A cardboard chair design challenge turns simple classroom materials into a real engineering problem. The goal is to build a chair from corrugated cardboard that can safely hold a teenager while using limited material. Students learn that strength is not just about how much material is used, but how the material is shaped, folded, layered, and oriented.
This project matters because it connects forces, structures, testing, and redesign in a hands-on way.
Key Facts
- Weight force is W = mg, where m is mass in kilograms and g is about 9.8 m/s^2.
- Stress is σ = F/A, where F is force and A is the load-bearing area.
- A wider base lowers the chance of tipping by keeping the center of mass over the support area.
- Corrugated cardboard is strongest when the flutes are oriented to resist compression and bending in the main load direction.
- Triangular braces and folded beams increase stiffness because they reduce bending and side-to-side motion.
- Fatigue testing checks whether a structure weakens after repeated loading, such as 10 sit-stand cycles.
Vocabulary
- Compression
- Compression is a squeezing force that pushes material together and can cause cardboard legs to crush or buckle.
- Corrugation
- Corrugation is the wavy inner layer of cardboard that helps it resist bending and crushing.
- Center of mass
- The center of mass is the average location of an object's weight and affects whether a chair stays balanced.
- Prototype
- A prototype is a test version of a design built to check how well an idea works before improving it.
- Fatigue
- Fatigue is weakening caused by repeated loading and unloading, even if each load is not enough to break the material at first.
Common Mistakes to Avoid
- Using flat single-layer panels as legs, which is wrong because thin sheets bend and buckle easily under compression. Folded tubes, columns, or layered panels carry vertical loads better.
- Ignoring corrugation direction, which is wrong because cardboard strength depends on how the flutes line up with the force. Flutes should be oriented to resist the main compression and bending loads.
- Placing supports too far apart, which is wrong because long unsupported spans bend more under the same load. Shorter spans, cross braces, and closer support spacing reduce sagging.
- Testing only once with a quick sit, which is wrong because a chair may pass a single load but fail after repeated use. A fatigue test such as 10 sit-stand cycles reveals weak joints and hidden damage.
Practice Questions
- 1 A 60 kg student sits on a cardboard chair. What weight force must the chair support? Use g = 9.8 m/s^2.
- 2 A chair leg supports 150 N of force and has a contact area of 0.003 m^2. What compressive stress is on the leg?
- 3 Two chair designs use the same amount of cardboard. Design A has four narrow vertical legs with no braces, and Design B has folded triangular legs with cross supports and vertical corrugation. Explain which design should be stronger and why.