Last reviewed 30 Sept 2026 · 6 min read
The long-span problem
As the span of a girder grows, its dead load grows faster than its capacity: a deep girder mostly carries its own weight. A cable carries load in pure tension, the most efficient way to use steel. Long-span bridges therefore hang the deck from cables that run to towers (pylons) or to anchorages. The main types are cable-stayed, suspension, and the intermediate extradosed form.
| Type | Typical main span | Key idea |
|---|---|---|
| Extradosed | 100–250 m | Shallow girder + short cables from low towers |
| Cable-stayed | 200–1000 m | Straight cables from towers to the deck |
| Suspension | 500–2000 m and more | Main cables hung between towers, deck hung on vertical hangers |
Cable-stayed bridges
A cable-stayed bridge has one or more towers from which straight stay cables run down to the deck in a fan, harp or semi-harp arrangement. The cables support the deck at intervals, so the girder behaves like a continuous beam on elastic supports and can be shallow.
Components
- Tower / pylon — H, A, inverted-Y or single-mast shapes; carries the cable forces in compression and bending.
- Stay cables — parallel wire or strand bundles, individually protected against corrosion; anchors at the deck and at the tower.
- Deck / girder — steel box, composite or PSC box; carries local traffic loads between the cable anchors and resists axial compression from the horizontal cable components.
- Back stays and anchor piers — balance the side span and stabilise the tower.
Cable arrangements
| Layout | Features |
|---|---|
| Fan | All cables converge near the top of the tower; efficient use of steel but crowded anchorage |
| Harp | Parallel cables spaced along the tower; neat appearance, more bending in the tower |
| Semi-fan | Compromise |
Behaviour
Each stay is a tension-only member. Under load, the vertical component of the cable force supports the deck; the horizontal component compresses the deck. The cable stiffness is affected by its own sag, which makes it slightly non-linear; the Ernst equivalent modulus corrects the elastic modulus for sag:
= weight per unit length of cable, = horizontal projected length, = area, = tension. The longer and more lightly stressed the cable, the softer it becomes.
Cable force adjustment is a defining part of construction: the cables are stressed to a target force distribution so that the deck gets a chosen deflected shape under dead load (zero-displacement or minimum-bending-moment state).
Suspension bridges
A suspension bridge hangs the deck from two main cables draped over towers and anchored at both ends. The cables carry the load in tension with the parabolic shape
for a uniformly distributed load on span with sag (sag-to-span ratio about 1/9 to 1/12). A smaller sag means a bigger and heavier anchorage. Hangers (suspenders) transfer the deck load to the cables; the stiffening girder (truss or box) distributes concentrated loads and resists wind.
- Towers are subject to axial force from the cables' vertical components.
- Anchorages — massive blocks of concrete or rock tunnels — resist the horizontal pull ( plus the back-span component).
- Erection: first the towers, then a light catwalk, spinning the main cables strand by strand (or lifting prefabricated strands) and finally hanging the deck sections in balance to avoid deforming the cable.