An extradosed bridge employs a structure which combines the main elements of both a prestressed box girder bridge and a cable-stayed bridge. 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.
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. 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.
The extradosal bridge form is mostly suited to medium- length spans between 100 metres (330 ft) and 250 metres (820 ft), and over fifty such bridges had been constructed around the world to 2012. 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. They have frequently been adopted when overall height, navigation clearance, or aesthetic requirements have made the cable-stayed or girder alternatives less feasible.
