Bridge Building Lesson Plan: Truss Design Challenge (Grade 4–5)

Grade Band: Upper Elementary (4–5)
Subject Area: Science

This free bridge building lesson plan helps students think like structural engineers. Students test what makes shapes strong, build and test bridges, then redesign a truss to improve strength while using limited materials.

Overview

Students learn how engineers design structures that are strong, stable, and efficient. The unit starts with quick hands-on tests using simple materials, then moves into a team bridge-building challenge. Students collect data, compare designs, and use what they learn about triangles, tension, and compression to redesign a truss bridge for better performance.

Learning Goals

Students describe what structural engineers do and explain why some shapes and designs are stronger than others. Students identify basic bridge types and use the ideas of tension and compression in simple, age-appropriate ways. Students plan, build, test, and improve a bridge design based on evidence from results.

Materials

• Gumdrops or mini marshmallows (or clay balls) for connectors
• Toothpicks
• Paper cups or small boxes for test supports
• Books or small weights for testing
• Drinking straws and masking tape
• Cardboard strips for a “road deck”
• Rulers or measuring tapes
• Student recording sheets (paper or digital)

Preparation

Set up a simple testing area with two stable supports (cups, blocks, or boxes) and a consistent way to add weight (books or small weights). Prepare a few photos or simple diagrams of bridge types (beam, arch, truss, suspension) for a quick introduction. Decide the constraints you will use for your classroom (example: number of straws, length of tape, and testing goal).

Teaching Procedure

Session 1

1. Introduce the role of a structural engineer and explain the day’s question: “What makes a structure strong without using a lot of material?” Students share ideas and you record them on chart paper.
2. Pairs build two small tabletop structures using toothpicks and connectors: one with only squares/rectangles and one that includes triangles. Students sketch both and predict which will resist pushing better.
3. Test rigidity by gently pressing each structure from the side. Students record what happens and write one sentence explaining what the triangles changed.

Session 2

1. Run a short design warm-up: each pair builds a “book-lift” structure that must hold one book at least 1 inch above the table for 15 seconds. Give a tight limit on materials to force planning.
2. Students do a gallery walk. Each pair leaves a sticky note with one “strength move” they notice on another team’s structure (example: wide base, triangles, short members).
3. Test structures publicly using the same timing and weight steps for every team. Students record results and identify one design choice that helped or hurt strength.

Session 3

1. Show quick visuals of bridge types (beam, arch, truss, suspension). Students identify what they notice about each type, especially what is holding up the load.
2. Introduce tension and compression with a simple demonstration: push gently on a ruler to show “squish” (compression) and pull on a rubber band to show “stretch” (tension). Students act it out with hands as you name each force.
3. Teams choose a bridge goal for the next session: longest span, best strength, or most efficient use of materials. Students sketch a plan that includes at least one triangle or truss pattern.

Session 4

1. Bridge build challenge begins. Teams build a bridge using straws and tape that spans between two supports and holds a cardboard road deck. Set clear constraints (example: 20 straws, tape only at joints, no tape longer than a student thumb).
2. Mid-build check: teams stop and label on their sketch where they believe compression happens and where tension happens. Students explain their thinking in simple terms.
3. Conduct the first test. Each team’s bridge is measured for span and tested for load (example: hold 3 matchbox cars or a stack of small books for 30 seconds). Students record results.

Session 5

1. Teams analyze their first test results and choose one specific improvement goal (example: stop sagging in the middle, strengthen joints, widen the base, add triangles to the sides).
2. Teams redesign and rebuild a truss version of their bridge. Require one labeled truss pattern on each side (example: a repeated triangle pattern).
3. Final test and reflection: teams test again using the same rules and compare performance. Students write two sentences: one describing the improvement and one explaining why it worked.

Assessment

• Observation of teamwork, planning, and safe building habits
• Student sketches and labeled plans showing a truss/triangle pattern
• Data recording from tests and ability to compare results
• Short written reflection explaining how redesign improved performance

Differentiation

Provide a simple truss template (a triangle pattern) for students who need extra support, and allow them to build directly from it. For advanced students, add a “cost” constraint by charging each straw and each taped joint, then challenge them to improve strength without increasing cost. Offer roles within teams (builder, tester, recorder, materials manager) to support students who need structure during group work.

Extension Ideas

Have students photograph their bridge and create a short captioned slide explaining three design features. Another option is a “bridge makeover” task where students take a weak design and annotate how they would reinforce it using triangles. If you want a math connection, students can graph class results (span or load) and discuss what design pattern appears most consistently in the strongest bridges.