arches, domes), more or thicker supports, shorter spans and firm joints. Materials get stronger when folded, rolled, corrugated or laminated (layered). The paper bridge challenge is a fair test (A1.2) with safety rules (A1.4).

How to help: Look for triangles and arches on bridges, playgrounds, roofs and towers. Open a cardboard box edge to show the wavy layer. Ask "What one thing are we changing?" before each test.

Common difficulties:

  • Thinking only the material matters. The same paper can be very weak or very strong depending on its shape.
  • Unfair testing. Placing coins in different spots or changing the gap. Help your child keep everything the same except the paper's shape.
  • Thinking bigger is always stronger. A tall, thin column can buckle; the shape and supports matter.

Safety: coins and small pieces are a choking hazard for younger children. Add books gently.