|This article needs additional citations for verification. (July 2007)|
|Ancestor||Box girder bridge, Cable-stayed bridge|
|Carries||Pedestrians, vehicles, light rail, heavy rail|
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, 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 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 three bridges in the United States use the extradosed moniker, the Pearl Harbor Memorial Bridge in Connecticut, which is currently under construction, and two Interstate 35 frontage road bridges over Brazos River in Waco, Texas, completed in 2014. 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 is 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.
Extradosed bridges by country
The Triplets are the first three extradosed concrete bridges in Bolivia. The construction of a beltway allowing traffic decongestion in the city of La Paz, Bolivia, has just been completed (2010). The new elevated road crosses three parallel valleys with signature bridges. These three consecutive bridges have similar features and as a result are called the Triplets. All three-span structures are made of concrete, with maximum span of 372 ft, featuring extradosed pre-stressed concrete. The structures are built using balanced cantilever segmental construction. The elevation of the three bridges reaches heights between 130 and 197 ft above the bottom of the valley, which made a cable-stayed bridge option, with pylons higher than 82 ft above the deck, inappropriate for the site. Therefore, it was proposed an extradosed bridge type, which reduces the height of the pylon, and a single plane of stays to allow a more transparent view. Bridges were designed by PEDELTA.
The North Arm Bridge, used by the Canada Line connecting Vancouver with its suburb of Richmond over the Fraser River has been billed as North America's first extradosed bridge. The Canada Line went into service on August 17, 2009.
The Golden Ears Bridge, also crossing the Fraser but between the municipalities of Pitt Meadows / Maple Ridge and Langley, is the longest extradosed bridge in North America. The bridge opened to traffic on June 16, 2009.
India's first extradosed bridge has been built by Gammon India Ltd. and the Delhi Metro Rail Corporation for the New Delhi Mass Rapid Transit System between Pragati Maidan and Indraprastha over Indian Railways tracks. This bridge is 196.3 metres long, with the main span over the railway lines 93 meters long. 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. This bridge is being designed and managed by French consultant, Systra. Prior to this the first such bridge was constructed in Japan.
The Southern Bridge over the Daugava River in Riga, Latvia is presently the biggest construction project in Latvia and its capital city, Riga. In terms of work volume it can only be compared to the Island Bridge that was built in the seventies. Work on the development of the Southern Bridge project started in 2002, when the City Development Department of the Riga City Council developed the design task for the route of the Southern Bridge, which would connect Vienības Anenue on the left bank of the Daugava River and the Slāvu Roundabout on the right bank of the Daugava River. The Southern Bridge over represents a multispan structure of 49.5 + 77 + 5 × 110 + 77 + 49.5 metres known as extradosed system with 6 traffic lanes. The total length of the bridge – 803 metres. The width of the bridge – 34.28 metres. The number of pylons – six, each at a height 13.33 metres above the roadway pavement. Each pylon has 8 pairs of cables.
The largest extradosed bridge in Europe, one of the largest in the world, is completed in July 2013 and is planned to open to traffic on 26 July  over Vistula River in Kwidzyn, Poland. Total length of the bridge is 808,5m with the main span of 204m. The bridge has been commissioned by General Directorate for National Roads and Highways,designed by Transprojekt Gdanski and is built by Budimex and Ferrovial Agroman at a cost of 90 million EUR. In addition to the bridge three more flyovers with total length of 1028,4m and two bridges over the rivers Młyńska Struga and Liwa will be formed. Parts of bridge: - The bridge over the Vistula River in length L = 808 m, range spans 79+130+2x204 + 130+70 m - El part L = 144.4 m, breadth spans 36+36++36+35,4 m - E2 part L = 479.6 m, breadth spans 59,4+6x59,4+59,3 m - E3 part L = 409.5 m, breadth spans 59,3+5x60+49,3 m above objects are construction of prestressed concrete box- - 3-span bridge over the river Liwa length 51 m - span 15,7 + 19,6+15,7 m - bridge over the Młynówka river; - 3 passes for animals spans the 14,3 m; - 1-local traffic viaduct - span of 30 m;
The bridge was opened on July 26, 2013.
Sri Lanka's Road Development Authority is planning to construct an extra-dosed bridge over Kelani River as part of a project aimed at improving traffic condition around existing New Kelani Bridge. The construction will commence in 2017.
The first extradosed bridge in the U.S. is the northbound span of the Pearl Harbor Memorial Bridge that carries Interstate 95 (Connecticut Turnpike) over the Quinnipiac River in New Haven, Connecticut, which opened to traffic on June 22, 2012. The southbound span is planned to open in 2014. Another extradosed bridge is planned to cross the St. Croix River between Oak Park Heights, Minnesota and St. Joseph, Wisconsin and be completed by 2016.
The second and third extradosed bridges in the U.S. are Interstate 35 frontage road spans over the Brazos River in Waco, Texas. Construction began in July 2012 and the bridges were dedicated in July 2014. They are the first extradosed bridges to be constructed in Texas.
- Benaim, Robert (2007). The Design of Prestressed Concrete Bridges (Google books (Preview)). Routledge. p. 519. ISBN 978-0-415-23599-0. Retrieved April 21, 2009. "Extradosed bridges are transitional between girders and cable-stayed bridges."
- "Protected Status". Bridge Design & Engineering (51): 53. Second Quarter 2008. ISSN 1359-7493.
- J.B. Smith (July 3, 2014). "New signature Brazos bridge design could be model for others nationwide". Waco Tribune-Herald. Retrieved July 3, 2014.