Chapter 4: The Construction Process of Segmental Bridges
cantilevering superstructures and install vertical prestressing tendons. Furthermore it is very common to make use of an additional temporary pier with vertical prestressing that is located close to the permanent one (Casas 1997). This pier helps withstanding overturning moments from unbalanced load cases on the bridge superstructure.
Several advantages have contributed to the success of the cantilevering method. Certainly the most important one is that no falsework or centering is required, leaving traffic under the spans widely unobstructed during construction. Access from the ground is only necessary for construction of the piers and abutments and in preparation for the start of cantilevering, which starts from these locations.
Only relatively little formwork is required due to the segmental nature of the superstructure. Cantilevering is a very feasible method if the bridge spans are too high above ground for e.g. economical use of falsework, and if the terrain under the spans is otherwise inaccessible or unfeasible, being e.g. a deep gorge with danger of flood events. Especially in these cases rapid construction can be achieved with cantilevering.
Fletcher (1984, p13) notes that especially in cantilevering “complete calculations are required for the construction stage[s] and these are complicated as many stressing effects are time- dependent.” In addition to this, the influence of stepwise construction needs to be considered. However, the statical system that needs to be analyzed is rather simple and in case of the cantilever prior to closure at midspan even statically determinate.
220.127.116.11 Precast Construction
Precast construction means that bridge members or segments are prefabricated at a location different that the site, transported to the site, and installed there. Mathivat (1983) gives the maximum economical span of bridges built in precast segment as about 150 m, since cost for the placement equipment increase considerably the longer the spans are. Construction with precast segments has several advantages in comparison with cast-in-place segmental bridges. Casting of the segments can be performed under controlled, plant-like conditions at the precasting yard.