Effects of butterfly web design on bridge construction

Abstract

A new type of bridge called "butterfly web bridge" has been constructed in Japan. In a butterfly web bridge, the panels used as the web are cut so as to appear pinched in the center, giving a butterfly-wing shape. For the butterfly-shaped web made of precast panels with a thickness of 150 mm, 80 MPa steel fiber-reinforced concrete is used. The design method was proposed after the shear tests of butterfly webs. In this paper, four projects of expressway bridges using this new technology will be shown. It has been shown that by a simplified construction process, construction time can be reduced. Then, future development is proposed in the last part of this paper by considering a 500-m-span extradosed bridge. The butterfly web technology will greatly affect bridge design and construction in the near future. K E Y W O R D Sbutterfly web, extradosed bridge, fiber-reinforced concrete | INTRODUCTIONOne of the recent innovations in bridge technology is the composite bridge. A variety of ideas have been applied in this area, including the use of steel members in the web, and attempts to find an effective method for utilizing the prestressed forces in the concrete members used for the deck slab and bottom slab. Examples include corrugated steel web bridges and composite truss bridges. In Japan, over 150 corrugated steel web bridges have been constructed since the first one to be built in 1993. The composite truss approach was first adopted in 2003, and there are now eight composite truss bridges in service. These composite bridges achieve reductions of 10-15% in girder weight compared with conventional concrete bridges, and the weight reduction has given them an advantage in Japan, where earthquakes are frequent.In a corrugated steel web bridge, the joints between the web and the concrete slabs run continuously along the longitudinal direction of the bridge, enabling the structural details of the joint to be simplified. However, fabricating the corrugated steel web requires folding of the steel sheets using a press, and the individual corrugated steel webs need to be welded together at the construction site. These processes can affect the economy and ease of construction. In contrast, composite truss bridges use conventional materials such as steel pipes that do not need to be joined on site, but because nodes are used for the joints with the concrete slabs, substantial forces concentrate at the nodes. Moreover, joint details become more complex for longer spans. These factors make it more difficult to achieve economy and ease of construction, and that is probably why there are only few examples of composite truss bridges around the world.Being involved in the design and construction of such bridges, the author considered the potential for structures that combine the benefits of these two types of composite bridges, and eventually developed the "butterfly web bridge" design 1 ( Figure 1) described in this paper. A butterfly web bridge, which uses butterfly-shaped panels instead of a double Warren truss, is a new structure...

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“…6) were carried out to establish the design method. [4] The analysis and tests showed that in addition to the behaviour of a pure double Warren truss, the butterfly web panel is subjected to in‐plane bending moment and stresses similar to a Vierendeel truss. For that reason, instead of uniformly distributing the prestressing strands in the tension zone, they are arranged with a higher density of strands at the outer sides (Fig.…”

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“…A real-case application of FRC in long-span bridges is found in [22]. FRC was used in the web design of the butterfly web bridges.…”

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