Bridge Building
Five ways to cross a gap, and why the span decides.
A bridge carries load across a gap and into supports at each end. Every type is a different answer to the same problem, and span length is usually the deciding factor.
The types
| Type | How it works | Typical span |
|---|---|---|
| Beam | Deck in bending: compression on top, tension underneath | Up to ~60 m |
| Arch | Load converted to compression along the curve, pushed outward into abutments | Up to ~250 m |
| Truss | Triangles resolving load into pure tension and compression in members | Up to ~150 m |
| Cantilever | Arms anchored at one end, meeting in the middle | Up to ~500 m |
| Suspension | Deck hung from cables in tension, carried over towers to anchorages | Over 2,000 m |
Why triangles
A rectangle of pin-jointed members collapses into a parallelogram under sideways load. A triangle cannot change shape without changing the length of a side, so it is rigid. That single property is why trusses are built from triangles, and it is the most demonstrable idea in structural engineering.
Tension and compression
Compression squeezes, tension stretches. Stone and concrete take compression well and tension badly, which is why masonry bridges are arches — the arch converts the load into compression. Steel takes both, which is what made long suspension spans possible: the main cables work purely in tension.
Failure modes
Bridges fail through overload, fatigue from repeated cycles, resonance, and scour — water eroding material from around the foundations, which is a leading cause of collapse and invisible from above. The Tacoma Narrows collapse of 1940 was aeroelastic flutter, not simple resonance, and it changed how decks are shaped.