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Removal of temporary supports and creep effects in the sequential construction of bridges


Structural conception of bridges is strictly related to the construction methods involved. A wide application of precasting technologies of reduced dimension elements assembled on site is today the most common choice. During construction, static schemes and strength characteristics of the elements generally change. The connection of different structural elements in subsequent stages, the introduction of delayed restraints and the use of provisional supports modify the structural configuration. Such transient phases can influence significantly the final behaviour of the bridge. In such a way the designer, acting during the constructive stages, can force the structural scheme towards the solution that optimizes elements functionality. In this paper a study about the influence of delayed strains due to creep on the structural conception of prestressed bridges is considered. The effects of additional restraints and support removal on the final solution are evaluated and discussed. The study permits characterizing the phenomena, to define possible simplifications, useful for developing simplified models in the hypothesis of linear viscoelasticity and homogeneous structures. These models allows the use of elastic solutions by means of the redistribution functions. A case study of an arch-portal bridge is presented. During construction a fundamental role is played by the removal of temporary piers after the introduction of delayed restraints, with a change of the static scheme from the continuous beam behaviour to the arch one.