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 first casting phase or 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 self-supporting core is completed, a partial prestressing force is introduced so that the scaffolding can advance to the next span.
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 is possible for the execution of this second casting phase.
This new procedure clarifies the distribution of loads between the MSS and the deck. The scaffolding should only support the weight of the first casting phase, while the weight of the second casting phase is supported by the self-supporting core.
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.
Results show that in some aspects the new sequence is better than the traditional. These aspects are ELU global bending moments and many ELU internal forces or transverse stresses at the top slab. On the other hand, some values are worse at the new sequence, as
ELU bending moments at the bottom slab or longitudinal stresses at the top slab over piers. Nevertheless, it can be concluded that better results in some aspects compensate worse results in others and the advantages of the constructive process itself has lead several designers to prefer the new sequence over the traditional one. One of the main issues concerning the new sequence, related to the need of placing trusses in order to avoid transverse deflections of the self-supporting core is not justified by the results that were obtained by this study so none truss is needed to avoid transverse deflections while construction.
It can be concluded that the new sequence results are as good as the traditional results and constructive advantages suggest that subsequent improvements of this sequence will make it the best choice when a viaduct design with movable scaffolding system is required.
