Cable-Stayed Bridge

How a cable-stayed bridge carries load

Each stay cable runs in a straight line from the pylon to a point on the deck. Because the cable can only take tension (steel strands have negligible bending stiffness), it pulls the deck up at its anchorage point with a vertical component, and pulls the pylon down/inward with a horizontal-and-vertical force. The pylon, in compression, transmits the combined axial load to its foundation.

                    Pylon
                      │
                  ╱╲ │ ╱╲
                ╱  ╲│╱  ╲     <- stay cables (tension)
              ╱    ╳    ╲
            ╱   ╱  │  ╲   ╲
   ────────────────●────────────  Deck (compression + bending)
                  Foundation

   Vertical load on deck → tension in stay → axial load in pylon → foundation

Crucially, a cable-stayed bridge is self-anchored. The horizontal pull of all the stays on one side of the pylon is balanced by the horizontal pull on the other side, transmitted through the deck as compression. There's no need for the giant gravity anchorages that a suspension bridge requires — making cable-stayed structures attractive on sites where bedrock anchorage is difficult or expensive (estuaries, island crossings).

Cable-stayed vs other bridge types

	Cable-stayed	Suspension	Cantilever (truss)	Arch	Beam / girder	Through-arch
Span sweet spot	200–1,100 m	500–2,000+ m	200–550 m	100–500 m	10–250 m	100–600 m
Deck loading	Compression + bending	Bending only (hangers vertical)	Bending + truss tension/comp	Compression along arch	Bending	Compression through arch
Anchorages	None (self-anchored)	Massive	Compression at piers	Thrust at abutments	Reactions at piers	Thrust at abutments
Aerodynamics	Stiffer than suspension	Most flexible (Tacoma risk)	Stiff truss	Stiff arch	Stiff	Stiff
Construction	Cantilever from pylon	Catwalk + main cable + hangers	Outward cantilever from piers	Falsework or cantilever	Falsework / launch	Falsework or cantilever
Cost / m of main span	Mid	High	Mid-high	Mid	Low	Mid-high
Iconic example	Russky (1,104 m)	Akashi-Kaikyō (1,991 m)	Quebec (549 m)	Sydney Harbour (503 m)	Most highway overpasses	New River Gorge (518 m)

Stay configurations

• Harp. All stays parallel, anchored at evenly spaced heights up the pylon. Visually clean, easier to detail at the pylon, but slightly less structurally efficient because lower stays make shallow angles with the deck. Used at Bonn-Nordbrücke and many German Rhine bridges.
• Fan. All stays anchored at a single point at the top of the pylon. Maximally efficient (steepest angles) but the cluster of anchorages at one point is hard to detail and can be a maintenance nightmare. Pure fans are now rare in long spans.
• Semi-fan. Stays anchor over a short length at the pylon top — a compromise that gives near-fan efficiency with cleaner anchorage. The dominant modern choice; Russky and Sutong both use semi-fan.
• Star (modified harp). Stays anchor at a single deck point but spread up the pylon — the visual inverse of a fan. Rare; used on aesthetic grounds, e.g. Erasmus Bridge in Rotterdam.

Worked example: deck deflection at midspan

Consider a simplified two-pylon cable-stayed bridge with a 400 m main span, semi-fan layout, and 20 stays per side per pylon. Take a uniform live load of 25 kN/m across the deck (typical highway loading plus crowd):

• Total live load on main span: 25 kN/m × 400 m = 10,000 kN
• Half carried by each pylon's stays: 5,000 kN per side
• Average vertical force per stay: 5,000 / 20 = 250 kN
• If the average stay angle to the deck is 35°, cable tension per stay: 250 / sin(35°) ≈ 436 kN

For the deck itself, modelled as a continuous beam on elastic supports (the stays act as springs of stiffness EA/L per stay), midspan deflection under uniform load is approximately:

δ_mid ≈ 5wL⁴ / (384 EI)   ×  reduction factor

where:
  w = 25 kN/m (live load)
  L = stay-to-stay spacing (20 m typical)  ← *not* the full main span!
  E = 200 GPa (steel deck), I = ~3 m⁴ (orthotropic deck)
  reduction factor ≈ 0.5–0.7 (cable spring action)

For 20 m stay spacing, intermediate-support deflection:
δ ≈ 5 × 25,000 × 20⁴ / (384 × 200×10⁹ × 3) × 0.6
  ≈ 5 × 25,000 × 160,000 / (2.3×10¹⁴) × 0.6
  ≈ 0.052 m = 52 mm between stays

The key insight: a cable-stayed bridge's deck does not deflect over the full 400 m main span — it deflects only between adjacent stay anchorages, typically 10–25 m apart. The bridge behaves like a beam on twenty closely-spaced elastic supports, not like a beam spanning 400 m. This is why cable-stayed bridges are so much stiffer than suspension bridges of similar length: a suspension bridge's hangers are vertical and only restrain the deck against gravity, while stays restrain it both vertically and against horizontal sway.

Iconic cable-stayed bridges

• Russky Bridge, Vladivostok, Russia, 2012 — 1,104 m main span. Single semi-fan configuration; A-frame pylons rise 320 m above water level. Built in record time for the APEC summit.
• Sutong Bridge, China, 2008 — 1,088 m. Held the world record for four years until Russky surpassed it.
• Stonecutters Bridge, Hong Kong, 2009 — 1,018 m, twin-mast pylons, navigation clearance for container ships.
• Tatara Bridge, Japan, 1999 — 890 m. Held the world record for nine years; aerodynamic deck profile developed in wind-tunnel tests.
• Millau Viaduct, France, 2004 — seven pylons, longest at 343 m, viaduct deck total 2,460 m. Designed by Norman Foster; the highest road bridge deck in Europe.
• Øresund Bridge, Denmark/Sweden, 2000 — 490 m main span carrying both rail and four-lane highway across an international border.
• Erasmus Bridge, Rotterdam, 1996 — 284 m, single asymmetric pylon, the "Swan" of Rotterdam; uses an unusual back-tilted star configuration.

Common failure modes

• Stay cable corrosion at anchorages. Where the cable enters the deck or pylon, water ingress past the seal causes pitting on the strands. Cathodic protection and re-greasing programs target this.
• Fatigue at anchorage forks. The steel forging that grips each cable strand sees stress reversals from traffic load; cracks initiate at the radius between socket and barrel. Periodic ultrasonic inspection finds them.
• Vortex-induced vibration of stays. Long stays in moderate cross-winds shed vortices at frequencies matching cable resonances; without dampers, amplitudes can reach ±0.5 m and fatigue the anchorage in months. Hydraulic dampers and helical fillets on the HDPE sheath are now standard.
• Pylon foundation settlement. Differential settlement between pylon and deck supports can detune the entire stay system; modern bridges include adjustable saddles or jackable bearings.
• Wind-induced deck flutter. Less catastrophic than for a suspension bridge, but a flat deck profile in high winds can develop torsional flutter. Wind-tunnel testing of deck cross-sections is mandatory.
• Sheath UV degradation. The HDPE sheath protecting the strands from corrosion degrades under sunlight; carbon-black additives and periodic recoat campaigns extend life.