Structures and forces

Bridges, chairs, phone stands and egg shells are all structures. Good structures carry loads without bending or breaking.

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Forces on structures

Five types of force act on structures:
ForceWhat it doesExample
TensionPulls or stretchesA rope in a tug of war
CompressionPushes or squashesThe legs of a chair when you sit
BendingMakes a part bend or curveA shelf with heavy books
TorsionTwistsWringing out a cloth, turning a key
ShearTwo forces push in opposite directions across a part, trying to slide it apartScissors cutting paper

Types of structure

Frame structures are made from parts joined together, like a skeleton: a bicycle, an electricity pylon, a climbing frame.
Shell structures are hollow, with a thin outer layer carrying the load: an egg, a drinks can, a car body.
Solid (mass) structures rely on their weight and bulk: a dam, a stone wall.
A static load does not move (a building's own weight). A dynamic load moves or changes (people walking over a bridge).

Making structures stronger

Triangulation: a triangle cannot change shape unless one side changes length. A square frame can be pushed into a parallelogram, but adding a diagonal brace turns it into two rigid triangles.
In a frame, a part in compression is a strut; a part in tension is a tie.
Gussets (small plates) strengthen joints. Folding, ribs and curved shapes (such as arches) also add strength. Corrugating card or sheet metal makes it stiffer.
Worked example

A square shelf bracket wobbles sideways. How could it be made rigid?

  1. A square frame can be pushed out of shape into a parallelogram.
  2. Adding a diagonal brace splits it into two triangles.
  3. Triangles cannot change shape without a side changing length, so the frame becomes rigid (triangulation).

Answer: Add a diagonal brace to triangulate it; triangles are rigid.

Key idea

Forces: tension (pull), compression (squash), bending, torsion (twist) and shear. Structures can be frame, shell or solid. Triangulation makes frames rigid; struts are in compression and ties are in tension.

Key words
Tension
A force that pulls or stretches.
Compression
A force that pushes or squashes.
Torsion
A twisting force.
Shear
Forces acting in opposite directions across a part, trying to slide it apart, as when scissors cut.
Triangulation
Using triangles to make a frame rigid.
Strut
A part of a frame structure that is in compression.

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Common mistakes
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Extra practice

2 more questions

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  1. A guitar string being pulled tight is under...

    1. Tension
    2. Compression
    3. Torsion
    4. Shear
    Show the answer

    Answer: Tension

    Pulling is tension.

  2. Which force squashes a material?

    1. Compression
    2. Tension
    3. Torsion
    4. Shear
    Show the answer

    Answer: Compression

    Compression pushes together.

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Sources and references

Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.