Bridges & structures

You walk across a bridge, twenty metres of air below you, and think nothing of it. Yet an invisible play of forces is at work under your feet: your weight pushes on the material, the material pushes and pulls back, and in the end everything lands safely in the ground. Once you understand the two basic forces, compression and tension, you will see bridges, cranes and pylons with completely different eyes.

Compression and tension: the two basic forces

In every structure, one of two things happens to a part: it gets squeezed together, which is called compression, or it gets pulled apart, which is called tension. A column under a balcony is in compression. The rope of a swing is in tension. Materials have their preferences: a rope is great in tension but completely useless in compression, try pushing with a rope. Concrete withstands enormous compression but cracks quickly when pulled. Good engineers therefore build so that every material gets exactly the force it handles best.

Why triangles do not wobble

Build a square from four rods whose corners can pivot, then push against one corner: it immediately collapses into a diamond, even though no rod breaks. The angles can change without any rod changing length. That is impossible with a triangle: once the three side lengths are fixed, the shape is fixed too. To deform a triangle you would have to squash or stretch a rod, and that is exactly what rods are strong against. This is why pylons, crane arms and truss bridges all share the same pattern: lots of triangles. Such a lattice of rods is called a truss, it makes structures stiff yet light, because between the rods there is only air.

The arch: load becomes compression

The ancient Romans built bridges from stone, which is weak in tension, and yet some still stand today. Their trick was the arch: it passes the weight from above, stone by stone, diagonally down and outwards until it reaches the foundations. Each stone only gets squeezed, and compression is exactly what stone withstands superbly. In return, the arch pushes outwards strongly at its ends, so it needs massive abutments to absorb that thrust.

The suspension bridge: load becomes tension

The suspension bridge flips the principle around. The deck hangs from steel cables, and these run over tall towers called pylons. The cables are entirely in tension, which steel is perfect for. The pylons carry the load as compression straight down into the ground. Every force ends up in the material that handles it best, and that is exactly why suspension bridges span the largest distances in the world: the longest span today measures about 2 kilometres, with no support in between at all.