Extradosed bridge
From Wikipedia, the free encyclopedia
| Extradosed Bridge | |
|---|---|
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| 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 is frequently described as a cross between a girder bridge and a cable-stayed bridge. This is somewhat deceptive, since with rare exception all 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 over 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 part of their tension causes longitudinal compression in the deck. Thus the cable stays also act as prestressing cables for a concrete deck, whether made of I-beam girders or a box girder. The deck of an extradosed bridge can be less deep than that of a comparable span beam bridge, but must be thicker than that of a comparable span conventional cable-stayed bridge.
Given its intermediate design, it is unsurprising that extradosed bridges are fairly expensive and material inefficient. Almost any span that could be bridged by an extradosed 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 is not to say that it 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; this is the Pearl Harbor Memorial Bridge in Connecticut, currently under construction. The term seems 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.
[edit] India's First World's Second
India's first extra-dosed bridge has been built by Delhi Metro Rail Corporation between Pragati Maidan and Indraprastha towering over Indian Railways tracks. This bridge is 196.3 metres long and the main span over the railway lines is 93 metres long and does not have any pier support. In addition, the bridge has a 302-metre radius curvature and the main span has been kept long to allow for future expansion of Indian Railways lines. Prior to this the first such bridge was constructed in Japan.
[edit] External links
- Diagrams of this bridge type and comparisons with its parent types
- List of extradosed bridges around the world (Structurae site)
- Himi Yumi bridge with good image.
