Test Data (pasted from the build journal)
| Load added | Observation | Photo |
|---|---|---|
| 0 g | Empty bucket hanging. Bridge straight, no deflection. | 📷 L20-01.jpg |
| 250 g | Very slight deflection visible on deck centre (~1 mm). No sounds. | 📷 L20-02.jpg |
| 500 g | Deflection ~3 mm. No creaks. | 📷 L20-03.jpg |
| 750 g | Small creak from rear truss. Deck slightly twisted. Bridge still holding. | 📷 L20-04.jpg |
| 987 g | FAILURE. Centre vertical on rear truss buckled sideways. Bridge collapsed. | 📷 L20-05.jpg |
- Bridge mass: 124 g
- Max load: 987 g
- Load-to-mass ratio: 987 ÷ 124 = 7.96
- Failure mode: compression buckle of rear-truss centre vertical
Written Evaluation (450 words)
Paragraph 1 — What worked
Our bridge performed well overall, holding 987 g on a total mass of 124 g — a load-to-mass ratio of 7.96 which exceeded our design target of 7.0. The triangulation on our front truss performed especially well; inspecting the photo at 750 g (L20-04.jpg), every diagonal on the front truss remained perfectly straight under load, showing that our Pratt diagonals successfully transferred tension to the bottom chord as predicted. The X-bracing we added at the centre two panels (Idea 3 from our folio) also held firm — no X-brace member deformed at any load. These elements confirmed our research from Source 6 (PopsicleBridges.com), which recommended X-bracing at the highest-stress centre panels.
112 wordsParagraph 2 — What failed
Failure began at the centre vertical of the rear truss when the hanging load reached 987 g. In the L20-05.jpg photo, this member is clearly visible bent sideways at about its midpoint, with one end pulled away from the bottom chord joint. The failure was sudden — within about one second of adding the final 250 g increment. Before failure we heard a small creak at 750 g (L20-04 captures this moment), which in hindsight was the first sign the vertical was beginning to deform. The front truss and the deck remained intact even after the rear truss buckled; the bridge as a whole only collapsed because the load became unsupported.
117 wordsParagraph 3 — Why it failed
The centre vertical was in compression when under load, as expected for a Pratt truss. Its failure was a compression buckle rather than a pure compression crush — the member was long and thin (60 mm tall by only ~2 mm thick), and at a critical load it bent sideways rather than simply shortening. This is a classic slenderness problem: Euler buckling is proportional to the cube of the thickness (depth), so thin vertical members are extremely vulnerable. The X-bracing we added helped the centre two PANELS but did not directly reinforce the centre VERTICAL itself. The rear truss failed before the front truss because during our build (photo L14), we noticed the rear truss had one slightly thinner vertical, and we did not replace it.
124 wordscompression buckle, slenderness, Euler buckling, member, panel vs vertical). Explains failure mechanism with a physical rule ("stiffness ∝ depth³"). Admits a build-time mistake — the rear truss had a known weak vertical. Honesty about decisions under pressure is exactly the engineering habit teachers want to see.
Paragraph 4 — One specific improvement
In a version 2 bridge, we would laminate the centre vertical into a double-thickness member (two sticks glued side-by-side with PVA, 24-hour cure). This roughly doubles the depth of the vertical, which by Euler buckling roughly 8× its bending stiffness. Based on our failure load of 987 g and the slenderness of the current vertical, we estimate version 2 could carry an additional 200-300 g before the new weakest point (probably the adjacent rectangles) failed. The cost is modest: 2 extra sticks and 20 minutes of cure time. We would also ensure both trusses use identically-thick sticks before gluing — matching our materials to our design more carefully. These two changes together should move our load-to-mass ratio from 7.96 toward a target of 9.5.
127 words · TOTAL 480 wordsMarkers' hidden tips
- 🎯 Always refer to a specific member or photo — not "the middle" or "a part of it". Name it. Bottom-chord-centre. Vertical-4-rear. Use the labels from your plan drawing.
- 🎯 Connect each observation back to the forces — "because it was in compression" / "because the diagonal was in tension" / "because the deck was bending".
- 🎯 Be honest about what you would change — vague promises ("we'd make it better") score low. Specific changes with numbers score high.
- 🎯 Word count targets: 400-600 words. Going under 400 often means you didn't have enough evidence to cite. Going over 600 usually means you're repeating yourself.
- 🎯 Paragraph structure is fixed: P1 worked · P2 failed · P3 why · P4 improve. Don't re-order.