Tacoma Narrows case study + 12 famous bridges data table

Do Now · 5 min

The classic rigidity test. Teacher hands each pair 4 popsicle sticks + tape.

1. Make a square. Push on one corner. What happens?
2. Add a 5th stick across the diagonal. Push again. What happens now?

I Do · ~10 min Teacher demonstration

Part 1 — The Tacoma Narrows collapse (Workbook Q6–Q7). Ms Gao plays the 1940 collapse video.

Why it collapsed: (1) the deck was too shallow and flexible → twisted in wind; (2) no lateral bracing; (3) the wind matched the bridge's natural frequency → resonance amplified every oscillation. Every modern bridge since has stiffening trusses AND aerodynamic testing in wind tunnels.

Part 2 — 4 truss types. Teacher draws each truss type; you copy into your workbook.

• Pratt truss — diagonals slope DOWN toward centre. Verticals in compression, diagonals in tension. Most common.
• Howe truss — diagonals slope UP toward centre. Verticals in tension, diagonals in compression.
• Warren truss — alternating diagonals form equilateral triangles (no verticals). Light and elegant.
• K-truss — diagonals meet at vertical midpoints forming K shapes. Used for tall/wide trusses where normal diagonals would be too long.

Sydney Harbour Bridge is a Pratt-style truss inside its arch. The Eiffel Tower uses Warren + K-trusses together.

We Do · ~15 min Guided practice

Photo ID round — Ms Gao projects 5 bridge photos; class calls out the truss type. Then we colour-code force direction on a Pratt truss together:

• Top chord — RED = compression
• Bottom chord — BLUE = tension
• Verticals — depends on truss type (Pratt: compression; Howe: tension)
• Diagonals — the opposite of verticals

You Do · ~35 min Independent pair work

Part 1 — Workbook Q6 + Q7 (10 min):

• Q6: Why did the Tacoma Narrows Bridge collapse? (2+ sentences using resonance, aeroelastic flutter, stiffening)
• Q7: What could have been done during design + construction to prevent it? (2+ sentences — think bracing, deck depth, wind testing)

Part 2 — Workbook Q8 — 12 Famous Bridges Data Table (15 min): use the PBS Wonders of the World Bridges browse page to fill in location, length, material, date for all 12 bridges (Akashi Kaikyo · Brooklyn · Charles River · Firth of Forth · Garabit · George P Coleman · Golden Gate · Iron · New River Gorge · Sunshine Skyway · Tacoma Narrows · Tower).

Part 3 — Truss diagnostic quiz (10 min): 10-question quiz — does NOT count, helps me re-teach.

1. Complete the 10-question quiz on your laptop (link below).
2. Sketch each of the 4 truss types to scale in your folio, using technical lettering for labels (remember Term 1 L10!).
3. Colour-code the Pratt truss — red for compression, blue for tension, black for the load.
4. With your partner: agree which truss type you think you might use for YOUR bridge. Write 2 sentences giving reasons.

Differentiation

Exit Ticket · 5 min

On a sticky note:

1. A triangle cannot change shape unless ___ breaks.
2. Which truss type did YOU pick for your bridge, and why?
3. One thing from today's quiz you got wrong and now understand.

📚 Resources for this lesson

• 📓 Bridge Workbook Folio — Q6, Q7, Q8 sections
• YouTube — Tacoma Narrows Bridge collapse (1940)
• PBS Building Big — Wonders of World bridges (for Q8 table)
• PBS Building Big — Bridge Challenge
• Truss types reference (on hub)
• Video — Why Triangles Are Structural Perfection

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