Structures

In a frame joined by pins, every member either pulls (tension) or pushes (compression). Students test a roof truss, read which members pull and push, find that a taller roof means smaller forces, and choose a roof height that keeps every member within its limit and the budget.

1. Which member pulls?
2. Try: test the roof › Test it
3. Try: test the roof › The pull
4. Try: test the roof › Why 707?
5. Worked example: height changes the forces › Three heights
6. Worked example: height changes the forces › Flatter
7. Vary: raise the roof
8. Practise: forces in a roof › A wider roof
9. Practise: forces in a roof › Pull or push
10. A roof to a brief › A brief
11. A roof to a brief › Evaluate

A square frame on pins folds flat when pushed sideways; a triangle can't change shape without a member stretching. Students brace frames with diagonals, use the counting rule (members = 2 × joints − 3) to find how many they need, and brace a tower with the fewest members.

1. The square
2. Try: brace it › Test the square
3. Try: brace it › Add a diagonal
4. Try: brace it › Which diagonal?
5. Worked example: count the members › The rule
6. Worked example: count the members › Find the mistake
7. Vary: a taller tower › Two storeys
8. Vary: a taller tower › Both diagonals
9. Practise: rigid or not? › A crane arm
10. Practise: rigid or not? › Rigid?
11. Practise: rigid or not? › A gate
12. Brace a tower › Three storeys
13. Brace a tower › Evaluate

Long members buckle under a push long before they snap under a pull, so good bridges keep pushed members short. Students find a deck on pins collapses, a big triangle buckles, then design a bridge that holds 1000 N within a budget and test it to failure.

1. Just a deck
2. Try: deck, then a big triangle › The deck
3. Try: deck, then a big triangle › A big triangle
4. Try: deck, then a big triangle › Why A-G?
5. Worked example: short pushes, long pulls › Buckling
6. Worked example: short pushes, long pulls › Shorter
7. Vary: a bridge that holds
8. Practise: bridges › Buckling
9. Practise: bridges › Safety
10. Practise: bridges › Cost
11. The cheapest safe bridge › The cheapest safe bridge
12. The cheapest safe bridge › Evaluate

A beam's supports share its load like a lever, and its stiffness depends on its shape far more than on how much material it has: depth counts cubed. Students compare a strip flat and on its edge, an I-beam and a tube, work out support forces, and choose a section to a brief.

1. Flat or on edge?
2. Try: four shapes › On its edge
3. Try: four shapes › How many times?
4. Try: four shapes › I-beam and tube
5. Worked example: supports and stiffness › Two rules
6. Worked example: supports and stiffness › Deeper
7. Vary: move the load, choose the shape › Shares
8. Vary: move the load, choose the shape › Least wood
9. Practise: beams › Shares
10. Practise: beams › Deeper
11. Practise: beams › I-beams
12. A shelf › A shelf
13. A shelf › Evaluate