Arch Bridges: Shape and Strength

Overview

In this second lesson of the six-part unit Bridges: Forces, Forms, Futures, students investigate how an arch transfers forces into its supports. They examine examples from Aotearoa and overseas, then design, build and test a small arch bridge.

Learning intentions

• WALT identify the main parts of an arch bridge.
• WALT explain how an arch transfers load towards its supports.
• WALT investigate how shape and materials affect bridge strength.
• WALT compare bridge designs for different places and purposes.

Success criteria

• I can label the arch, deck, piers and abutments.
• I can use arrows or words to show where forces travel.
• I can build and test an arch bridge safely and fairly.
• I can use evidence from testing to explain which design worked best.

Curriculum links

• Science: Physical world—investigating forces, movement, stability and how structures respond to loads.
• Science: Nature of science—gathering evidence through practical investigations, making fair comparisons and communicating explanations.
• Technology connections: understanding how materials, design features and structure help meet a need.
• Key competencies: thinking; managing self; participating and contributing; using language, symbols and texts.

Lesson structure (45 minutes)

1. 0–5 minutes – Hook and prior learning Open with the arch bridge introduction and hook showing an arch bridge carrying a heavy load. Ask: “Why might a curved shape hold more weight than a flat piece of card?” Briefly revisit that a force is a push or pull. Invite students to predict where the force travels.

2. 5–12 minutes – Observe and label bridge parts Use the Aotearoa and overseas bridge examples to compare an arch bridge in Aotearoa with one from another country. Discuss location, materials, likely cost, size and purpose. Students complete the labelled diagram on the arch bridge investigation sheet, identifying the arch, deck, piers and abutments. Clarify that piers are upright supports and abutments are the supports at the ends of a bridge.

3. 12–18 minutes – Model force transfer Demonstrate with a strip of card: hold it flat between two books and gently place a small object on it. Then curve the card into an arch, keeping its ends against the books, and repeat. Explain that the load pushes down, while the arch redirects much of the force sideways and down into the supports. Students use arrows or labels to show “load down” and “force into supports” on their worksheet. Avoid suggesting that the arch removes forces; it transfers them.

4. 18–32 minutes – Team build and test Organise 22 students into four or five mixed-ability teams. Give each team blocks or books for supports, cardboard or folded newspaper for an arch and deck, and a small set of identical test weights such as counters or books. Teams build an arch bridge spanning a set gap, then predict how many weights it will hold. They test one weight at a time, record the result on the arch bridge investigation sheet, and observe safety rules: keep faces and fingers clear, add weights slowly, and stop if the structure becomes unstable. Teams may use the Forces Arrow Diagram Cards to help discuss and show force directions.

5. 32–39 minutes – Improve and compare designs Teams identify one feature that helped or weakened their bridge, such as the curve, support position, material thickness or deck placement. Allow one quick redesign and retest, changing only one feature where possible. Ask: “What evidence shows your redesign was stronger?” and “Would this bridge suit a river, a road, an earthquake-prone area or a temporary crossing?”

6. 39–45 minutes – Share and reflect Invite each team to share its strongest design choice and test result. Return to the opening question and ask students to explain, using the words load, arch, supports and force. Students complete the final reflection on the arch bridge investigation sheet: “An arch can strengthen a bridge because…” Collect worksheets as students leave.

Resources

• the arch bridge introduction and hook covering the hook, bridge examples, labelled parts, force-transfer model, build instructions, discussion prompts and plenary.
• the arch bridge investigation sheet with a labelled bridge diagram, prediction, results table, force arrows and reflection.
• Blocks, wooden cubes or books for piers and abutments.
• Cardboard strips, paper or newspaper.
• Identical test weights, counters or small books.
• Rulers or tape measures for setting a consistent span.
• Pencils, coloured pencils and clipboards if working on the floor.
• the Forces Arrow Diagram Cards for force-direction discussion.
• Clear floor or table space; a camera or tablet for recording designs if available.

Assessment

• Listen for accurate use of load, force, arch, pier and abutment during discussion and team building.
• Check worksheets for correctly labelled parts, a sensible force explanation, a recorded test result and evidence-based reflection.
• Observe whether students make a fair comparison, follow safety instructions and use test evidence to justify a design decision.

Differentiation

• Provide a partially labelled diagram, a word bank and sentence frames such as “The load travels…” and “Our evidence shows…”.
• Pair students strategically and assign roles: materials manager, builder, tester, recorder and reporter.
• Offer pre-folded card or a simple arch template for students needing fine-motor or planning support; allow oral explanations or photographs instead of extended writing.
• Extend confident students by asking them to compare two materials, control the span length or explain why strong sideways forces require substantial abutments.