Build and test model bridges of different designs (for example, truss types made from popsicle sticks or plastic straws) by gradually applying a load until failure, then compare performance using strength-to-weight ratio. This demonstrates how design, materials, span, and joints influence a bridge’s ability to carry weight.

1. Select materials (e.g., popsicle sticks with wood glue and binder clips, or plastic straws with tape) and build at least two truss-style bridges.
2. For glued bridges, clamp joints and allow a full cure before assembly steps continue; keep both side trusses parallel and square when adding cross pieces.
3. Weigh each finished bridge to the nearest gram to enable strength-to-weight calculations.
4. Set up two equal-height supports, place a loading block at midspan, and suspend a container below the deck with rope or a hook.
5. Add weight to the container steadily (sand, water bottles, or metal weights) while observing for deflection, creaking joints, or buckling; continue until failure.
6. Record the total load at failure and identify the first point or member to fail; photograph the intact and failed bridge.
7. Compute strength-to-weight ratio as (mass of breaking load)/(mass of bridge) and compare across designs.
8. For fair tests, change only one variable at a time (design, material, span, or joint method) while keeping all others constant.

Making a Popsicle Bridge & Testing It! - StructurePlanet:

📄 The Effect of Bridge Design on Weight Bearing Capacity - Terik Daly and Andrew Olson: https://www.sciencebuddies.org/science-fair-projects/project-ideas/CE_p011/civil-engineering/the-effect-of-bridge-design-on-weight-bearing-capacity

📄 How to Test Your Model Bridge - Garrett Boon: https://garrettsbridges.com/testing/how-to-test-your-model-bridge/

• Set as a challenge for groups of students to do with limited materials.
• Keep design constant but change material (popsicle sticks vs. straws) and compare strength-to-weight.
• Keep materials constant but change bridge type (Warren vs. Howe vs. Pratt) to see which performs best.
• Build identical designs with different spans to study how length affects capacity.
• Compare joint methods (glue vs. tape vs. lap joints) on otherwise identical bridges.
• Use a top-loading method (stacked weights) and compare results to the hanging-bucket method.

• Keep feet, hands, and faces clear of the loading zone; loads can fall suddenly.
• Use sturdy, equal-height supports and stabilize the test area to prevent tipping.
• Lift weights with proper posture and avoid overreaching around the setup.

• Which bridge had the highest strength-to-weight ratio, and why? (Likely the design that best channels forces into compression/tension along straight members with strong joints.)
• Where did failure start, and what does that reveal about the load path? (Failure often begins at highly stressed joints or slender compression members that buckle.)
• How does increasing span length change capacity? (Longer spans increase bending moments and typically reduce load capacity for the same cross section.)
• Why is it important to keep supports level and load applied at midspan? (Uneven supports or off-center loading introduce asymmetry and premature failure.)
• How could you improve your best design for the next iteration? (Stronger joints, thicker or doubled members in compression, better lateral bracing, or optimized truss geometry.)