What is a structure? Three types
A structure is something that keeps its shape and holds up a load (a weight or a push). Nature makes structures (trees, bones, shells, spider webs). People make them too (bridges, chairs, towers, boxes).
Solid (mass) structures
One big heavy piece. Its own weight keeps it in place. Examples: a dam, a stone wall, a brick pillar. Strong, but heavy and uses lots of material.
Frame structures
Thin bars (called members) joined at their ends. Light and uses little material. Examples: a cycle frame, a roof truss, a mobile tower, a chair.
Shell structures
A thin curved skin that carries load by its shape. Examples: an egg, a car body, a helmet, a dome, a drinks can.
Many real things mix types: a house has a concrete frame, brick (solid) walls and maybe a curved roof.
Loads and forces inside a structure
Loads come from outside. A static load does not move (the weight of the roof). A dynamic load changes or moves (wind, people walking, a passing truck).
Loads make forces inside each part. There are five kinds:
- Compression: squeezes a part shorter (a table leg).
- Tension: stretches a part longer (a rope, the cable of a lift).
- Bending: compression on one side and tension on the other (a shelf with books).
- Shear: two forces slide layers past each other (scissors cutting paper, a bolt in a joint).
- Torsion: twisting (turning a key, wringing a towel).
In the 3D, the beam's top strip turns red (compression) and the bottom turns blue (tension). Forces are measured in newtons (N). A 1 kg mass weighs about 10 N on Earth.
Why triangles make frames rigid
A square of four bars with loose joints can change shape: push the top sideways and it folds into a slanted shape. A triangle cannot do this. If its three sides keep their length, its angles are fixed. So engineers add a diagonal brace to turn a square into two triangles. This is called triangulation.
A truss is a frame made only of triangles. Bridges, cranes and roof frames are trusses. In a truss some members are in tension (called ties) and some are in compression (called struts).
Stability and strength
Stability means the structure does not tip over. Every object has a centre of gravity (CG): the point where its weight seems to act. Draw a line straight down from the CG. If the line falls inside the base, the object comes back when tilted. If it falls outside the base, the object falls.
To make something more stable:
- make the base wider (a tripod spreads its legs),
- keep the centre of gravity low (put heavy parts at the bottom),
- fix it to the ground (bolts, deep foundations, guy ropes).
Strength means it carries the load without breaking or bending too much. Strength depends on:
- material (steel is stronger than wood; wood stronger than card),
- shape of the part (a plank on its edge, an I-beam, a tube or a folded sheet is much stiffer than a flat sheet),
- joints (glued, bolted, welded or nailed; a weak joint breaks first),
- size (thicker parts carry more).
Building model structures
Use the design process: problem → ideas → sketch → build → test → improve.
- Materials: card, paper straws, lolly sticks, wooden dowels, glue gun, tape, string.
- Fold card into L, U or tube shapes to make it stiff.
- Triangulate every square panel with a diagonal or a gusset (a small triangle of card at the corner).
- Make the base wide and add weight low down.
- Test with a measured load (coins or 100 g masses) and record the load at which it fails.
- Measure efficiency: load carried ÷ mass of the structure. A lighter model that holds the same load is better.
Stay safe: cut away from your hand and let glue cool.
Try it at home
Make a square from four paper straws and pins at the corners. Push one corner: it folds. Pin in one diagonal straw: it stops folding. Then lay a sheet of paper across two books and put a coin on it: it sags. Fold the same sheet into a zig-zag (like corrugated card) and try again: it holds many coins.
Key formulas and definitions
- Weight W = m × g (g ≈ 9.8 N/kg ≈ 10 N/kg)
- Stable when the vertical line from the centre of gravity falls inside the base
- Tipping angle (box of base b, CG at height h): tan θ = (b/2) ÷ h
- Structural efficiency = load carried ÷ mass of structure
- Compression = squeeze, tension = stretch, shear = slide, torsion = twist, bending = squeeze + stretch
Worked examples
1. Name the type of structure: (a) a brick wall, (b) a cycle frame, (c) a hard hat.
(a) Solid: one heavy mass. (b) Frame: thin tubes joined in triangles. (c) Shell: a thin curved skin.
2. A shelf holds books. Which force acts on the top surface and which on the bottom?
The shelf bends. The top surface is squeezed (compression). The bottom surface is stretched (tension).
3. A model bridge has mass 50 g and holds 2.5 kg before it breaks. Find its efficiency.
Efficiency = load ÷ own mass = 2500 g ÷ 50 g = 50. It carries 50 times its own mass.
4. A tall box has a base 0.6 m wide. Its centre of gravity is 0.9 m high, in the middle. At what tilt angle will it tip?
It tips when the CG is right above the edge. tan θ = (0.6 ÷ 2) ÷ 0.9 = 0.3 ÷ 0.9 = 0.333. θ ≈ 18°.
5. The same box now has its CG lowered to 0.45 m. New tipping angle?
tan θ = 0.3 ÷ 0.45 = 0.667, so θ ≈ 34°. Lowering the CG almost doubles the safe tilt.
6. A lift cable holds a 400 kg cabin at rest. What force is in the cable and what kind is it?
W = m × g = 400 × 10 = 4000 N. The cable is stretched, so it is in tension of about 4000 N.
Common mistakes
- Thinking a square frame is rigid. Without a diagonal brace it folds; only triangles hold their shape.
- Mixing up tension and compression. Tension stretches (rope), compression squeezes (pillar).
- Thinking a heavier structure is always more stable. What matters is a low centre of gravity and a wide base.
- Judging strength by material alone. A thin sheet folded into a tube or on its edge is far stiffer than the same sheet lying flat.