Forces, Stresses and Reinforcement in Timber

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

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?

A timber shelf is loaded with heavy books and bows downward in the middle. Reading the description of the bent shelf, tap the part that is being STRETCHED (in tension).

• The loaded shelf bends into a shallow curve.
• Along its top surface the fibres are pushed together.
• Through its middle layer the fibres are barely stressed.
• Along its bottom surface the fibres are stretched apart.

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

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

Put the steps of making a laminated (glulam) beam in the right order.

• Cut and prepare the thin timber layers (laminates)
• Spread adhesive between the layers
• Stack the layers, often over a curved former to add a camber
• Clamp under pressure and leave the glue to cure
• Machine and finish the cured beam to its final size

Fix the structure

Three timber structures each have a weakness. Choose the best way to reinforce each one.

• A tall timber shelving unit leans (racks) sideways when it is pushed. What is the best fix?
• A long floor joist sags in the middle under the weight of the room. What is the strongest response?
• In a triangular roof truss the two sloping rafters are pushing the tops of the walls outward. What best resists that?

Compression and tension

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 _____.

Stop the bench sagging

A designer is making a long timber garden bench. When someone sits in the middle it sags, and the whole frame wobbles side to side. Recommend how to reinforce the bench and justify your choices.

• Name the stress acting on the seat rail when someone sits in the middle (which surface is in tension?)
• Recommend a method to stop the sag (for example lamination or a deeper section) and say why it beats just using more nails
• Recommend a method to stop the side-to-side wobble (for example a diagonal brace) and explain how it works
• Mention one timber-specific point: how the wood's natural growth/drying forces might affect the finished bench