Chapter 4: The Construction Process of Segmental Bridges
Secondly, part of the midspan superstructure can be designed as a suspended span sitting on bearings between the cantilevers. In this configuration the deflection angle between the shorter cantilevers and the suspended span will be much smaller, and “differential settling of the supports” can better be accounted for (Podolny and Muller 1982, p38). Still, the connections require special details in the structural system.
Finally, the whole superstructure can be made continuous at midspan. Achieving this statically indeterminate system is the most common way in building cantilever bridges for several reasons. Mathivat (1983, p41) names specifically that the deflections in the stiffer continuous superstructures are “indeed far smaller than those met in hinged structures” and both visual appearance and drivers’ comfort are better than in hinged superstructures. He also notes the necessity for expansion joints in very long continuous multi-span superstructures and advises to provide expansion joints about “300 to 600 m apart” (Mathivat 1983, p45) in points of small moments in the superstructure. Horizontal movements of the bridge superstructure can be accommodated “by the flexibility of the piers themselves, or by using elastomeric bearings or sliding supports” (Mathivat 1983, p45).
Continuity is generated by casting a closure segment into the gap at midspan, through which continuity tendons, the so-called integration cables as mentioned in Section 4.1 run in the bottom part of the box girder (Mathivat 1983). Prior to casting this segment, misalignments of the two superstructure halves are corrected with hydraulic jacks. It should, however, be tried to keep these additionally imposed stresses small by paying close attention to the correct alignment including camber when casting the superstructure halves. Additionally, as mentioned in Section 220.127.116.11, the two girders are often jacked apart to compensate for future effects “of long-term creep and shrinkage of the superstructure on the substructure” as Matt et al. (1988, p37) report. They further mention casting the closure segment of their bridge project at night to avoid problems from temperature gradients in the superstructure. For casting and curing of the midspan closure segment the girders need to be fixed in their position. Finally, continuity tendons can be inserted into the newly cast segment and post-tensioned. Upon closure of midspan internal stress redistribution takes place, shifting the moments from the supports more towards midspan. The formerly free cantilevers are now restrained in deflection and rotation. Podolny and Muller