A suspension bridge does something almost paradoxical: it spans thousands of feet using structural elements that have zero bending stiffness. The main cables are just bundled steel wires — they can't resist a moment at all. Yet they carry the entire deck load. The trick is that a cable in pure tension, loaded uniformly along its horizontal span, naturally takes the shape of a parabola. (An unloaded cable hanging under its own weight forms a true catenary — cosh(x) — but once the deck load dominates, the shape is parabolic. Engineers usually design to the parabolic approximation.)

The load path is beautifully linear: deck → vertical suspenders → main cable → tower tops → anchorages. Everything above the deck is in tension. The towers see almost pure vertical compression, because the cable tensions on both sides pull them symmetrically inward and the horizontal components cancel. The anchorages — massive concrete blocks buried in bedrock — resist the horizontal pull of the cable at each end. On the Golden Gate, each anchorage weighs about 60,000 tons.

The key geometric parameter is the sag-to-span ratio (f/L). Typical values are 1/10 to 1/12. For a uniformly distributed load w across span L with mid-span sag f, the horizontal cable tension at any point is:

• H = wL² / (8f)

The maximum tension (at the tower) is H / cos(θ), where θ is the cable angle at the tower. Rule of thumb: a shallower sag means a lighter, cheaper deck but dramatically higher cable tension and anchorage forces. Halve the sag, double the cable tension. That's why designers don't just make cables taut — the anchorages would become impossibly expensive.

Worked example: A 1000 m span carries 200 kN/m of dead + live load, with sag = 100 m (f/L = 1/10). Horizontal tension H = (200 × 1000²) / (8 × 100) = 250,000 kN. That's 25,000 tons of horizontal pull the anchorage must resist — forever, on both sides.

The other subtle failure mode is aerodynamic instability. Tacoma Narrows (1940) taught engineers that a slender, torsionally flexible deck can couple with wind to produce self-exciting oscillations (flutter). Modern suspension decks use deep trusses or aerodynamically shaped box girders (Great Belt, Akashi Kaikyō) tested in wind tunnels. The main cable itself is stable; the deck is what dances.

Compare to cable-stayed bridges: stays run straight from tower to deck, putting the tower in bending as well as compression, but eliminating the anchorage. That's why cable-stayed dominates the 300–1000 m range and suspension dominates above 1000 m — anchorage cost gets amortized only over very long spans.