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Adjustable prestressing for construction stages of incrementally launched bridges


Incrementally launched bridges, which are generally built with the static scheme of a continuous beam on supports, change their scheme several times during launching. In these different configurations, internal forces vary and bending moments of opposite signs occur in the same sections for different positions of deck during advancement. In these bridges, two different kinds of prestressing are necessary: a temporary one during launching and a definitive one in service life. In this study, the possibility of implementing automatic adjustable prestressing for the launching stages is investigated; the launch prestressing is partially supplied by a system which can change the tension value in the prestressing tendons according to the change in the loads acting on the structure. This adjustable prestressing can be either semi-automatic or totally automatic (auto-adjustable), following the concept of "organic prestressing." The conditions of activation and deactivation of automatic prestressing through a control system of stresses and deformations are given, giving indications to designers on the additional tendons to be applied in the advanced part of the deck and on the control sections which system implementation needs. A numerical example is given to clarify the concept of adjustable prestressing, details being provided in the case study examined on the evaluation of stresses, on the choice of control sections and on the prestressing tendons, through the aid of diagrams of activation and deactivation of the adjustable prestressing. The results of the study highlight the fact that adjustable prestressing is useful for the advanced part of the deck, in the launching stages of bridges with several spans with similar lengths, especially for the bottom fibres, when sections are placed in the midspan, or for upper fibres when they pass over piers, in the case of limited nose lengths.