PSI - Issue 62

Giuseppe Colombo et al. / Procedia Structural Integrity 62 (2024) 377–384 Colombo, Mariani, Lambrugo, Giardina/ Structural Integrity Procedia 00 (2023) 000 – 000

382

6

Against this background, historical analyses have been carried out that have not allowed us to find any document capable of justifying the reasons that led to the creation of the prestressing system designed in 1988, realized in 1992, and which therefore made it necessary to carry out a numerical analysis of the structure and an analysis of its state of conservation (taking into account the lack of effectiveness of the reinforcement system installed. Since the result of the above analyses highlighted a situation in which this reinforcement is not necessary to guarantee the safety of the structure, with the aim of eliminating elements of significant weight and low effectiveness, it was decided to remove the existing prestressing system by proceeding as follows: • De-tensioning of the prestressing cables by means of a single-strand jack, due to the difficult accessibility with equipment of large loss and size. • Removal of the cables from the anchor and deviation blocks and away from the structure. 2.2. Slab strengthening Following the numerical analysis of the structure, a modest lack of longitudinal reinforcement was highlighted in correspondence with the support on the piers of the box girders. In this section, the box girders, which accommodate the buffers beams resting on the Gerber saddles at their ends, have the maximum negative bending moment produced by gravitational and traffic loads. The reinforcements present before the intervention were not adequate to guarantee safety according to the indications of the NTC 2018, in the face of the current conditions of the structure and its geometry significantly modified by the increase in thickness and therefore in weight of the upper slab.

Fig. 5. Installation of supplementary rebars and connecting needles over the slab The project therefore foresaw the need for an additional longitudinal reinforcement to be laid in the following ways: • Construction of longitudinal recesses in the slab by mechanical means, and installation of supplementary rebars  22 at a pitch of 0.33 m. • Connection of the supplementary rebars to the underlying structures of the slab by using sewing needles with an "L" shape  18 at a pitch of 0.33 x 0.40 m fixed to the deck with two-component epoxy resin. The vertical bars were also sized to absorb the shear between the original slab and the casting made in 1992, which was not adequately connected (fig. 5). • Solidarization of the integrative reinforcements with the existing structure with special mortar with additives, also to restore the cross-section prior to the intervention. • In correspondence with the buffer beams, put in place of the vertical needles  18 in the same way as described for the box girders, to make the connection between the original slab and the additional casting of 1992.

2.3. Gerber saddles reparation

The structures of both the box girders and the buffer spans, in the areas near the Gerber saddles, were heavily damaged. The deteriorations can be summarized as follows: