Chapter 4: The Construction Process of Segmental Bridges
in the balanced cantilevering method to stabilize the structure against tipping over from construction loads. They can be additionally strengthened with prestressing rods to withstand the forces that are imposed on them during construction. Other applications are found during incremental launching, where together with the launching nose they serve to keep the range of bending moments small. Instead of using temporary towers as supports Liebenberg (1992, p156) also mentions stiffening girders “by means of prestraining with a king post and adjustable inclined ties.”
4.2.5 Span-By-Span Erection
Levintov (1995, p47) lays out the characteristics of the span-by-span erection method as assembling all segments for a span in a set, which is “then aligned, jointed, and longitudinally post-tensioned together to make a complete span.” The principle of span-by-span erection is shown in Figure 4-9. Span-by-span erection is typically limited to bridges that consist of box girders with constant depth. The actual construction can have several variants, the segments can be assembled on the ground and lifted in place as a group by a heavy-duty crane or they can all be put into their final position on erection girders along the spans to be completed. The second method was e.g. used for constructing the Biloxi Interstate I-110 Viaduct. In this project different types of erection girders were used. The authors report that on some spans triangular trusses were implemented, and on the other hand steel box girders came into use, which left more clearance for traffic underneath (Phipps and Spruill 1990). Erection girders were supported at their ends “by steel falsework resting on the footings at each pier” (Phipps and Spruill 1990, p130). After completion of a span the erection girders were set forward to the next span and adjusted. By then, the precast segments for this span had already been supplied and would be lifted in place by crane. Fine adjustments of the segments on the erection girders were possible by means of variable individual supports. Finally, post-tensioning would be performed to link all the segments together to form a complete span. The structure became self-supporting after casting of the closure joints with the pier table segments had been done and the longitudinal prestressing force was induced. With the method described, an erection speed of one span per about 3.5 days could be achieved (Phipps and Spruill 1990).