Squash, stretch and the power of triangles

Look at any bridge, goalpost or playground frame. It looks still, but inside it there is a silent tug of war. Every part is being pushed or pulled, all the time. Engineers design structures so those pushes and pulls are balanced and nothing breaks.

The four forces

Bending is two forces at once

When a plank bends under your weight, its top gets squashed and its bottom gets stretched. That is why a shelf often cracks on its underside first. Wood, steel and even paper straws are usually better at one of these jobs than the other. A straw is strong in tension but buckles quickly in compression.

Why triangles win

Push the corner of a square frame made of straws and it folds flat into a diamond. Push a triangle and it holds its shape. A triangle can only change shape if one of its sides gets longer or shorter. So engineers turn squares into triangles by adding a diagonal piece called a brace. A frame made from many triangles is called a truss.

Triangles in Aotearoa

The Auckland Harbour Bridge, opened in 1959, has a steel truss full of triangles under its highest span. On the North Island Main Trunk railway, the Makatote Viaduct stands on tall steel legs criss-crossed with diagonal bracing, so trains can cross a deep gorge in the bush. Next time you pass a power pylon, a crane or a roof frame, count the triangles.

Strong and light

Anyone can make something strong by making it solid and heavy. The clever part is making it strong and light. Triangles let engineers put material only where the forces are, so a bridge can span a river without collapsing under its own weight.