Build a Bridge

  • Challenge:  Using only craft sticks, clothespins, and binder clips, design and build a bridge that can hold as many books as possible.

Materials

  • Craft sticks
  • Clothespins
  • Binder clips
  • Stack of books for testing
  • Flat testing surface (two equal-height supports like boxes or cups)

Challenge Instructions

  1. Set Your Span
  • Place two supports about 8–10 inches apart.
  • Your bridge must stretch from one support to the other without touching the table in between.
  1. Build Your Bridge
  • Use craft sticks as your main building pieces.
  • Use clothespins and binder clips to connect sticks together.
  • Try different shapes—triangles, layers, beams, arches.
  • You may rebuild or adjust your design at any time.
  1. Test Your Strength
  • Carefully place one book at a time on the center of your bridge.
  • Count how many books it can hold before it bends or collapses.
  • Record your number.
  1. Improve Your Design
  • Look at what worked and what didn’t.
  • Strengthen weak spots using more sticks or better connections.
  • Test again to see if your bridge holds more weight.

Extensions:

  1. Structural Variations (The “What If” Phase)

Most students naturally gravitate toward a simple beam bridge. Challenge them to incorporate specific geometric shapes to see how force is redistributed.

  • The Power of the Triangle: Compare a square frame to a triangular truss. Why do triangles prevent “shearing” (the leaning-over effect)?
  • Arch Theory: Use flexible materials (like thin cardboard) to create an arch bridge. Have students observe how the arch pushes the load outward toward the “abutments” (the ends) rather than just straight down.
  • Suspension Extensions: Introduce string or yarn. Can they build a bridge where the deck is supported from above rather than below?
  1. Real-World Constraints

Engineering isn’t just about strength; it’s about resources. Add these “Professional” layers to the activity:

  • The Budget Challenge: Assign a “price” to every material (e.g., $10 per popsicle stick, $5 per inch of tape). The goal is to build the strongest bridge with the lowest cost-to-strength ratio.
  • Environmental Factors: Introduce a “wind” test using a hair dryer or a “seismic” test by shaking the table. How does height or base width affect stability?
  • The “Gap” Variable: Once a bridge succeeds at a 10-inch span, force them to clear a 20-inch span. This usually requires a total redesign of the support system.
  1. The Physics of Failure

Don’t just measure how much weight the bridge holds—analyze how it fails. This is where the real science happens.

  • Tension vs. Compression: Use a thick sponge to demonstrate these forces. When you bend it, the top bunches up (compression) and the bottom stretches out (tension). Ask students to identify which parts of their bridge are being pulled vs. pushed.
  • Slow-Motion Analysis: Film the “Stress Test” (adding weights) on a phone in slow motion. Watching the exact moment a joint snaps helps students identify the “weakest link” for their Version 2.

 

Standards Alignment: NGSS PS2-1:  Plan and conduct an investigation to compare the effects of different strengths or different directions of pushes and pulls on the motion of an object.