Forces, stresses and structural reinforcement
Design & Technology · GCSE · The School
Five forces act on structures, and each has a name
Tension stretches a material apart. Compression squeezes it together. Bending curves it, putting one side in tension and the other in compression at once. Shear slides one layer past another. Torsion twists it along its length. Naming which force is acting is the first step in choosing how to resist it.
Triangulation resists bending in frame structures
A rectangular frame collapses sideways easily under a sideways push, because its joints can flex. Adding a diagonal brace turns the rectangle into two triangles, and a triangle cannot change shape without one of its sides changing length. This is why bridges, pylons and roof trusses are built from triangles rather than rectangles.
A worked example: a wobbly shelf bracket
An L-shaped shelf bracket bends downward under load because the joint between its two arms can flex. Adding a diagonal strut between the two arms — closing the L into a triangle — resists that bending directly, because the strut is now in compression, and a strut resists compression far better than an unbraced joint resists bending.
Webbing and ribbing reinforce a flat panel
A flat sheet bends easily under load because it is thin in one direction. Adding a rib — a raised strip of the same material running across it — increases its resistance to bending in that direction without adding much extra mass, because the rib moves material further from the panel's centre, where it resists bending most effectively.
Lamination builds strength from thin layers
Laminating glues several thin layers together, often with their grain or fibres running in different directions, so a weakness along one layer's natural grain is compensated by the next layer running a different way. Laminated timber beams and plywood are both stronger and more resistant to warping than a single piece of the same total thickness.
Matching the reinforcement to the actual force
A structure needing to resist bending is triangulated or ribbed. A structure needing to resist splitting along its grain is laminated. Choosing a reinforcement method WITHOUT first naming which force is the problem is choosing blind — the technique only makes sense once the force it counters is identified.