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... Read More