Extradosed Bridge

An extradosed bridge employs a structure that is frequently described as a cross between a girder bridge and a cable-stayed bridge. The name comes from the French word extradossé, which is derived from the word extrados. Extrados is defined as the exterior curve of an arch.

This description is somewhat deceptive, since many cable-stayed bridges have some sort of box-girder deck. The difference is one of degrees.

A typical cable-stay bridge has a tower with a height above the deck at least half the span to the next support, since the cables are the vertical support and must come at a relatively high angle.

In an extradosed bridge, the deck is directly supported by resting on part of the tower, so that in close proximity to the tower the deck can act as a continuous beam. The cables from a lower tower intersect with the deck only further out, and at a lower angle, so that their tension acts more to compress the bridge deck horizontally than to support it vertically. Thus the cable stays act as prestressing cables for a concrete deck, whether made with I-beam girders or a box girder. The deck of an extradosed bridge can be thinner than that of a comparable span-beam bridge, but must be thicker than that of a conventional cable-stayed bridge of comparable span.

Extradosed bridges are relatively expensive and material inefficient. Almost any span that could be bridged by an extradosed bridge could be spanned more inexpensively with a continuous girder, or more efficiently (but at even greater cost) with a cable-stayed. In most cases the spans are short enough that the use of cables at all is an aesthetic rather than engineering-necessitated choice. This does not imply that is a "bad" choice, since in some cases the difference in cost and efficiency is small, and the extradosed type is a very elegant form.

It is debatable whether an "extradosed" type even exists; several notable designs amount to extradosed bridges, but have never been described as other than "cable-stayed". For example, Christian Menn built two notable bridges in Switzerland that fit the extradosed description: Ganter Bridge and Sunniberg Bridge. They are consistently described as "hollow box cable-stayed" or "low-tower cable-stayed". Only one bridge in the United States uses the extradosed moniker, the Pearl Harbor Memorial Bridge in Connecticut, which is currently under construction. The term appears to be more popular in East Asia and Latin America. It is still a very rare form, with Structurae listing only 36 entries, with more than half either in planning or construction rather than completed and in use.

In structural perspective, main differences between cable-stayed and extradosed bridge types are load participation ratio, which affects design aspective of cable members. Since cable-stayed bridge totally rely on their vertical load to cable members, extradosed bridge usually rely their load on only 20% to 50% to cable, and remaining portion is covered by the girder which is more stiff than the stiffening girder in a cable-stayed bridge. It's usual that the main design constraint in a cable-stayed bridge is fatigue of cable and anchorage system. In extradosed bridges, fatigue is not a big concern since live load usually creates only a small amount of stress variation in the cables because of the stiffness ratio between a cable and the girder. Cost wise, allowable stresses for cables in cable-stayed bridges are always smaller than extradosed bridge types - in may design code because of fatigue concern and types of anchor can be chosen for external tendon anchorage systme in extradosed bridge rather than cable-stayed bridge type which way more expensive. Generally, extradosed bridges do not require tension re-adjustment (tune-up) before service because its cables act as external-tendons.