Chapter 4: The Construction Process of Segmental Bridges
Tendon dimensions and layout.
Properties of each cable stay including area, modules of elasticity, and location of their connections to the pylon and deck.
Coordinates of the model nodes and definition of the segments.
Definition of all the construction loads applied during erection.
Definition of the support and restraint conditions of all of the structural elements.
Definition at every construction phase of segments and stays to be assembled, tendons to be stressed, construction loads to be applied, and boundary conditions to be implemented.”
Incremental launching basically functions by incrementally pushing the superstructure from the shore where it is produced over the bridge piers to cross the obstacle. The tip of the cantilever with the launching nose will consecutively be free cantilevering in the spans and be supported by the piers. This erection method causes complete stress reversal in the girder, which makes considerable top and bottom prestressing necessary. If the range of bending moments from this launching process is depicted along the superstructure, a moment envelope results, which is e.g. provided by Basse et al. (1985) for the example of the Aichtal Bridge. Reinforcement and prestressing tendons need to withstand these stresses at the respective locations.
Balanced cantilevering needs to be safe against overturning moments from construction loads as described in the previous Section 4.3.1 until closure of span. The stresses within the cantilevering girder do no change their sign, but with advancement of the form travelers to cast a new segment or placement of a new segment the cantilever stresses will increase. Ways to alleviate danger of overturning are use of additional temporary towers with vertical prestressing to withstand vertical compression and tension from unbalanced cantilever arms, and temporarily fixing hinged bearings that are located on top of the piers. Mathivat (1983) gives an example of vertical prestressing within the pier table segment to rigidly attach the superstructure to the pier. The piers need to be designed strong enough to withstand bending moments that might occur from the most unfavorable combination of actions on the growing cantilever. A brief list of possible causes for overturning moment is taken from Mathivat (1983, pp159f) and adapted in Table 4-4 for easier reading: