How you can explain it
The weight pulls on some beams and squeezes others. Triangles help a bridge keep its shape. A long, thin beam can bend when it is squeezed too hard.
BeamOne of the straight bars that joins two dots.
Bridge Lab
Explain what makes a bridge stable
Help your child see how triangles and beam length influence a model bridge.
Nallo’s guide for parents
Before the first test
The setup is ready. Ask your child what they expect, then start the test.Steps
- Ask your child: “Where might the bridge change shape under the test weight?”
- Start the prepared test after discussing your predictions.
- Compare which beams are pulled, compressed or fail, using the force display if helpful.
Starter bridge is ready. Change one thing, then test it.
0.8 m model span · changes show on the bridge
Blue: pulled
Red: squeezed
Yellow: near its limit
Broken: failed
Test weightStart light, then try the challenge.
250 N ≈ 25.5 kg under Earth gravity
An activity for you to guide
- Make two supports from stacks of books with a small gap between them.
- Lay paper across the gap. Compare a flat sheet with the same sheet folded into an accordion.
- Add coins slowly while your child observes. These are tabletop models, never bridges to stand on.
Explanation guide for parents
Stay with your child throughout the experiment. Read the task together, compare your predictions and start the test. Operate the controls, then discuss what changed.
Support your child’s reasoning
Say “pulled” and “squeezed” first; introduce tension and compression later. Ask your child to point to a triangle. On touchscreens, use a large drawing area for moving dots.
Ask: “What did you change? What did you notice? What will you try next?” A different result is a clue, not a bad score.
Screen and real life
These loads belong to a small computer model. Never use its numbers to decide whether a real bridge is safe to stand on.
Use the simulation to discuss patterns. You remain responsible for choosing and supervising any physical activity; real results can differ.
A question to take it further
Why might a triangle keep its shape better than a rectangle with flexible corners?
A common misunderstanding
A stable-looking outline can still contain a long, thin compression member that buckles. The displayed loads apply only to this small model.
How the model works
Equilibrium check
No result yet.
Model assumptions
- 0.8 m tabletop-scale, 2D pin-jointed truss.
- Beams carry axial force only.
- Compression includes Euler buckling.
- Beam self-weight is applied as half-weight at each pin joint; joint failure is not modeled.
- Educational model — not structural certification.
Force tableFor curious builders and teachers0 beams
| ID | Dots | Material | Length | Force | State | Capacity | Stress |
|---|
Quick guide: s = seconds · m = metres · cm = centimetres · g = grams · kg = kilograms.
Why triangles?
Press your hands together in a square shape: it can lean. Add a diagonal and it becomes two triangles that hold their shape.
Try it: move a white dot, then test again. A red beam is being squeezed; a blue beam is being pulled.
LEARN FROM YOUR OWN BRIDGE
The colors show where the weight goes
Blue beams are pulled. Red beams are squeezed. Long squeezed beams can buckle. Change one thing, retest, and learn what actually helped.
Pulled
Engineers call this tension.
Squeezed
Engineers call this compression.
Triangles
They stop many bridge shapes from folding.
BUILD 20 · PARENT GUIDE · 29 SEP 2026