Chapter 4: The Construction Process of Segmental Bridges
Liebenberg (1992, p164) also gives a very clear statement of the main difficulty of the incremental launching method: “During launching, the section undergoes complete stress reversals as it progresses from a cantilever to the first support and thereafter over the following spans to its final position.” Clearly, this erection sequence generates a bending moment envelope in the structure depending on the span lengths that needs to be accounted for in designing the cross-section properties and the amount of reinforcement and prestressing tendons. The stresses due to the aforementioned high bending moments require much longitudinal prestressing both at top and bottom of the cross-section. Another disadvantage is the large workspace that is needed for the casting bed at the abutment and the adjacent storage areas (Podolny and Muller 1982).
The Aichtal Bridge in Germany that was built mainly between 1981 and 1983 serves as a good example of the incremental launching method. According to Basse et al. (1985) this bridge with its total length of 1,161 m is the longest one ever built with incremental launching. It crosses two valleys at a maximum of 48 m and 50 m above ground, respectively. A fixed bearing is located at the pier between the two valleys. At the same location a second jacking system was installed for use in later construction stages.
The normal pier spacing for the 21 spans of the Aichtal Bridge is 51 m, reaching a maximum of 80 m and 84 m respectively at the deepest parts of the valleys. These wide spans required use of temporary towers that were braced with stay cables from the ground to resist the horizontal forces from launching. The whole bridge superstructure consists of two parallel single cell box girders that are 3.50 m deep, 5 m wide at the soffit and carry 13.50-m wide decks. After completion of one girder all construction equipment was relocated for the second box girder.
An enclosed 25.50-m long casting bed with adjacent assembly yard for the reinforcement cages was erected behind the abutment with the launching jacks. For winter construction work another 50-m long tent with large heaters was set up for proper curing and the piers were built with thermally insulated climbing formwork. Casting of segments was done in a weekly cycle. Both longitudinal and transverse limited prestressing was implemented. Basse et al. (1985, p23) give information on the sequence of casting steps that is compiled in Table 4-3: