Extradosed bridge

An extradosed bridge employs a structure which combines the main elements of both a prestressed box girder bridge and a cable-stayed bridge.^[1][2]:85[3] The name comes from the word extrados, the exterior or upper curve of an arch, and refers to how the "stay cables" on an extradosed bridge are not considered as such in the design, but are instead treated as external prestressing tendons deviating upward from the deck. In this concept they remain part of, (and define the upper limit of), the main bridge superstructure.^[4]:1

Compared to a cable-stayed or cantilever-girder bridge of comparable span, an extradosed bridge uses much shorter stay-towers or pylons than the cable-stayed bridge, and a significantly shallower deck/girder structure than used on the girder bridge.^[2]:85-86 This arrangement results in the typical extradosed "look" of a fan of low, shallow-angle stay cables, usually with a pronounced "open window" region extending from the sides of each tower.^[4]

The extradosal bridge form is mostly suited to medium-length spans between 100 metres (330 ft) and 250 metres (820 ft), and over fifty such bridges had been constructed around the world to 2012.^[4]:16-26 Whilst incurring many of the construction costs of both the cable-stayed and girder bridge types, extradosed bridges can deliver material savings to offset much of this penalty.^[1]:387-388 They have frequently been adopted when overall height, navigation clearance, or aesthetic requirements have made the cable-stayed or girder alternatives less feasible.^{[4]:15, 135-136}

History

Ganter Bridge, Switzerland 1980.
Main span 174m.

Odawara BlueWay Bridge, Japan.
Main span 122m, 1994.

Sunniberg Bridge, Switzerland
Main spans 140m, 1998

Pearl Harbor Memorial Bridge, U.S.
Main spans 157m, 2012-2015

]

The earliest bridge known to incorporate some of the features of an extradosed design is the Ganter Bridge, constructed in Switzerland in 1980. Designed by Christian Menn of Zurich, it preceded by several years the 1988 publication on the design philosophy of such bridges by Jacques Mathivat, who is credited with inventing the extradosed terminology and its design concepts.^[4]:3-5 The Ganter Bridge is a modified prestressed concrete cantilever girder design, where the longitudinal "continuity" tendons are raised significantly above the deck height at the ends of each main span and supported on short towers. These tendons are encased in fin-like blade walls on each side of the towers for protection, a design arrangement now often referred to as a cable-panel bridge.^[4]:2

Whilst little is known of the origin of Menn's design, Mathivat developed a theoretical basis for such a concept during 1982-1983 while preparing a tender proposal for the Autoroute A64 viaduct across Arret-Darre in France. Mathivat's design replaced the internal tendons normally located in the top of the box girder with "external" tendons running at a shallow angle from the deck surface in one span up over short towers and back to the next span. These he called extradosed tendons, as they connected to the extradose (or upper surface) of the bridge's spanning structure. The shallow angle of these tendons resulted in them transmitting a large compression force component into the bridge deck structure, allowing them to function in a similar manner to conventional "flexural" bridge prestressing. The extradosed tendons were continuous over the towers and were stressed from the deck level, unlike stay cables which normally terminate at the top of each pylon.^[4]:5

The first example of a bridge constructed using Mathivat's concepts appears to be the 1993 Ponte dos Socorridos^[5] at Camara de Lobos, Portugal, with a main span of 106 metres (348 ft). This was quickly followed by the Odawara Blueway Bridge on the Seisho Bypass, Japan, designed by Akio Kasuga and completed in 1994.^[4]:6 This bridge has a 122m main span, a 16.2m deck width and utilises four pylons of 10.7m height each supporting two sets of eight exposed extradosal tendons in a modified fan arrangement.^[6] These tendons pass over saddles at the top of each pylon, however it is unclear as to whether they form a continuous tendon between the two spans, as the designers' documentation refers to them being anchored outside each saddle to restrain slip.^[2]:86 Numerous bridges have subsequently been constructed in Japan to similar designs, the largest being the twin, 33 metres (108 ft) wide Ibigawa and Kisogawa "Twinkle" bridges, spanning 271.5 metres (891 ft) and 275 metres (902 ft) across the Ibi and Kiso Rivers respectively, and completed in 2001.^[2]:87[7][8]

In 1996, a short 54 metres (177 ft) span bridge of the extradosal style was constructed to pass the A43 Autoroute across the Maurienne River at Saint-Remy-de-Maurienne, France. This was followed in 1998 by the curved, 140 metres (460 ft) multi-span Sunniberg Bridge in Switzerland, also designed by Christian Menn and utilising low, outward-leaning pylons to minimise its visual impact on the surroundings.^[4]:10[9]

In North America, a small 104 metres (341 ft) fin-backed prestressed concrete bridge was built across Barton Creek near Austin, Texas in 1993 to service a private development. This is similar in concept to the original Ganter Bridge, except that the stay cables are encased within triangular blade walls connecting the deck to the towers.^[4]:5-6 The first "true" extradosal design on the continent is the North Arm Bridge at Vancouver, Canada, which spans 180 metres (590 ft) and was completed in 2008.^[10]

Since the mid-2000's the style has grown appreciably in popularity with over fifty bridges with extradosal characteristics being recorded as-of 2012. They have been constructed in many countries, although the significant majority of them are located in Japan and South Korea. The longest-span example remains the 2001 Kisogawa Bridge at 275 metres (902 ft).^[4]:16-22

Design characteristics

Ibigawa Bridge, Nagoya Japan
Main span 271.5m, 2001

Kisogawa extradosed bridge
(Schematic)

Shin-Meisei Bridge, Japan
Main span 124m, 2003

Visually, extradosed bridges typically have the appearance of a cable-stayed bridge with very short towers (pylons), with cable stays of shallow angle which may not extend along the full length of the deck, and (frequently) with a more substantial deck superstructure. Developed as a hybrid structure between the classic cable-stay and cantilever-girder bridge types, they can offer cost saving and aesthetic opportunities for medium-length bridges in the 100 metres (330 ft) to 250 metres (820 ft) span range.^[4]:1,26

Their hybrid nature can lead to significant additional complexity in their design, as the response of the bridge to applied loads is determined by the interactions between:^[4]:46-50

• the flexural stiffness of the deck/girder superstructure;
• the axial stiffness of the cable stays;
• the height and longitudinal stiffness of the towers/pylons;
• the lengths of adjacent spans and back-spans;
• the degree of fixity between the superstructure, the towers and the substructure;
• the flexural stiffness of the main support piers.

Although differentiated from cable-stayed bridges in a number of areas, the principal and defining extradosal characteristic is the low height of the main towers or pylons, expressed as a proportion of the main span length.^[4]:1 Classic cable-stayed designs employ a tower-height to main-span ratio of around 1:5 (or 0.20).^[11]:61 In comparison, extradosed bridges have towers with height:span ratios of between 1:8 and 1:15 (0.125 to 0.067), with around 1:10 (0.10) being most common.^[1]:393 This lower tower height results in a much flatter cable angle, typically ~ 17° to the horizontal, and a correspondingly much higher axial compression force within the bridge superstructure due to the greater horizontal force component within each cable stay.^[4]:1

From a structural perspective, the second extradosal characteristic is the reduced proportion of superstructure live load that is carried by the cables. On cable-stayed bridges, the stays commonly support at least 80% of the loading from vehicular traffic acting on the bridge superstructure, whereas on extradosed bridges, the stay cables typically support only between 20% and 60% of this load. In both cases, the remainder of the load is carried by the longitudinal girder element in spanning-action between the main bridge supports.^[2]:92–93 This characteristic derives from the relative stiffnesses between the cable stays and the girder element. Cable-stay designs typically incorporate flexible deck structures without a stiff girder element, and such systems transfer the majority of any applied deck loads directly into the nearest stay cables. In comparison, extradosal designs utilise a substantially stiffer deck/girder structure, allowing the girder to support a significant proportion of any deck loads, and facilitating the distribution of the remaining load between a larger number of individual stay elements.^[1]:388

Thirdly is the magnitude of fatigue loading experienced by the external cables, and their subsequent treatment by design codes. Unlike a bridge's near-constant dead load, its live load can be highly variable both in time and position, resulting in fluctuations in the stress level experienced by the bridge's structural elements as the live loading varies. The lower proportion of live load carried by the extradosed cables results in a reduced magnitude of stress fluctuations within the cables, down from typically 100 megapascals (15,000 psi) or above for cable-stay bridges to around 50 megapascals (7,300 psi) for extradosed designs.^[4]:13 This has a direct benefit in reducing the detrimental effects of fatigue experienced by the stay cables and their end anchorages, and bridge design codes allow extradosed stay cables to operate at a significantly higher design stress level and therefore material efficiency level as a result.^{[1]:394[2]:85}

