ABSTRACT
Two different procedures of construction are introduced: the traditional sequence and the new sequence. The traditional sequence consists in a first casting phase that is formed by the lower slab and webs and a second casting phase that is formed by the top slab of the deck. Once the first and second casting phases are completed, the total prestressing force is introduced. The new sequence consists in a self-supporting core that is formed by the lower slab, webs and top slab cantilevers and a second casting phase that is formed by the central zone of the top slab. Once the selfsupporting core is completed, a partial prestressing force is introduced so that the scaffolding can advance to the next span. Both sequences are described with their constructive peculiarities and issues related to the constructive easiness and cost reductions.
The new sequence has these advantages:
The second casting phase of the section is outside the critical path.
The use of more simple auxiliary means.
This new procedure clarifies the distribution of loads between the MSS and the deck.
However, there are issues associated with this solution:
Need to control the transverse bending of the selfsupporting core.
Need two shifts of prestressing.
Need of long splices between casting phases to provide long overlap lengths as Codes prescribe.
The new sequence presents some advantages that permit a faster construction without critical path. These advantages (second casting phase outside critical path, more simply auxiliary means, clear distribution of the top slab weight distribution) are important enough to consider the new sequence as an improvement over the traditional one and to design bridges with this sequence.
The isues related to the new sequence have been faced in different ways. Most designers have not considered replacing long splices by loop joints, so the extraction of the inner formworks is more difficult. Many of them have considered the need of placing stiffening elements in order to prevent transversal deflections of the self-supporting core while several have considered that it is not necessary. Finally, it was observed that none of them considered reducing the number of prestressing stages per span to only one. All of them disposed two prestressing stages.
Although issues related to the new sequence, it use has spread in Spain in recent years because its advantages are considered as more important than its issues. This papers is focused in highlighting both the advantages and issues and show so that designers can afford the viaduct design with a new possibility in order to reduce time and economic costs.
