Bridge Physics — Tension, Compression and Resonance
A bridge is a frozen force diagram: every beam, cable and arch exists to route enormous loads safely into the ground. The failures are as instructive as the successes.
Tension vs compression members
Bridge parts do one of two jobs: cables and hangers pull (tension), while arches, piers and truss top chords push (compression). Designers assign each member the job its material does best — steel loves tension, concrete loves compression.
Trusses: triangles that cannot deform
A triangle is the only polygon that cannot change shape without changing a side length, so truss bridges tile triangles to make rigid spans from slender members. Each member carries pure tension or compression — no bending — which is why trusses are so material-efficient.
Suspension bridges and the catenary
A freely hanging cable forms a catenary curve; loaded by the deck through hangers, it approaches a parabola. The main cables carry the entire deck's weight in pure tension to the anchorages — the towers merely hold the cables up.
Resonance: the Tacoma Narrows lesson
The 1940 Tacoma Narrows collapse was driven by aeroelastic flutter — wind vortices shedding at a frequency that fed the bridge's torsional oscillation until it tore itself apart. Modern decks are aerodynamically shaped and damped so wind can never again play a bridge like an instrument.
Try this yourself
- Build a paper bridge between two books and load it with coins — then fold the paper into a corrugated (wavy) shape and compare the strength.
- Pluck a ruler clamped to a desk at different lengths: the pitch rises as the free length shortens — the same frequency-length physics as bridge vibration.
- Watch footage of the Tacoma Narrows collapse and identify the torsional (twisting) mode that destroyed it.