As a result of these characteristics, the "stay cables" on extradosed bridges are not treated as such by the design codes, but are instead considered as external post-tensioning tendons which have been deviated upwards from the deck to the towers to increase the superstructure's load capacity over the main supports.^[2]:1 Extradosed bridges frequently extend this approach by making the extradosed tendons continuous over the towers via saddle supports and using anchorages only at the deck connections, significantly simplifying the tensioning operations.^[4]:33,61

By country

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 was completed in 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 feet, featuring extradosed pre-stressed concrete.^[12] The structures are built using balanced cantilever segmental construction. The elevation of the 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. The bridges were designed by PEDELTA.

Canada

North Arm Bridge, Canada 2009

The North Arm Bridge is a transit-only bridge of SkyTrain Canada Line, connecting Vancouver with its suburb of Richmond crossing over the Fraser River. The bridge went into service on August 17, 2009, coinciding with the opening of the Canada Line.^[13]

The Golden Ears Bridge, crosses the Fraser River between the municipalities of Pitt Meadows / Maple Ridge and Langley. It is the longest extradosed bridge in North America.^[14] The bridge opened to traffic on June 16, 2009.

India

India's first extradosed bridge was built by Larsen & Toubro Ltd. for the Second Vivekananda Bridge Toll corporation over river Hooghly, Kolkata. This bridge is 880 metres long, with a span of 110 metres.

Latvia

The Southern Bridge over the Daugava River in Riga, Latvia, is presently the biggest construction project in Latvia and its capital city, Riga. In work volume it can only be compared to the Island Bridge that was built in the 1970s. 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 bridge, which would connect Vienības Anenue on the left bank of the Daugava River and the Slāvu Roundabout on the right bank. The Southern Bridge over represents a multispan extradosed structure of 49.5 + 77 + 5 × 110 + 77 + 49.5 metres with six traffic lanes. The total length of the bridge is 803 metres. The width is 34.28 metres. There are six pylons, each at a height 13.33 metres above the roadway pavement. Each pylon has eight pairs of cables.^[15]

Pakistan

Pakistan inaugurated its first extradosed bridge for traffic on August 21, 2014. The Earthquake Memorial Bridge was built in four years on the Jhelum River in Muzaffarabad, the capital of Azad Jammu Kashmir region of Pakistan. The bridge is 60 metres above the river and 246 metre long. The cable-stayed extradosed bridge was designed and funded by the Japan International Cooperation Agency at the cost of approximately Rs 1.5 billion. The bridge connects Naluchi and Chattar areas of Muzaffarabad reduced commuting duration by an hour.

Poland

402-meter bridge in Mszana near Wodzisław Śląski is the widest extradosed bridge in the world.

The largest extradosed bridge in Europe, one of the largest in the world, was completed in July 2013 and was planned to open to traffic on 26 July over the Vistula River in Kwidzyn, Poland.^[16] The total length of the bridge is 808.5 metres (2,653 ft) with the main span of 204 m. 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 1028.4 m 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+ 2×204 + 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-

• three-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.^[17]

The widest extradosed bridge in the world is Most autostradowy w Mszanie (MA 532) on A1 autostrada (Poland).

Sri Lanka

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.^[18] The construction will begin in 2017.

United States

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 opened on August 19, 2015.^[19] Another extradosed bridge is planned to cross the St. Croix River between Oak Park Heights, Minnesota and St. Joseph, Wisconsin and be completed by 2017.

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.^[20][21]

Thailand

Maha Chetsada Bodinthranuson Bridge (Nonthaburi 1 bridge) is the first extradosed bridge in Thailand. The 4.3-kilometre Nonthaburi 1 Bridge links the western side (Bang Krang and Bang Sri Muang subdistricts) with the eastern part of the Nonthaburi province (Suan Yai and Talat Kwan subdistricts) and a new road connecting with Ratchaphruek Road.

References