Forces, Stresses and Reinforcement in Timber
Timber has to resist being squashed, stretched and sheared - and it carries its own hidden forces from the day the tree grew. Learn the three stresses, what makes timber special, and how lamination, bracing and tie bars stiffen a structure.
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The forces inside a timber structure 🪵
A wooden shelf, a chair leg, a roof beam - every piece of timber in use is being **pushed, pulled or twisted** by the loads on it. If it cannot resist those forces, it bends, cracks or breaks. There are **three main stresses** a material has to cope with: **compression, tension and shear**. Timber also has a twist of its own - forces locked inside it from when the tree grew. This module covers both, and how designers **stiffen** timber to make it stronger.
Three stresses to know 🔑
These three act on every material, timber included. Learn what each one does - the next steps ask you to spot them.
Match each stress to what it does 🔗
- Compression
- Tension
- Shear
- Pushes the material together and tries to squash it
- Pulls the material apart and tries to stretch it
- Makes one part slide across another in opposite directions
Which stress is this? 🔍
A heavy person sits on a wooden stool. The forces in the vertical legs are mainly...
- Compression - the legs are being squashed downwards
- Tension - the legs are being stretched
- Shear - the legs are sliding apart
- No force at all
What makes timber special ⚖️
Compression, tension and shear act on every material. But timber carries **extra forces of its own** that metals and plastics do not - and knowing the difference is what separates a top answer.
Timber's hidden force 🌳
Why can a plank of timber warp or split even before any load is put on it?
- Natural forces built up as the tree grew, released as the wood dries unevenly
- Shear from the loads sitting on it
- Compression from the air pressure around it
- It cannot - unloaded timber never moves
Where is the tension? 📍
An interactive activity.
Stiffening timber: the toolkit 🛠️
Timber can be made stronger and stiffer without simply using a bigger, heavier piece. The main methods are: - **Lamination** - bonding thin layers together (plywood, glulam), often with the grain crossing, so the piece resists bending and warping. - **Bracing** - adding a **diagonal** to a frame so it cannot lean over (triangulation).\n- **Tie bars** - a member in **tension** that stops two parts being pulled apart.\n- **Frame structures** and good **fabrication/assembly** (strong joints), plus **embedding composite materials** for extra stiffness.
Match each method to how it helps 🧩
- Lamination
- Diagonal brace
- Tie bar
- Frame structure
- Bonds thin layers so the piece resists bending and warping
- Triangulates a frame so it cannot lean or rack sideways
- Sits in tension to stop two parts being pulled apart
- Forms a rigid skeleton that carries loads efficiently
Spot the reinforcement ✅
Select the TWO methods that would genuinely stiffen or reinforce a timber structure.
- Laminating thin layers into a glued beam
- Adding a diagonal brace across a rectangular frame
- Sanding the outer surface smooth
- Painting the timber a darker colour
Build a laminated beam 🪜
An interactive activity.
Fix the structure 🧭
An interactive activity.
The summary 📝
A force that squashes timber is _____, and one that stretches it is _____. Timber also carries natural forces from when the tree _____, which can warp it before any load is added. Bonding thin layers to resist bending is called _____, and a member in tension that stops parts being pulled apart is a _____.
Your turn ✍️
An interactive activity.