How do beam, arch, truss, cantilever and suspension bridges each carry load?

A bridge has to do one job: get weight across a gap. The five classic designs differ in how they pass that weight along, and in whether each part is being squeezed (compression) or stretched (tension). A beam bridge is the simplest: a span resting on a support at each end, like a plank over a stream; under load it sags slightly, so its top is squeezed and its bottom stretched. An arch is a curved bridge under compression, and it pushes outward at its feet. A truss is a frame of triangles, with the top bars usually squeezed and the bottom bars usually stretched. A cantilever reaches out from a support with nothing underneath. A suspension bridge hangs its deck from cables, and any load on the deck turns into tension in those cables.

The very first beam bridges were probably copied from nature. Prehistoric people saw fallen trees lying across streams and did the same on purpose. Modern beam bridges are built of steel or reinforced concrete, but the idea is the same.

Why so many designs? Because each one suits a different gap and a different material. A beam gets weaker the further apart its supports are, so single beam spans rarely exceed about 80 metres. An arch needs strong abutments at each end to resist its outward push, but it lets weak-in-tension materials like stone do the work by keeping them squeezed. Suspension bridges turn the tables: their steel cables are stretched, and that is what lets bundled wires reach spans of about 2 kilometres.

So when you look at a bridge, ask the same question of every part: is it being pushed or pulled? The answer tells you why it has the shape it does.