Extradosed bridge

Extradosed bridge

Diagram of an extradosed bridge
Ancestor	Box girder bridge, cable-stayed bridge
Related	Low-tower cable-stayed
Descendant	none
Carries	Pedestrians, vehicles, light rail, heavy rail
Span range	Medium
Material	Concrete
Movable	No
Design effort	High
Falsework required	no

An extradosed bridge employs a structure that combines the main elements of both a prestressed box girder bridge and a cable-stayed bridge.^{[1][2]: 85 [3]} The name comes from the word extrados, the exterior or upper curve of an arch, and refers to how the "stay cables" on an extradosed bridge are not considered as such in the design, but are instead treated as external prestressing tendons deviating upward from the deck. In this concept, they remain part of (and define the upper limit of) the main bridge superstructure.^[4]: 1

Compared to a cable-stayed or cantilever-girder bridge of comparable span, an extradosed bridge uses much shorter stay-towers or pylons than the cable-stayed bridge, and a significantly shallower deck/girder structure than used on the girder bridge.^{[2]: 85–86} This arrangement results in the typical extradosed "look" of a fan of low, shallow-angle stay cables, usually with a pronounced "open window" region extending from the sides of each tower.^[4]

The extradosed bridge form is mostly suited to medium-length spans between 100 m (330 ft) and 250 m (820 ft), and over fifty such bridges had been constructed around the world as of 2012.^{[4]: 16–26} Whilst incurring many of the construction costs of both the cable-stayed and girder bridge types, extradosed bridges can deliver material savings to offset much of this penalty.^{[1]: 387–388} They have frequently been adopted when overall height, navigation clearance, or aesthetic requirements have made the cable-stayed or girder alternatives less feasible.^{[4]: 15, 135–136}