What does extradosed mean?

The extrados is the upper face of an arch or beam. In an extradosed bridge the prestressing cables run above the deck instead of inside it, carried over short towers, so they act as external prestressing raised above the girder.

Is an extradosed bridge a cable-stayed bridge?

It looks like one but works differently. The tower is about half as tall, the cables are flatter, and the stiff girder carries most of the traffic load, so the cables behave like prestressing tendons with a small stress range rather than like stays.

What span suits an extradosed bridge?

Main spans of roughly 100 m to 250 m. Shorter spans are cheaper as girder bridges and longer spans usually favour a full cable-stayed bridge.

Which was the first extradosed bridge?

The Odawara Blueway Bridge in Japan, completed in 1994 with a 122 m main span, is generally cited as the first. The Ganter Bridge in Switzerland (1980) is regarded as its forerunner.

How tall is the tower of an extradosed bridge?

Typically about one tenth of the main span above the deck, in a range of roughly 1/7 to 1/13, against about 1/5 for a cable-stayed bridge.

Bridge topics · Bridge types

Extradosed Bridge: What It Is, How It Works and When to Use It

A prestressed girder with its tendons lifted over short towers: stiffer than a cable-stayed bridge, shallower than a box girder, and at its best for spans of about 100 m to 250 m.

What an extradosed bridge is

An extradosed bridge is a prestressed concrete girder bridge whose prestressing tendons have been lifted out of the deck and carried over short towers. The cables look like stay cables, but they behave much more like external prestressing: they push the girder up and squeeze it horizontally, while the girder itself stays stiff enough to carry most of the traffic load in bending. The name comes from the extrados, the upper face of an arch or beam. Ordinary tendons sit inside the section, below the extrados. Extradosed tendons sit above it.

The type sits between two familiar families. A box girder built by balanced cantilever carries every load through the depth of its section, which becomes very deep at the piers as spans grow. A cable-stayed bridge does the opposite: a tall pylon and steep stays carry almost all the load, and the deck can be slender. The extradosed bridge takes a little of each. Short towers and shallow cables relieve the girder, so it can be roughly half as deep as a girder bridge of the same span, yet the deck still does a large share of the work, so the cables see small stress changes and can be detailed like prestressing tendons.

Same main span, two systems. The extradosed tower is about half as tall and its cables are flatter, so the girder stays deeper and carries most of the traffic load itself.

How it differs from a cable-stayed bridge

The differences are proportions, and they follow from one another.

Feature	Cable-stayed	Extradosed	Girder (balanced cantilever)
Tower height above deck / main span	about 1/5	about 1/8 to 1/13, typically 1/10	no tower
Cable angle to the deck	steeper, typically 25° or more	flat, around 15° to 20°	tendons inside the section
Share of traffic load carried by the cables	80% or more	roughly 20% to 60%	none
Live-load stress range in the cables	100 MPa or more	around 50 MPa	small
Girder span / depth at the support	shallow deck, depth set by local bending	about 25 to 35, typically 30	about 16 to 20
Typical main spans	200 m to over 1,000 m	about 100 m to 250 m	up to about 250 m, economic below 150 m

Because the cables are flat, their vertical component is modest and their horizontal component is large. The deck therefore receives a strong axial compression from the cables, exactly what a prestressing tendon would provide. Because the girder is stiff, traffic load deflects the deck only a little, and the cable force changes only a little. That small stress range is the key to the whole type: fatigue does not govern the cables, so codes and guidelines let designers use a higher working stress than for true stay cables, closer to the limits used for prestressing tendons. Fewer and smaller cables follow.

The flip side is that the cables cannot be tuned to carry the deck the way stays are. Cable-stayed bridges are built by adjusting stay forces until the deck geometry and bending moments are right. In an extradosed bridge the girder takes the permanent load mostly in bending and prestress, and the cables are sized like tendons, so there is far less stay-force adjustment during construction.

Span range and proportions

Most extradosed bridges have main spans between about 100 m and 250 m. Below that range a constant or haunched girder is cheaper. Above it, the girder would have to become deep and heavy again, and a full cable-stayed solution with a taller pylon usually wins. Published reviews of built examples give these working proportions for a first sketch:

tower height above deck: main span / 10 (range about /7 to /13);
girder depth at the tower: span / 30 (range /25 to /35), often haunched to about span / 50 at midspan;
side span: around 0.55 to 0.6 of the main span; much shorter side spans cause uplift at the anchor piers;
cables anchored in a saddle at the tower and fanned or harped along the side and main spans, leaving a cable-free zone near midspan and near the piers.

