Structures

A structure is anything that holds its shape and supports a load, from a bridge or a tower to an egg or a bird’s nest. Engineers make structures strong, stable and light by choosing the right shape as well as the right material.

Key facts

  • Frame structures are skeletons of parts joined together (pylons, bicycle frames, roof trusses). Shell structures are hollow outer layers (eggs, cans, helmets). Solid structures are built up from solid material (walls, dams).
  • Compression squashes, tension stretches and torsion twists. A strut is a part in compression; a tie is a part in tension.
  • A triangle is rigid: it cannot change shape unless a side bends or breaks. A square frame can be pushed into a slanted shape.
  • Adding a diagonal strut turns a square into triangles (triangulation). Gussets (corner plates) make joints stronger.
  • Folding (corrugating), rolling into tubes and making arches all make thin materials much stronger.
  • A structure is stable when it has a wide base and its heavy parts are low down.

Worked examples

Example 1

Why does a wooden gate have a diagonal bar across it?

  1. Without the bar, the corners of the square can turn, so the frame leans over into a slanted shape and the gate sags.
  2. The diagonal bar splits the square into two triangles.
  3. A triangle cannot change shape without a side changing length, so the gate stays rigid.

Answer: it turns the square frame into triangles, which cannot change shape, so the gate does not sag

Example 2

A load presses down on the top of this triangle. Which sides are struts and which is a tie?

  1. The load pushes the top corner down, so the two sloping sides are squashed: they are struts, in compression.
  2. The sloping sides push the bottom corners outwards, so the bottom side is stretched: it is a tie, in tension.

Answer: the two sloping sides are struts (in compression); the bottom side is a tie (in tension)

Example 3

Sort these into frame, shell and solid structures: an egg, an electricity pylon, a brick wall, a drinks can, a climbing frame, a dam.

  1. A skeleton of joined bars: the pylon and the climbing frame are frame structures.
  2. A thin, hollow outer layer: the egg and the drinks can are shell structures.
  3. Built up from solid material: the brick wall and the dam are solid structures.

Answer: frame: the pylon and the climbing frame; shell: the egg and the drinks can; solid: the brick wall and the dam

Example 4

A sheet of paper resting on two books holds 5 g. Folded into a zigzag, it holds 60 g. How many times as much is that, and why?

  1. Divide the new load by the old load. 60 ÷ 5 = 12
  2. Each fold stands up like a little wall, so the paper is much harder to bend.
  3. Corrugated cardboard and metal roofs use the same idea.

Answer: 12 times as much; the folds make the paper much stiffer, so it does not bend