Build It Strong

A small town is cut off by a gap - a stream, a ravine, a washed-out road - and needs supplies to get across. Your job is to build a bridge or raised platform that spans the gap and holds as much weight as possible without bending too far or collapsing.

Every structure that holds something up is solving your bridge's problem: carry a load across a distance without bending or breaking. The same ideas hold up a bookshelf and a highway overpass.

• Bridges
• |Shelves
• |Roofs
• |Tables
• |Playground structures

Structure

Something built to hold itself up and carry a load, like a bridge or a shelf.

Load

The weight a structure has to carry, including the weight of the structure itself.

Support

A part that holds up the load and passes the force down to the ground.

Triangle bracing

Adding triangle shapes to a design, because triangles keep their shape under load better than squares.

Material strength

How much force a material can take before it bends or breaks. Shape can make a weak material act strong.

• Popsicle sticks
• Straws
• Index cards
• Tape
• String
• Cups or books for supports
• Coins, washers, or small weights

Materials behave differently. Use this to decide which material does which job.

Engineers plan before they build. Work through these before you pick up any tape.

1. 1Draw your idea before you build it. A rough sketch is fine.
2. 2Label the strongest part of your design, and the part you are least sure about.
3. 3Predict what might fail first when you test it.
4. 4Decide which material goes where, and why that material fits that job.

These are guides, not rules. Two students can follow them and build very different designs - that is the point.

1. 1

   Set up your gap first: place two cups or book stacks about 20 cm apart. That gap is what your structure has to cross.

2. 2

   Build the deck - the flat part supplies rest on - so it reaches from one side to the other.

3. 3

   Add supports underneath or along the sides to carry the weight down to the table.

4. 4

   Try adding triangles: three sticks joined into a triangle where the deck meets the supports.

5. 5

   Leave the middle testable. You will load weight there, so make it strong without overbuilding it.

A design is only finished when it passes a fair test. Run each test the same way every time.

• Load test

  Add coins one at a time to the center of the deck, counting as you go.

  Measure: Number of coins held before it collapses

• Bend test

  Watch the deck as weight is added and mark when it starts to sag.

  Measure: How many coins before it bends more than a finger's width

• Triangle test

  Compare a square-braced version to a triangle-braced version with the same materials.

  Measure: Which design holds more, and by how many coins

Write down what happens each time you test. Your numbers tell you what to improve. Print this page or copy the table into your notebook.

Your first version is a prototype, not the final answer. Change one thing based on what failed, then test again.

• If it bent in the middle, add a support or a triangle brace under that spot.
• If a joint pulled apart, add tape or string to hold that connection tighter.
• If it twisted sideways, connect the two sides so they move together.
• If one material kept failing, swap it for a stronger shape - a folded card or a tube - and test again.

1. 1Where did your structure bend first?
2. 2What part carried the most weight?
3. 3Did triangles help?
4. 4Which material worked best?
5. 5What would you change with fewer materials?

• Rebuild to hold the same load using fewer craft sticks.
• Make the bridge span a wider gap without adding a support in the middle.
• Design a platform that spreads the load so a stack of coins does not tip.

Running this lesson? The parent & teacher guide has setup, materials prep, safety notes, common failure points, and questions to ask.