Compared with a balanced cantilever girder of the same span, the deeper girder is replaced by a shallower one plus a short tower and a modest quantity of cable. The usual result is a lighter deck, smaller substructure loads, and lower vertical clearance demands under the bridge, which matters when shipping or traffic clearance is tight.

Built examples

Ganter Bridge, Switzerland (1980). Christian Menn's bridge on the Simplon Pass road has a 174 m main span. Its prestressed stays are encased in concrete walls above the deck, and it is widely cited as the forerunner of the type.
Odawara Blueway Bridge, Japan (1994). Main span 122 m. Generally recognised as the first bridge designed and named as an extradosed prestressed concrete bridge.
Sunniberg Bridge, Switzerland (1998). A curved 526 m viaduct near Klosters with five spans, the longest 140 m, and very short splayed towers. It shows how the type suits curved alignments where a deep girder would look heavy.
Pearl Harbor Memorial Bridge, New Haven, USA. The first extradosed bridge in the United States, with a 515 ft (157 m) main span and a girder depth of about span / 31 at the towers.

Japan and Korea have built the largest number of extradosed bridges, and the type now appears across Europe and the Americas as well, usually where a span of 120 m to 200 m is needed with a modest structure depth.

Construction

Most extradosed decks are concrete boxes built by balanced cantilever with form travellers or precast segments, exactly as a girder bridge would be. The difference is that from a certain segment onward a pair of cables is installed and stressed as the cantilever grows, replacing the top cantilever tendons that a girder bridge would need. Towers are short, so they are usually cast in a few lifts with the pier table. The cables pass through steel saddles in the tower, which keeps the tower slender and allows replacement. Corrosion protection follows stay-cable practice: individually sheathed strands inside an HDPE pipe, and dampers where the cable length and wind exposure call for them.

When to choose an extradosed bridge

Span 100 m to 250 m and a depth limit. Navigation or road clearance below, or a low approach gradient above, rules out the deep haunch of a girder bridge.
Tower height is restricted. Airports, heritage settings and approach paths often limit how high a pylon can rise. Extradosed towers are roughly half the height of cable-stayed pylons.
A contractor who knows balanced cantilever. The method, equipment and most of the details are those of a cantilever girder bridge, which lowers construction risk compared with a full cable-stayed bridge.
Visual lightness. On curved or multi-span viaducts the low cable planes give rhythm without the dominance of tall pylons.

It is the wrong choice when the span grows past about 250 m, when the deck must be very slender (the girder still needs real depth), or when the cables would be exposed to large live-load cycles, for example a light deck carrying heavy rail traffic, in which case the design drifts toward a true cable-stayed bridge anyway.

MethodHow extradosed bridges are built ToolFCM size ToolSpan recommender ToolStay-cable catenary BookTypes of bridges MethodStay cable installation TopicSelf-anchored suspension bridges

Questions engineers ask

What does extradosed mean?: The extrados is the upper face of an arch or beam. In an extradosed bridge the prestressing cables run above the deck instead of inside it, carried over short towers, so they act as external prestressing raised above the girder.
Is an extradosed bridge a cable-stayed bridge?: It looks like one but works differently. The tower is about half as tall, the cables are flatter, and the stiff girder carries most of the traffic load, so the cables behave like prestressing tendons with a small stress range rather than like stays.
What span suits an extradosed bridge?: Main spans of roughly 100 m to 250 m. Shorter spans are cheaper as girder bridges and longer spans usually favour a full cable-stayed bridge.
Which was the first extradosed bridge?: The Odawara Blueway Bridge in Japan, completed in 1994 with a 122 m main span, is generally cited as the first. The Ganter Bridge in Switzerland (1980) is regarded as its forerunner.
How tall is the tower of an extradosed bridge?: Typically about one tenth of the main span above the deck, in a range of roughly 1/7 to 1/13, against about 1/5 for a cable-stayed bridge.

Sources

Updated 2026-10-25 · written by a practising bridge engineer

Extradosed bridges