ABSTRACT
With its 13 spans and a total length of 1028 m, Colle Isarco Viaduct is among the most important structures of the Italian Brenner freeway. The three longest spans are 91 m, 163 m and 91 m long, which create a structure made of pre-stressed concrete box girders of various heights (Fig. 1). Elevation and cross-section of Colle Isarco Viaduct. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig41_1.tif"/>
In 1988 and 1999, the viaduct underwent maintenance work to restore its original static and functional features. Today, with a growing traffic load, there is the need to increase the life of the viaduct based on the current structural codes. Therefore, it was decided to retrofit the viaduct with a structural intervention. The main work includes: thickening of the upper slab of the girders, installation of an external post-tensioning system and increasing the torsional stiffness through steel truss diaphragms (Autostrada del Brennero SpA 2013). Since summer 2014, the bridge deflection has been continuously monitored by two topographic total stations and 72 prisms placed on the decks of the 163 m-long span. In 2016, the main spans of the viaduct will be further instrumented with fiber optic sensors to measure the strain, thermocouples to measure the temperature, and load cells to monitor the tension in the new post-tensioning cables.
In order to investigate the main causes of the past behavior and to predict the future one, a simplified beam model and a refined 3D finite element model have been built. The former can be used to perform inference analyses and to understand the global deformation of the bridge. The time-deflection profile of the bridge can be detected by means of the well-known Principle of Virtual Work and by assuming the curvature as a function of the difference in strain between the upper and the lower slabs. Three contributions to the curvature are accurately computed: elastic, viscous, and drying. The concrete creep Model B3 (Bazant and Baweja 2000), proposed by Bazant in 1989 and differing from that of the Italian and European design codes, was used. The main advantage in using such a complete and refined creep model for the study of Colle Isarco Viaduct is the presence of an additional component that, given a constant stress, never stops growing. It was observed that the time-deflection profile obtained using Model B3 matches the measured values with a high degree of accuracy.
The 3D finite element model is useful to evaluate the values of local stresses. Using the software Ansys v. 12.1, the concrete body of the bridge was implemented using SOLID186 elements, whereas the 414 cables were modeled through 8059 BEAM188 elements, for a total number of 260000 degrees of freedom. Concrete creep Model B3 was implemented with a user-defined Fortran subroutine. The post-tensioning was modeled by an equivalent thermal gradient to be applied along the whole of each steel cable, and the construction phases of the 45 bridge segments were modeled by including the variation of the boundary condition. The results of the analysis were then compared with those obtained from the simplified beam model.
