IL METODO DEGLI SCHEMI ELASTICI PARZIALI NELLA COSTRUZIONE A SBALZO DEI PONTI AD ARCO

In questo lavoro viene presentato uno studio sulle sequenze di tesatura degli stralli nei ponti ad arco costruiti per sbalzi successivi con l’ausilio di tiranti provvisori. In particolare viene presentato il metodo degli schemi elastici parziali, proposto per la determinazione degli sforzi iniziali nei cavi. Tale metodologia è stata dapprima applicata sui ponti strallati e viene estesa in questo studio al caso dei ponti ad arco costruiti per sbalzi successivi. La trattazione teorica è seguita da un’applicazione numerica su un caso studio di un ponte ad arco in calcestruzzo

CREEP EFFECTS AND STRESS ADJUSTMENTS IN CABLE-STAYED BRIDGES WITH CONCRETE DECK

In construction stages of cable-stayed bridges with prestressed concrete deck, the influence of creep on stresses and strains is very important in order to foresee the final patterns of internal forces and displacements. In cantilever construction, the concrete deck can be considered, in each stage, as a continuous beam resting on elastic restraints, which modify with successive additions of new segments, until the last one has been assembled. In these stages stress relaxation in concrete occurs as well as vertical displacements increase. Ehen structure has been closed by inserting midspan segment, stress redistribution begins, due to creep. Deformation and internal force development in construction and service life modify stay stresses such as deck and pylon final profiles. It is necessary to prevent undesirable deformed shape of deck and pylon and to control the final stress pattern of deck and stays. The requested final geometry of the bridge is reached by adjusting stay axial forces during construction. A study is presented in which, by taking into account creep effects, the optimization in terms of deck and pylon deformed shape can be achieved through a sequence of stay force adjustments during construction stages. The presented analysis is based on the theory of aging linear viscoelasticity in order to give a useful tool for the conceptual design of cable-stayed bridges with concrete deck. The proposed procedure allows engineers to design by reducing and avoiding creep effects instead of calculating them with refined models since the first design step

A parametric study of curved incrementally launched bridges

The structural behaviour of incrementally launched bridges in the construction stages depends on different parameters involving deck, nose, supports and guide devices, because static schemes vary continuously with the advance of the deck above the piers. For this reason temporary stresses in the deck, during launching, are rather different from those occurring in service life. Horizontally curved launched bridges also present the effects of torsion induced by geometric curvature. A parametric study is presented in order to analyse the influence of design parameters on the construction of these bridges. Analyses were carried out by extending to curved beams a procedure based on the Transfer Matrix Method, already known for straight continuous decks. Effects of curvature, nose–deck ratios of length and load, bending and torsional stiffness ratio were taken into account. The results show that maximum torsion values increase with the decrease in the curvature radius R and with the decrease in the ratio between bending and torsional stiffness. Moreover, with variation in the nose length ratio, the value ln/l = 0.60 with respect to the span length, is confirmed as the optimal value, as happens for straight bridges. With variation in the nose weight, a significant increase in bending moment and torsion can only be appreciated in the cantilever stages of launching. Dimensionless diagrams and related expressions are given for numerical evaluation of the maximum values of bending moment and torsion in the construction stages, with variation in the stiffness ratio and the radius of curvatur