Construction methods · Cable-supported and arch bridges

Extradosed bridge construction

A balanced cantilever box girder with short pylons and stiff external stays that act as prestressing more than as suspension. Built exactly like a cast-in-place cantilever with the stays added in the cycle; spans of 100-275 m with pylons only a tenth of the span high, and stays that hardly fatigue.

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 the piers with the short pylons (10-25 m) by climbing forms; cast the pier table.
Form-traveller cycle as for balanced cantilever; install a stay pair every two to three segments and stress it.
Close at midspan; the stays are adjusted lightly because their force varies little with live load.

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).

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.

Examples

Odawara Blueway (the first, 1994), Kiso and Ibi rivers, Golden Ears, Korean Seohae approaches and many rail bridges.