Build it, test it, explain it.

Objective

Carry as much load as you can for the bridge’s own mass across a 300 mm clear span, then explain the failure in terms of tension, compression and joints.

Before you start

• No engineering coursework needed. It helps to know that members of a structure carry tension or compression; the truss on the Civil & Structural card shows where each occurs.
• Tools: scissors, a ruler, a kitchen scale that reads to 1 gram, a light container, and two supports of equal height, such as stacks of books.

Safety. Test over a tray or towel, keep the drop height low, and load with water or sand rather than heavy metal weights.

Constraints

• Materials: up to four sheets of ordinary printer paper (A4 or US Letter) and up to 1 metre of standard clear tape. No glue, card, string or other materials.
• Span: the bridge rests on two supports with a 300 mm clear gap between them. It may sit on the supports but must not be taped or clamped to them. A4 paper is 297 mm long and US Letter 279 mm, both shorter than the span, so most designs will need joints.
• Size: no more than 400 mm long and 120 mm wide.
• Loading: hang or place a light container at mid-span and fill it gradually with water, sand or coins until the bridge fails.
• Failure: the moment the bridge can no longer hold the load, because it collapses, tears or lets the container touch the surface below.

What to document

1. A photo or sketch of the design, with its main dimensions.
2. The mass of the bridge, in grams.
3. The failure load: the mass of the container and its contents at failure, in grams, and how you measured it.
4. The ratio of failure load to bridge mass.
5. Where and how the bridge failed, such as a buckled top edge, a torn joint or a peeled tape seam, with a photo if you have one.
6. One prediction you made before testing, and how the result compared.
7. What you would change in the next version.