Chapter 4: The Construction Process of Segmental Bridges
construction are experienced. In comparison with incremental launching and balanced cantilevering, a simpler flow of forces takes place between superstructure and the piers. No horizontal forces are introduced in the piers and no unbalanced bending moments have to be withstood by the piers. It is therefore possible to immediately install the permanent bearings (Mathivat 1983).
Some disadvantages of the progressive placement method need to be dealt with during design and construction. As construction only progresses at the tip of one cantilever, progress is slower than in balanced cantilevering. Progressive placement resembles incremental launching in that the superstructure undergoes stresses very different from the permanent service conditions, including even stress reversals. In both cases the structure needs to incorporate temporary prestressing tendons to account for these stresses. Mathivat (1983) also points at the difficulty in erecting the first span with progressive placement. Other construction methods may have to be employed for this stage.
18.104.22.168 The Linn Cove Viaduct
An interesting example for use of the progressive placement method is the Linn Cove Viaduct (Anon. 1984), shown in Figure 4-3. Its location in an environmentally sensitive area in North Carolina, the inaccessibility of the sloping site at the mountain face, and the highly curved alignment of the viaduct provided a set of difficult conditions for this project. Match-cast prefabricated segments for the highly curved viaduct were delivered by truck directly to the end of the already completed part of the structure, where they were placed and attached with temporary thread bars and tendons. Deviating from the method outlined above, no cables were used above the spans, but steel bents were installed under the spans during construction to provide support. Only drilling so-called microshaft piles for the pier foundations needed to be done directly on ground, as the piers themselves also consisted of precast segments that were craned into position from above and post-tensioned vertically. Implementation of the progressive placement method in connection with precast pier segments that were lowered into place from