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Day 3: Bridge Challenge — Design Phase

Lesson Overview
Time 50 minutes
Objectives Apply the Engineering Design Process (EDP) phases 1-2: Define the problem and research/sketch 2 bridge design options before building
TEKS d(1)(C)
Deliverable Approved bridge sketch showing two design options with labeled structural elements (truss, arch, or beam)
Materials Chromebooks, printed Bridge Design sketch page, projector, PBS Design Squad videos, bridge materials for display only (not yet built)

Warm-Up (5 min)

WARM-UP: Think about the strongest bridge you have ever seen. What made it strong, the shape, the materials, or the size? Write down your best guess.

Collect 2-3 guesses. Bridge to the idea that shape matters more than most students expect: a triangle holds its shape under load, while a square collapses into a parallelogram.

Activity 1: The Challenge + Engineering Design Process (10 min)

Announce the challenge clearly so students understand the constraints:

The Challenge: In teams of 2-3, design and build a bridge that spans a 12-inch gap between two desks. The bridge must hold as much weight as possible without collapsing. The winning bridge holds the most weight per gram of materials used (strength-to-weight ratio).

Material limits per team:

  • 40 plastic drinking straws
  • 3 feet of masking tape
  • 5 index cards
  • Scissors

Introduce the Engineering Design Process. Civil engineers follow this same process in real projects.

  1. Define the Problem: What exactly are we solving? (Span 12 inches, hold maximum weight.)
  2. Research & Ideate: How have others solved this problem? What designs work?
  3. Prototype: Build it.
  4. Test: Measure performance.
  5. Redesign: Fix what failed.

Today's goal: Complete Phases 1 and 2.

Facilitation Tip

Do not distribute materials today. Students who get materials immediately start building without thinking. The point of Day 3 is forcing them to plan first, just like a real civil engineer does before pouring a single cubic yard of concrete.

Activity 2: Bridge Type Research (15 min)

Project a quick 3-minute visual tour of bridge types on the screen, use PBS Design Squad or a static comparison slide:

  • Beam Bridge: simplest. A flat deck with supports on each end. Weak unless the beam is thick.
  • Truss Bridge: triangular framework. Distributes load across many small pieces. Common in railroads.
  • Arch Bridge: curved. The weight pushes outward into the ground. Very strong but hard to build.

After the tour, teams research bridge types online (PBS Design Squad, BLS civil engineering page, or a teacher-curated image set). They identify:

  • Which type fits a 12-inch span with straws?
  • What is the key structural element of each?
  • Which uses triangles and why are triangles strong?

Why Triangles?

A triangle is the only shape that does not deform under load without breaking a connection. Push on a square and it becomes a parallelogram. Push on a triangle and nothing moves unless one of the sides breaks. This is why trusses are made of triangles.

Activity 3: Design Two Options + Peer Critique (17 min)

Each team sketches two different bridge designs on the provided sketch page. The sketches must include:

  • Top-down view of the bridge
  • Side view showing the structural shape
  • Labels for each structural element (truss, arch, beam, support column)
  • Predicted weak points: where do they think it will break first?

After sketching, teams swap sketches with another team for a 2-minute peer critique. The critiquing team answers: "Where do you think this design will fail first, and why?"

Teams then pick one design to build on Day 4 based on the peer feedback.

DOK 2: Why is a truss stronger than a simple beam bridge? Use your sketch to explain.

Facilitation Tip

Approve each team's final design before they leave class. If a team picked a flat beam bridge with no triangular reinforcement, prompt them with: "What shape holds its form under load?" and send them back to revise. Avoid saying "That won't work", let them engineer their own realization.

Exit Ticket (3 min)

EXIT TICKET (Short Constructed Response) · Printable PDF:

  1. My team's bridge type (circle one): Truss / Arch / Beam / Other: _____

  2. Space below, sketch the chosen design (top view OR side view) with at least 3 structural elements labeled:
  1. ONE design feature I picked specifically to MAKE IT STRONGER:

Feature: _____. Why it makes it stronger in one sentence:

  1. The WEAK POINT I expect to fail first: _____. Why I expect that in one sentence:

(d(1)(C))

Differentiation

  • Support: Provide truss and arch bridge templates with basic geometry already drawn. Students adapt the template for the 12-inch span rather than designing from scratch.
  • Extension: Research cable-stayed and suspension bridges, why are these used for very long spans? How do they differ from truss bridges?
  • ELL: Visual bridge type poster with Spanish labels: Truss = Armadura, Arch = Arco, Beam = Viga, Support = Soporte, Load = Carga. Bridge-building is highly visual and accessible across language levels.