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Day 4: Bridge Challenge — Build and Test

Lesson Overview
Time 50 minutes
Objectives Apply EDP phases 3-4: Build the bridge prototype from the approved Day 3 design; test weight capacity incrementally; record max weight held
TEKS d(1)(C)
Deliverable Completed bridge prototype + test results sheet (max weight held)
Materials Bridge-building materials (40 straws, 3 ft tape, 5 index cards, scissors per team), 12-inch gap test station, weights (pennies in cups, washers, or a digital kitchen scale with textbooks), stopwatch, test results sheet

Warm-Up (5 min)

WARM-UP: What is one thing that could go wrong while you build your bridge? How will you know if it is happening?

Quick share. This primes students to self-monitor during building rather than just rushing. Civil engineers call this "quality control", checking as you go, not just at the end.

Activity 1: Build Phase with Checkpoints (22 min)

Distribute materials to each team. Emphasize: follow your approved sketch from Day 3. This is the "Prototype" phase of the EDP.

Use three visual checkpoints to keep teams on pace. Walk with a clipboard and mark each team's progress.

Checkpoint 1 — by minute 8

Base structure complete. Two long support rails (straws taped together) should span roughly 12 inches. Most teams are still shaky here, that is fine.

Checkpoint 2 — by minute 15

Triangular reinforcement added. Diagonal members (making truss triangles) should be visible connecting the two support rails. Teams with no triangles need a reminder, point to their Day 3 sketch.

Checkpoint 3 — by minute 22

Deck surface complete. Index cards or taped straws form a flat deck where weight can be placed. The bridge should stand on its own without being held.

Facilitation Tip

The most common build failure is students taping straws end-to-end (flimsy) instead of bundling 3-4 straws together with tape (strong). Walk by each team in the first 5 minutes and prompt them: "Are those straws bundled or just connected in a line?" One nudge early saves a collapse at testing.

Activity 2: Weight Testing Phase (15 min)

Set up the test station at the front of the room: two desks (or stacks of books) 12 inches apart with the bridge spanning the gap.

Each team brings their bridge forward for testing. Use one of these methods:

  • Method 1 (simple): Add pennies to a cup sitting in the center of the bridge, one at a time. Define "failure" before testing starts so teams cannot argue in the moment: STOP at the first visible sag OR the first taped joint that separates, teacher's call is final. Record the penny count at the stop.
  • Method 2 (precise): Use a digital kitchen scale placed on the bridge, and stack textbooks until failure. Record weight in grams.

Testing protocol:

  1. Team places bridge on the test station
  2. Weight is added slowly (one unit every 2 seconds)
  3. Teacher calls "STOP" at first visible failure (buckling, collapse, or deck breaking)
  4. Teacher records max weight on a class-visible whiteboard

Each team records their own result on their test results sheet along with notes on where the bridge failed first (usually the deck, a support rail, or a taped joint). Teams waiting their turn at the test station should start Activity 3 (redesign reflection) at their desks instead of sitting idle. This keeps the test queue moving and distributes the 15-min activity time across testing and writing.

DOK 4: What design features correlated with the strongest bridges in our class? Use evidence from at least two teams' results.

Activity 3: Redesign Reflection (5 min)

Before clean-up, each team writes a quick redesign statement on their test results sheet:

  • What failed? Name the specific element that broke first.
  • Why did it fail? Was it the material, the shape, or the connection method?
  • What would you change? Describe one specific change that would make the bridge stronger.

This reflection is the "Redesign" phase of the EDP. Real civil engineers rarely get it perfect on the first try. They iterate.

Connection to Infrastructure Imagination

Remind students that this bridge challenge is a simplified version of the Los Lomas Infrastructure Imagination project. Real civil engineers work at a much bigger scale and with much longer test cycles, but the process is the same.

Exit Ticket (3 min)

EXIT TICKET (Ranked Justification) · Printable PDF:

Rank these 4 design factors from MOST important (1) to LEAST (4) based on what made class bridges STRONG or WEAK today.

  • Triangular reinforcement (trusses): rank ____
  • Bundled straws (3+ straws taped together): rank ____
  • Deck thickness (index cards doubled up): rank ____
  • Tape placement (joints reinforced): rank ____

My bridge's MAX WEIGHT HELD: ____

First element that failed: _____

For EACH rank, write ONE piece of evidence (from my bridge OR another team's):

  • Rank 1 (most important): _____________

  • Rank 4 (least important): _____________

Bottom line: ONE change I would make in a redesign:

(d(1)(C))

Differentiation

  • Support: Provide pre-bundled support rails (3 straws taped together) for teams who need a head start. Their challenge becomes the truss design rather than the material prep.
  • Extension: Teams who finish early get a second set of materials (or use materials from other teams who finished) and build a stronger V2 based on their redesign statement. Test and compare.
  • ELL: Testing is visual and universally accessible. Pair ELL students with bilingual peers during the reflection writing. Provide sentence stems: "My bridge failed at the _ (En la ). I would change __ (Cambiaría _____)."