- Difficulty
- Beginner
- Build time
- 35-55 min
- Estimated cost
- $0-$3
- Age range
- 10-16
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The bridge spans 20 cm, supports at least 20 coins at midspan for ten seconds, and shows a documented failure location.
Learning goals
- Identify how downward coin load at midspan produces internal bending resistance across the span.
- Construct and explain a static load-to-beam deflection system.
- Measure how the cross-section shape changes performance.
- Diagnose losses caused by paper creasing and support slip.
Before you build
Materials, tools, and safety
Reuse-material cost: $0-$3 with reused materials. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Pencil
- Scissors
- Masking tape
Low-cost swaps
- Use clean recycled packaging whenever it has similar stiffness.
- Substitute paper clips, binder clips, or twist ties for specialty fasteners.
- Use identical newspaper rectangles and labeled washers when printer paper or coins are unavailable.
Project-specific safety
- Wear eye protection when stretched elastic, magnets, or spinning parts are present.
- Test at floor or tabletop height and keep the path clear of people.
- Keep supports low, catch falling coins in a tray, and wash hands after handling shared currency.
Orient the build
Place the build so downward coin load at midspan is on your left and internal bending resistance across the span is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Set the test span
Place two equal supports exactly 20 cm apart on a nonslip surface.
Mark the centerline between them.
Step 2
Test a flat control
Lay one unfolded sheet across the gap and place the empty cup at center.
Add coins one at a time until failure.
Step 3
Fold an accordion beam
Make equal 2 cm alternating folds along the second sheet.
Keep every crease parallel to the span.
Builder checkpoint: After fold an accordion beam, the first subassembly should stay aligned when handled gently.
Step 4
Test the accordion
Center it on the same supports with the same cup.
Load at the same rate and record failure.
Watch for: If this stage binds or drifts, inspect asymmetric loading before adding more parts.
Step 5
Fold a channel beam
Turn up two 3 cm side walls on the third sheet.
Keep the flat web and both flanges straight.
Step 6
Test the channel
Repeat the exact loading process and note deflection at ten coins.
Stop after failure or forty coins.
Builder checkpoint: After test the channel, operate the build slowly and confirm that internal bending resistance across the span begins without binding.
Step 7
Inspect failure modes
Circle the first buckle, crease, or tear on each sheet.
Photograph or sketch the side view.
Step 8
Compare designs
Calculate load-to-paper ratio and explain why shape changed strength.
Keep all sheets for evidence.
Builder checkpoint: At the final checkpoint, The bridge spans 20 cm, supports at least 20 coins at midspan for ten seconds, and shows a documented failure location.
See the engineering
Why it works
- Input
- downward coin load at midspan
- Output
- internal bending resistance across the span
- Motion
- static load-to-beam deflection
- Energy losses
- paper creasing, support slip, local buckling, asymmetric loading
Why this works
Beam depth and bending stiffness
Folding paper into a deeper cross-section places material farther from the neutral axis, increasing resistance to bending even though the paper mass stays the same.
Look for: Watch whether failure begins by overall sag, top-edge buckling, or a crease near the support.
Where the energy goes
Efficiency and losses
The ideal model leaves out paper creasing, support slip, local buckling, asymmetric loading. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where paper creasing becomes visible or audible.
Math bite
Compare load improvement
Formula: improvement = folded capacity / flat capacity
- Flat bridge = 4 coins
- Folded bridge = 24 coins
Substitute: improvement = 24/4 = 6
Result: The folded section carries six times the flat-sheet load.
The paper amount did not change; geometry changed stiffness and stability.
Coin placement and crease quality affect capacity.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Load the flat sheet first as a control.
Success looks like: At least one folded design spans 20 cm and supports 20 coins for ten seconds.
Measure: Failure load, deflection at ten coins, and failure location.
Change: the cross-section shape
Keep constant: paper size, span, supports, coins, cup, and loading rate
- flat sheet
- accordion
- channel
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| The bridge slides off | Support contact is small or surface slick | Push sideways before loading | Add nonslip paper under supports |
| One side collapses first | Folds or load are asymmetric | Measure edges and cup position | Refold evenly and center the cup |
| The loading cup punches through | Contact area is too small | Inspect a local dent under the cup | Add one identical load-spreader card |
| Results vary widely | Coins are added at different speeds or positions | Review the test video or record | Use one loader and fixed count rhythm |
Choose your tradeoff
Increase section depth without creating unsupported thin walls that buckle. Sharp consistent folds help, but crushing a crease during setup permanently weakens the beam.
Keep experimenting
Try another version
Two-shape comparison
Test only flat and accordion sections.
Mass efficiency
Divide supported mass by bridge mass.
Longer span
Predict capacity when span grows to 30 cm.
Build together
Classroom and access options
Classroom version
Teams can compare the cross-section shape while keeping paper size, span, supports, coins, cup, and loading rate. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Offer pre-cut parts and tactile or high-contrast measurement marks.
- Split roles so one builder can hold, another assemble, and another measure.
- Pre-mark fold lines and assign folding, loading, counting, and observing roles.
Reflect on the design
- How did the cross-section shape change the measured result?
- Where did paper creasing affect the build most strongly?
- What evidence shows that beam depth and bending stiffness explains the motion?
- Which change would improve internal bending resistance across the span without creating a new problem?
Glossary
- Beam depth and bending stiffness
- Folding paper into a deeper cross-section places material farther from the neutral axis, increasing resistance to bending even though the paper mass stays the same.
- Input
- The action or energy supplied to a system; here it is downward coin load at midspan.
- Output
- The useful response produced by a system; here it is internal bending resistance across the span.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
One build can start the next.
Share what you learned, change one variable, and help another builder understand what worked.
Explore more guidesSources and build notes
A familiar household engineering activity implemented with original instructions and controlled tests.
- Classroom engineering basis: A common educational challenge implemented with original dimensions, tests, diagrams, and instructions.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.