An extradosed bridge employs a structure that is frequently described as a cross between a girder bridge

Girder bridge

A girder bridge, in general, is a bridge built of girders placed on bridge abutments and foundation piers. In turn, a bridge deck is built on top of the girders in order to carry traffic. There are several different subtypes of girder bridges:...

 and a cable-stayed bridge

Cable-stayed bridge

A cable-stayed bridge is a bridge that consists of one or more columns , with cables supporting the bridge deck....

. The name comes from the French

French language

French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 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

Beam (structure)

A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...

. 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

Prestressed concrete

Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete...

 cables for a concrete deck, whether made with I-beam

I-beam

-beams, also known as H-beams, W-beams , rolled steel joist , or double-T are beams with an - or H-shaped cross-section. The horizontal elements of the "" are flanges, while the vertical element is the web...

 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

Christian Menn

Christian Menn is a bridge designer from Switzerland. He owned his own Engineering Company in Chur, Switzerland from 1957-1971. From 1971 until his retirement in 1992 he became a professor of Structural Engineering at ETH Zurich specializing in Bridge design...

 built two notable bridges in Switzerland that fit the extradosed description: Ganter Bridge

Ganter bridge

Ganter Bridge is a reinforced concrete road bridge which is currently the longest spanning bridge in Switzerland, located along the Simplon Pass road in the canton of Valais about south of Brig. It was designed by renowned Swiss civil engineer Christian Menn and completed in 1980...

 and Sunniberg Bridge

Sunniberg Bridge

The Sunniberg Bridge is a cable-stayed road bridge near Klosters in Switzerland. It is notable because of its innovative design and aesthetically pleasing appearance....

. 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

Pearl Harbor Memorial Bridge (Connecticut)

-Bridge plans bring controversy:The existing Q-Bridge opened with a design capacity of 90,000 vehicles per day , but as of 2006 more than 150,000 vehicles cross the span daily...

 in Connecticut

Connecticut

Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...

, 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 left portion is covered by girder which is more stiffer than stiffening girder in cable stayed bridge. It's usual that main design constraint in cable stayed bridge is fatigue of cable and anchorage system. In extradosed bridge, fatigue is not a big concern since live load usually create only small amount of stress variation in cable because of stiffness ratio between cable and their girder. In cost wise, allowable stress for cable in cable stayed bridge is always smaller than extradosed bridge type 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. Furthermore, usual extradosed bridge doesn't require tension re-adjustment (tune-up) before service because cables act as external-tendon.

Extradosed bridge in Latvia

The Southern Bridge

Southern Bridge

The Southern Bridge crosses the Daugava river in Riga, the capital of Latvia. The bridge was constructed between 2004 and 2008, and was opened on November 17, 2008...

 over the Daugava River in Riga

Riga

Riga is the capital and largest city of Latvia. With 702,891 inhabitants Riga is the largest city of the Baltic states, one of the largest cities in Northern Europe and home to more than one third of Latvia's population. The city is an important seaport and a major industrial, commercial,...

, Latvia

Latvia

Latvia , officially the Republic of Latvia , is a country in the Baltic region of Northern Europe. It is bordered to the north by Estonia , to the south by Lithuania , to the east by the Russian Federation , to the southeast by Belarus and shares maritime borders to the west with Sweden...

 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

Island Bridge, Riga

The Island Bridge is a bridge that crosses the Daugava river in Riga, the capital of Latvia. The bridge was built in the period from 1975 to 1977.- References :...

 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

Riga City Council

Riga City Council is the government of Riga City, the capital of Latvia. It is located in the Riga Town Hall at the Town Hall Square in the very heart of Riga....

 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.

India's first extradosed bridge

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 metres 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

Systra

SYSTRA is an international engineering and consulting group specializing in rail and public transport. SYSTRA is known worldwide for its work in transport modes that offer a sustainable alternative to cars and trucks...

. Prior to this the first such bridge was constructed in Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

.

Extradosed bridges in Canada

The North Arm Bridge

North Arm Bridge

The North Arm Bridge is an extradosed bridge in Metro Vancouver, British Columbia, Canada. It spans the north arm of the Fraser River, linking Vancouver to Richmond. It is used by trains on the Canada Line, which opened in August 2009...

, used by the Canada Line

Canada Line

Canada Line is a rapid transit line in the Metro Vancouver region of British Columbia, Canada. Opened in August 2009, it is the third line in TransLink's SkyTrain metro network, servicing Vancouver, Richmond, and the Vancouver International Airport...

 connecting Vancouver

Vancouver

Vancouver is a coastal seaport city on the mainland of British Columbia, Canada. It is the hub of Greater Vancouver, which, with over 2.3 million residents, is the third most populous metropolitan area in the country,...

 with its suburb of Richmond

Richmond, British Columbia

Richmond is a coastal city, incorporated in the Canadian province of British Columbia. Part of Metro Vancouver, its neighbouring communities are Vancouver and Burnaby to the north, New Westminster to the east, and Delta to the south, while the Strait of Georgia forms its western border...

 over the Fraser River

Fraser River

The Fraser River is the longest river within British Columbia, Canada, rising at Fraser Pass near Mount Robson in the Rocky Mountains and flowing for , into the Strait of Georgia at the city of Vancouver. It is the tenth longest river in Canada...

 has been billed as North America's first extradosed bridgehttp://www.joconl.com/article/id24546. The Canada Line went into service on August 17, 2009.http://www.canadaline.ca/OPENING/opening.html

The Golden Ears Bridge

Golden Ears Bridge

The Golden Ears Bridge is a six-lane extradosed bridge in Metro Vancouver, British Columbia. It spans the Fraser River, connecting Langley on the south side with Pitt Meadows and Maple Ridge on the north side. The All Electronic Toll bridge, owned by TransLink, has a clearance of , and a total...

, also crossing the Fraser but between the municipalities of Pitt Meadows

Pitt Meadows, British Columbia

Pitt Meadows is a city in southwestern British Columbia, Canada and a member municipality in Metro Vancouver. Incorporated in 1914, it has a land area of 85.38 square kilometers and a population of 17,410 .-Geography:...

 / Maple Ridge

Maple Ridge, British Columbia

Maple Ridge is a District Municipality in British Columbia, located in the northeastern section of Metro Vancouver. Maple Ridge has a population of approximately 68,949.-History:...

 and Langley

Langley, British Columbia (city)

The City of Langley is a municipality in Metro Vancouver. It lies directly east of the City of Surrey, adjacent to Cloverdale, and surrounded on the north, east and south by Township of Langley.-History:...

, is the longest extradosed bridge in North Americahttp://www.b-t.com/projects/goldenears.htm. The bridge opened to traffic on June 16, 2009.

Extradosed bridges in Bolivia

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. http://www.pedelta.es/en/Proyectos/Puentes/Carretera/trillizos. 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 a 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.

External links

• Latvia's first extradosed bridge
• List of extradosed bridges around the world (Structurae site)