📋 Cover Page
Bridge It 2026 — Design Folio (Part A)
Team name: Bradfield (named after JJC Bradfield, designer of Sydney Harbour Bridge)
Team members: Ms Gao (exemplar author)
Class: 7TECJ
Date: Week 7, Term 2, 2026
Teacher: Ms Y. Gao
1. Design Brief
We are designing and building a Pratt truss bridge made from popsicle sticks and PVA wood glue. It will span a gap of 350 mm between two tables and must safely carry a hanging load of at least 1000 g applied to the centre of the deck.
Our target users are Year 7 students learning engineering, and our design priority is a high load-to-mass ratio (aim 8:1 or better) — we want a bridge that is strong for its weight, not just strong overall. The project must be complete by Lesson 20, using only the supplied materials, and we will document every stage in a shared Google Doc.
Marker's note: notice this brief answers WHAT, WHO FOR, WHY, CONSTRAINTS — and adds a measurable target (8:1 ratio). That's A-band thinking.
2. Research — 6 Annotated Sources
| # | Source | Topic | Annotation (1 sentence) |
|---|---|---|---|
| 1 | Engineers Australia — "Understanding Truss Bridges" | Truss types | Clear diagrams of Pratt vs Howe with force arrows — I'll use for the compression/tension labelling on my isometric. |
| 2 | Britannica — "Truss bridge" | History | Explains Pratt origin (1844) and why it beat Howe in the USA — useful for my design rationale. |
| 3 | Practical Engineering — "Why Triangles" | Forces | Video shows square deforming vs triangle holding — confirms my choice of full triangulation. |
| 4 | Structurae — Sydney Harbour Bridge profile | Famous bridge | Key stats: 503 m span, 52,800 t steel, built 1923–32, Pratt-style trusses inside the arch. |
| 5 | NSW Engineers Heritage — JJC Bradfield biography | Engineer career | Bradfield was a civil engineer from Queensland who fought for 20 years to get the Harbour Bridge built — great role model. |
| 6 | PopsicleBridges.com (hobbyist) — "100 stick truss" | Build technique | Demonstrates lamination at centre verticals to triple bending stiffness — I'll apply this. |
Marker's note: 6 sources, each with a specific 1-sentence annotation linked to the design. That's the structure the rubric rewards.
3. Three Thumbnail Sketches
Idea 1 — Pratt truss (standard)
Pros: simple, predictable, diagonals in tension so less buckling. Cons: uses more sticks than Warren.
Idea 2 — Warren truss (zigzag)
Pros: fewest sticks, elegant, best load-to-mass potential. Cons: alignment harder; no verticals for bucket-hang stability.
Idea 3 — Pratt with X-bracing (hybrid)
Pros: extra strength at highest-stress panels. Cons: heaviest of the 3, more work.
Chosen design: Idea 3 — Pratt with X-bracing
We chose Idea 3 because our target is load-to-mass ratio. The centre X-bracing adds maybe 8 extra sticks (15% mass increase) but should add 30–40% more load capacity, because centre-panel failure is the most common failure mode in Pratt trusses (per Source 6). Net ratio improvement: ~20%. Trade-off: more build time, but we've scheduled for it.
Marker's note: the RATIONALE here is evidence-backed and quantified. That's what separates A-band from B-band thinking.
4. Scaled Drawing — Plan + Elevation at 1:5
ELEVATION (side view) — real bridge 450 mm → drawing 90 mm.
PLAN (top view) — real 450 × 150 mm → drawing 90 × 30 mm.
Marker's note: both views use the same scale. Dimension lines in a different colour. Title information in the plan.
5. Rendered Isometric Drawing
Marker's note: 3-tone shading shown. Labels in technical lettering (caps). Title block included.
6. Materials List
| Item | Quantity | Use |
|---|---|---|
| Popsicle sticks (114 mm full length) | 64 | Chords (top + bottom × 2 trusses) + deck |
| Popsicle sticks cut to 60 mm | 22 | Verticals + cross members |
| Popsicle sticks cut to 72 mm | 24 | Diagonals (Pratt) |
| Popsicle sticks cut to 85 mm | 8 | X-bracing at centre panels |
| Card gusset plates (15 × 15 mm) | 8 | Corner reinforcement |
| Total stick count | ~118 + 20% contingency = 142 | |
| PVA wood glue | ~150 g | All permanent joints |
| Hot glue sticks | 3 × 7 mm | Fast-cure tack joints (supervised) |
| Masking tape | 1 m | Temporary clamping |
7. WHS Risk Assessment
| Hazard | Likelihood | Consequence | Risk | Control |
|---|---|---|---|---|
| Scissor cut | Medium | Low | Low | Cut away from body; pass handle-first |
| Hot glue burn | Medium | Medium | Med → Low | Supervised use only; cool-down mat; announce "hot glue" |
| PVA in eyes | Low | Low | Low | Cap bottle when not in use; wash hands |
| Bridge collapse at test | High (designed to!) | Low | Low | 1 m exclusion zone; teacher operates bucket; eye safety optional |
| Finger pinch in taped clamps | Low | Low | Low | Careful tape application; take time |
8. Pair Agreement
- Partner A (Ms Gao) leads drawing and research. Partner B leads construction and quality control.
- Each partner contributes at least 40% of total build time. Journal photo captions record who did what.
- Disagreements: 2-minute discussion. If unresolved, each draws a sketch of their preferred option; teacher breaks tie.
- If one partner is absent, the present partner photographs and dates their own work. Absent partner catches up next lesson.
Signed: [Partner A] · [Partner B] · Date: Week 3 Lesson 5