Twinkle Kisogawa Bridge
The Twinkle bridge is the longest extradosed bridge in the continent of Asia. 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.
The Ibigawa Bridge and its twin Kisogawa form a composite bridge known as the Twinkle Bridge. These twin bridges, which are the first extra-dosed bridges in the world to use pressed concrete (PC) and steel conglomerate, are located in the city of Kuwana in Mie prefectureJapan. The two bridges are part of the Second Meishin Highway. CTI was in charge of the design of the 1,400 meter-long twinkle Bridge.
The extradosed system is a hybrid design that is a marriage between a concrete cable stressed girder bridge.
EXTRADOSED BRIDGE
An extradosed bridge employs a structure that is frequently described as a cross between a girder bridge and a cable-stayed bridge.This description is somewhat deceptive, since many cable-stayed bridges have some sort of box-girder decks. 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.
Given its intermediate design, the Twinkle Kisogawa bridge is 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 formof a bridge.
With a typical cable stayed bridge most of the load (weight of the bridge deck, girders, cars and trucks) is carried through the cables, up to the top of the towers and then down through the towers to the foundations. In an extradosed bridge both the girders and the wire cables carry the load. A portion of the load is carried back through the girders to the towers.
The remaining portion is then carried by the cables up to the top of the towers and then back down through the towers to the foundations. The result is a very efficient load-carrying bridge with superior aesthetic qualities due to the exposed cables and concrete towers above the bridge roadway surface.
CONSTRUCTION
Work on the development of the Twinkle Kisogawa bridge project started when the City Development Department developed the design task for the route of the roundabout on the right bank of the river Daugava. The bridge represents a multispan structure of 49.5 + 77 + 5 × 110 + 77 + 49.5 metres with 6 traffic lanes. The total length of the bridge –is 803 metres. The width of the bridge – 34.28 metres. The number of pylons is six, each at a height of 13.33 metres above the roadway pavement. Each pylon has 8 pairs of cables.
The wire carrying cables were placed outside the girders and up on the main concrete towers as opposed to inside the concrete boxes. However, the cables were not placed high up on tall towers, as would be the case with a conventional cable stayed bridge.
An extradosed bridge lends itself to longer spans (the distance between foundation supports) than a conventional girder bridge. This allowed the bridge foundations to be built without interfering with the existing bridge foundations, which had to remain in service to carry traffic during construction.
The tower height above the bridge deck is much less than that of a cable stayed bridge. This was important for the bridge in that the towers could not restrict flight traffic .The stay cables (wire strand cables which go up to the concrete towers) need no tension adjustment as would be required for a conventional cable stayed bridge. This will result in reduced future maintenance costs for the extradosed bridge.
CONCLUSION
This extradosed bridge has a very long main span which benefits boat and barge navigation below the bridge by providing a wider navigation channel. The girder depth of the bridge is less than that of a standard girder bridge. This allows for a longer span length while not impacting the profile of the bridge deck roadway.
PROFFESSIONAL TEAM
Design: Michel Virlogeux
Architect: Alain Spielmann
Owner: Ministère de l'Equipement, des Transports, du Logement, du Tourisme et de la Mer
Checking engineering: Ingérop
Structural engineering: Etablissements J. Richard Ducros
Main contractor: DV Construction
