1 - Getting Started ~5 mins
Think of a stone arch like many older Irish road bridges, versus a flat beam bridge or a modern cable-stayed bridge such as the Samuel Beckett Bridge in Dublin. Why do engineers bother with different shapes?
Today we will build four paper bridges across the same gap and find out which shape holds the most load before it fails.
If you have photos, show one stone arch road bridge and one modern Irish example (for example the Samuel Beckett Bridge). Keep the hook light: one curiosity question and the challenge ahead. Do not hand out card or set the gap yet. The pupil text already names real bridges so the opener works even without photos.
Nature of STEM: civil engineers in Ireland design bridges so the shape and materials share the load safely. Shape is not decoration; it is how the structure carries weight.
Key question: Does the shape of a bridge change how much it can hold?
2 - Question and Predict ~6 mins
Our question: Which bridge shape holds the most load before it fails?
Here are the four forms we will build (sketch them quickly on the board if it helps):
- Flat — a plain strip laid straight across, like a simple plank.
- Folded — a strip with zigzag ridges along it (like corrugated roofing) that still reaches both ends.
- Arched — a strip bent into a smooth curve, like a rainbow over the gap.
- Trussed — a top path with triangles taped under or beside it to share the weight.
We build all four from one group card budget (a fixed pile of equal strips you share). We keep the same span (the gap between the book stacks) and the same way of adding weight. Flat, folded and arched each use about one strip; the truss may use the leftover strips as braces. So we are testing shape and how the card is arranged, not four bridges with identical amounts of card.
Predict with your group: which shape will hold the most? Say why you think so.
Quick board sketches of the four forms before talk turns to the fair test. Agree the rules together before anyone builds:
- Compare: four forms (flat, folded, arched, trussed) built from one group strip budget.
- Measure: how many coins or washers the bridge holds just before it fails (the last count that still held).
- Keep the same: span (20 cm between supports), same card type and a fixed strip budget per group, same supports, same way of placing load at the centre one piece at a time.
- Material note (say aloud): flat, folded and arched each use about one strip; the truss may use the remaining strips as braces. We are comparing shape and arrangement from one budget, not equal card on every bridge.
- Failed means: the bridge sags to the table or collapses. Agree this as a class so every group judges the same way.
Pupil design choice (Stage 4 agency): groups do not invent a new question. In the next step, once the how-to is on screen, they decide how they will form the folded ridges and the truss triangles from their strip budget (for example how many zigzags, or whether triangles sit underneath or on the sides). That choice is written on the Investigation Journal page before loading. Shape names stay fixed so the class can still pool four shapes fairly.
Write the four shapes on the board. Take a quick show of hands for predictions. Predictions are never wrong; they start the science.
Misconception to head off: some pupils think a thicker or longer piece of card is allowed for the "strong" shapes. Everything comes from the same strip budget; only the form and how the strips are arranged change.
Key concepts for you (not for the board as a full table):
| Concept | Why it matters | Example |
|---|---|---|
| Load — the weight a bridge has to hold up | Engineers size and shape a bridge for the load it must carry safely | Cars and people on a road bridge, or coins or washers on our paper bridge |
| Span — the clear gap between two supports | A fair test keeps the span the same so form is the main change | Two book stacks 20 cm apart |
| Bridge shape — how the card is formed (flat, folded, arched, trussed) | Shape helps the structure share push and pull, not just look different | A zigzag or triangle-braced strip often holds more than a flat strip |
3 - Build the Four Bridges ~22 mins
In your group, build four bridges from your equal card strips across the same gap. All four bridges, plus any braces or end wedges, must come from your group's strip budget only.
Strip plan (keep this in mind so the truss is not left short): about 1 strip each for flat, folded and arched; keep about 5 strips for the truss deck, triangle braces and any tiny arch wedges.
Before you cut or fold, write one design choice on your Investigation Journal page: how will you form the folded ridges, and how will you make the truss triangles from your leftover strips?
- Flat: one strip laid straight across the supports.
- Folded: fold one strip into even zigzag ridges along its length (like corrugated roofing), so the strip still reaches both supports at full span. Do not accordion-fold it into a short fan that no longer crosses the gap. Set the ridged strip across the supports.
- Arched: one strip bent into a smooth curve. Rest the ends on the supports. If the ends slip, tuck a small thin wedge of scrap card under each end so the arch stays put (not thick blocks or extra layers).
- Trussed: one strip as the top path, with triangle braces cut from the remaining strips and taped underneath or along the sides. Triangles should touch the top strip and the ends near the supports.
