The impact of forces and stresses (compression, tension, shear) on natural and manufactured timbers and the techniques used to reinforce and stiffen them, including frame structures, lamination, bracing and composites.

What this dot point is asking

This is Edexcel 7.4, on the impact of forces and stresses on timber and how it can be reinforced and stiffened. Edexcel names the forces and stresses (7.4.1) and the reinforcement and stiffening techniques (7.4.2). In Section B this is examined with Explain and Evaluate questions set in a product context, often asking how to stop a timber product bending, sagging or failing. The forces here echo the tensile and compressive strength in the core timbers content.

Forces and stresses on timber

Edexcel also names the natural forces within the timber as it grows (internal stresses from growth that can cause warping once cut) and pre-stressed construction beams (engineered to carry load by building stress in before use). A loaded beam or shelf experiences all three forces at once: the top in compression, the bottom in tension, and shear within.

Reinforcing and stiffening timber

• Frame structures: triangulated frames (triangles do not distort) and adding depth (an L or T section, a front lip on a shelf) greatly increase stiffness for little extra material.
• Suitable construction processes: strong joints (mortise and tenon, dowel, housing) resist the forces at a junction better than a simple butt joint.
• Lamination: gluing several thin layers (laminates) into a thicker board or a curved shape gives strength, stability and the ability to form curves, as in laminated beams and bentwood furniture.
• Braces and tie bars: a diagonal brace stops a frame racking (leaning), and a tie bar holds two parts together against a tension force, as in a roof truss.
• Embedding composites: adding a stronger material (a steel rod, a composite strip) into the timber resists tension or bending where timber alone is too weak.