Extradosed bridge construction

How bridges get built · Cable-supported and arch bridges · Extradosed bridge construction

Also called: extradosed prestressing, low-pylon cable-stayed

How it is done

Build foundations and piers, then extend each pier into its short pylon by climbing formwork, casting stay anchor boxes or saddle guides to tight geometric tolerances.
Cast the pier table on brackets or falsework and fix it to the pier (or prop it) for the unbalanced cantilever stage.
Mount form travellers and cast segments in balanced cantilever as for an ordinary box girder, stressing internal cantilever tendons each cycle.
Install each stay pair once its deck anchorage segment is complete, typically every one to three segments, threading strands through the saddle or into the pylon anchorage, and stress to the stage force.
Survey deflections and stay forces each cycle and adjust casting levels; the stiff girder carries most of the load, so the profile is controlled mainly through segment levels.
Cast side-span and main-span closures and stress continuity tendons (and external tendons where specified).
Adjust stay forces after superimposed dead load, fill saddles and anchorages with corrosion protection, and fit dampers if the stays are long.

Key numbers

Main spanabout 100–275 m (economic about 120–200 m)
Pylon height above deckabout span/8 to span/12
Deck depth at the pierabout span/30 to span/40, against span/16 to span/20 for a plain cantilever box
Stay service stressup to about 0.6 GUTS where the live-load stress range is small, against about 0.45 GUTS for cable-stayed bridges
Cycle5–9 days per segment pair (balanced cantilever pace)
DeckUSD 2,500–4,000 per m² (indicative)

Where it fits

Spans 100-275 m where a full cable-stayed bridge would be too tall (airport approaches, aesthetics) and a plain cantilever too heavy; rail bridges (stiff), river crossings with height limits.

Where it does not

Spans under about 90 m (a cantilever box is cheaper), spans over 300 m (a true cable-stayed bridge is lighter).

Choosing it

Choose it over a plain cantilever box for spans of about 120–250 m: the stays allow a girder roughly half as deep, saving concrete and tendons.
Choose it over a cable-stayed bridge where pylon height is limited (airport approaches, protected views) or a stiff deck is needed (railways) and spans are under about 250–275 m.
Prefer a cable-stayed bridge beyond about 275–300 m and a plain cantilever box below about 100 m.
Use pylon saddles to simplify anchorages only with a tested saddle system; separate anchorages make stay replacement simpler.

Plant, pace and money

PlantAs for balanced cantilever plus stay equipment.

ProductivityCantilever pace: 5-9 days per segment pair.

CostDeck USD 2,500-4,000 per m2.

Risks and controls

What goes wrong

The same as balanced cantilever plus stay anchorage congestion at the saddle or anchor box in the short pylon.

Quality assurance

As for cantilever plus stay force records; saddle friction tests where saddles are used.

Origins

Christian Menn's Ganter Bridge (Switzerland, 1980), with its stays encased in concrete walls, is the forerunner; Jacques Mathivat coined the term 'extradosed' in 1988, and the Odawara Blueway Bridge (Japan, 1994) was the first built to the concept, followed by many Japanese bridges.

Examples

Odawara Blueway (the first, 1994), Sunniberg (1998), Kisogawa and Ibigawa (2001), Golden Ears (2009), Pearl Harbor Memorial in New Haven (2012–2015) and many Japanese rail bridges.

Case studies

Odawara Blueway BridgeJapan · 1994Generally regarded as the first extradosed bridge; 122 m main span with low pylons on a prestressed box girder.

Kisogawa BridgeJapan · 2001Multi-span extradosed bridge with 275 m main spans and a hybrid concrete and steel deck, among the longest extradosed spans.

Golden Ears BridgeCanada · 2009Extradosed crossing of the Fraser River with 242 m main spans and a steel-concrete composite deck.

Pearl Harbor Memorial Bridge (Q Bridge), New HavenUSA · 2012–2015First extradosed bridge in the US, 157 m main span, built in two stages to keep I-95 traffic running.