Set your supports 20 cm apart. Rest each bridge only on the ends. Label each shape. Do not start loading yet.
Before the lesson
- Cut card into equal strips long enough to rest on both supports across a 20 cm gap (A4 cut lengthways works: each strip is about 29.7 cm long). Give each group a fixed budget of 8 matching strips.
- Mark a 20 cm gap with tape on each group table, or use identical book stacks as supports.
- Put scissors on each group table for cutting truss pieces and thin wedges.
- Print the Investigation Journal page.
Material budget rule (say this aloud)
Everything for all four bridges comes from the 8 strips: deck strips, folded ridges, arch curve, truss triangles, and any tiny end wedges for the arch. Rough guide: 1 strip each for flat, folded and arched; about 5 strips left for the truss (deck plus braces) and thin wedges. No extra card from the scrap box. Thin arch wedges are only to stop ends slipping; they must not become thick piers or stacked layers. Tape is for joins only, not for wrapping extra strength into one shape.
Build how-to (support for every group — no separate templates needed)
- Flat: one strip, no folds, ends on supports only.
- Folded: make parallel zigzag ridges along the strip (corrugated stiffening) so the finished piece is still long enough to seat on both supports at 20 cm. The ridges run along the span and stiffen the middle; the strip must not be concertinaed into a short pack that fails to reach both ends.
- Arched: bend one strip into a curve; ends rest on supports; optional thin scrap wedges only if ends slip.
- Trussed: top strip as the deck; cut remaining strips into shorter pieces with scissors; tape them into triangles under or beside the deck so each triangle shares the load toward the supports.
One set of four bridges per group is enough. Circulate and check span with a ruler. If a group stalls on the truss, point to the board how-to and ask them to make at least two clear triangles under the middle third. If they have already used most strips on the first three shapes, remind them of the 1+1+1+5 guide and rebuild the order next time if needed.
Worked model (build only — no loading in this step):
- I set the gap at 20 cm with the ruler.
- I take one strip for the flat bridge and rest it only on the two supports.
- I show zigzag ridges on a second strip for the folded shape, checking it still spans both supports.
- I bend a third strip into a gentle arch and tuck tiny end wedges only if it slips.
- I tape two simple triangles under a fourth strip for a basic truss, using cut pieces from the leftover strips.
- I label all four. We do not add coins or washers until the next step, so every shape is ready for the same fair test.
Then groups finish building all four shapes before any comparative loading.
4 - Fair-test the Load ~14 mins
This is the exciting bit — listen for the creak and watch which shape gives way first.
Test one shape at a time. Place coins or washers gently in the centre of the bridge, one at a time. Count how many it holds just before it fails (sags to the table or collapses).
Keep the same: 20 cm span, same card budget, load always in the middle, same counting rule. Change the form you test.
As you finish each shape, write the load count on your Investigation Journal page straight away.
Aim for one clean trial of each shape. If you have time for a second trial of a shape, add the two counts and halve them, then record that number.
Method to insist on: load only at the centre; add one piece at a time; stop and record the last count that still held when the bridge fails. Stand back slightly as structures fail so eyes and fingers are clear.
Primary expectation: one clean trial per shape in the time available. Systematic repeats of all four are unlikely in about 14 minutes. If a group manages a second trial on one shape, they add the two counts and halve them (you may call this the mean). Do not say "middle" for two values.
If a group runs short of time, prioritise one clean trial of each of the four shapes rather than many repeats of one shape.
What to expect: flat usually holds least; folded and trussed often beat flat; a well-made arch or truss often holds most. Results vary with build quality, which is useful science-talk later (did the strongest shape also use more of the strip budget?).
Safety reminder mid-test: no leaning faces over the bridge as weights drop.
Quick teacher model of loading (flat strip only, once shapes are ready): show adding coins or washers one by one at the centre and stopping at failure so the counting rule is visible before groups begin.
5 - What the Results Mean ~4 mins
Your load counts should already be on your Investigation Journal page from the tests.
Now add one short note on the same page: which shape held most in your group, and whether that matched your prediction. Were you surprised, or did the winner match what engineers might expect? Keep what you measured separate from what you think it means.
This step is the meaning beat, not a second recording of the counts. Counts were written live in step 4. The Investigation Journal page is where design choice, load counts and this short meaning note all live — circulate for a quick units check (simple count of coins or washers) and that the prediction-match note is one or two sentences, not a full rewrite of the table.
If a group still has a blank row, give them 30 seconds to fill the missing count from memory of the trial, then move on to the meaning note.
Optional prompt while circulating: "Did the strongest shape also use more card from the budget?"
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