Bridge Building Lesson Grade 6–8: Design and Test a Model Bridge

Grade Band: Middle School (6–8)
Subject Area: Mathematics

This free bridge building lesson introduces students to how math and engineering work together in real construction projects. Students research bridge types, design a plan, build a model bridge, and evaluate how well their design performs under load.

Overview

Students work in small teams to act as engineering companies hired to design a bridge. They investigate real bridge structures, create scale drawings, estimate materials, and construct a physical model. The project ends with testing each bridge’s strength and analyzing why some designs performed better than others.

Learning Goals

Students apply measurement, ratio, and proportional reasoning to a real problem. They use geometric thinking to design a stable structure and explain how forces act on a bridge. Students also communicate mathematical ideas through diagrams, calculations, and presentations.

Materials

• Toothpicks, straws, or craft sticks
• White glue or school glue
• String or thread
• Graph paper
• Rulers and measuring tape
• Coins, washers, or weights for testing
• Small cardboard boxes or books to create a testing gap

Preparation

Prepare a testing station with a gap between two desks or boxes to represent a river span. Gather weights for strength testing. Decide a maximum length for bridges and a limited quantity of building materials per group so students must plan carefully before construction.

Teaching Procedure

Session 1

1. Introduce the challenge: each team must design a bridge that spans a fixed distance and supports weight. Show photos of real bridges and ask students what makes them strong.
2. Students research common bridge types (beam, arch, suspension, truss) and sketch at least two possible designs in their notebooks.
3. Teams discuss ideas and choose one design to develop further.

Session 2

1. Teach how to create a scale drawing using graph paper and measurements.
2. Teams draw a detailed plan of their bridge including length, height, and supports.
3. Students estimate the number of building pieces required and revise their plan if needed.

Session 3

1. Review construction safety and demonstrate strong joints and triangular supports.
2. Teams begin building the bridge according to their drawings.
3. Students record daily progress and note any design changes they make.

Session 4

1. Teams complete construction and allow glue to dry.
2. Each group predicts how much weight their bridge will hold and explains why.
3. Students prepare a short explanation of their design and mathematical reasoning.

Session 5

1. Place each bridge on the testing span.
2. Gradually add weights while students record how much the bridge supports.
3. After testing, students compare results and discuss which structural features increased strength.

Assessment

• Accuracy of scale drawing
• Use of measurements and estimates
• Participation during construction
• Bridge performance during testing
• Explanation of design decisions

Differentiation

Provide a sample bridge diagram for students who need support with planning. Allow advanced students to calculate strength-to-weight ratios or redesign their bridge after testing. Students who struggle with writing may explain their design orally.

Extension Ideas

Students can redesign their bridge to improve strength using fewer materials. Another option is comparing their design to a famous real-world bridge and explaining similarities. Classes may also create a class chart showing the relationship between structure type and load supported.