Chapter 4: The Construction Process of Segmental Bridges
Mathivat (1983) also gives information on means of accelerating this process. Use of special form travelers “with lateral main beams or self-supporting carriages” (Mathivat 1983, pp201f) will leave the bottom slab widely unobstructed and make construction easier. Secondly, “increasing the length of segments” can be considered during the design phase while keeping in mind the increasing weight and cost of bigger form travelers. Stepwise construction of the box girder is also possible, with relatively simple formwork for the top slab following a few segments behind the main form traveler. With this method less concrete has to be placed in one single step of concrete placement. Yet it requires careful structural design of the vertically segmented superstructure. Finally, an example of a segmental bridge is given for combination of cast-in- place and precast segment sections. Webs of the superstructure box girder of the Brotonne Bridge in France were precast and placed into the form travelers, which were used to fabricate the remaining cast-in-place parts of the cross-section (Mathivat 1983).
220.127.116.11 Balanced Cantilever Construction
Balanced cantilever construction denotes building a bridge superstructure from both sides of the pier table in a scales-like fashion. This erection method is also known under the name free cantilever construction (Podolny and Muller 1982). Fletcher (1984) gives information that the pier table element, serving as a base from which cantilevering is begun, is usually between 6 and 12 m long. In order to balance the weight of both arms of the cantilever superstructure the segments will be about equally placed at both ends. Actual placement of new segments will hardly proceed exactly at the same times as Mathivat (1983) expresses. Therefore the pier can undergo overturning bending moments and needs to be designed accordingly. Temporary towers with vertical prestressing or counterweights can provide additional support. Figure 4-1 schematically shows a typical construction stage in Balanced Cantilever Construction.