Issue 46

F. Bazzucchi et alii, Frattura ed Integrità Strutturale, 46 (2018) 400-421; DOI: 10.3221/IGF-ESIS.46.37 413 responsible for the support loss of the Gerber deck and the rupture of the deck due to bending in its plane (yellow, Fig. 18a). An asymmetric fall of the deck causes and inevitable impact with the intermediate deck support that would find themselves dragged into the collapse. Finally, the observation of the Tower 9 ruins allows an additional comment: many of the parts of the tower broke with planar surfaces, evidencing low or nonexistent process zone and plastic hinge creation. These planar surfaces represent probably the casting phases of the vertical structure. This would imply that (i) no sufficient rebars content was present in that zones, or (i) the boxed section had not sufficient plasticity reserve (red, Fig. 18b). In the end, at this level of knowledge, the lack of structural robustness can be addressed to these factors:  the low number of stays elements that causes zero redundancy in deck support;  the low rotation capacity of the A framed lateral tower and its limited redistribution;  the statically determined scheme for the entire viaduct, which causes higher displacements (and cracking as a consequence) and domino effect collapse;  the low ductility at member scale;  the intrinsic sudden loss of strength of prestressing technology. Lastly, from a mechanical point of view, concrete dissipation energy in compression is highly dependent on the size of the structure. Experimental results always evidenced a dependency in evaluating the compressive strength to the specimen size and the testing machine (that is why today we use standard) [25]. We know from literature that the post-peak branch of the load-displacement curve is governed by macrocracking, after the coalescence of the initial micro-cracks. The energy dissipation is a surface-dominated phenomenon and damage localization occurs in small concentrated zones, but this is true only for “small” specimen size; nevertheless, for larger structural dimensions the energy dissipation should be a volume-dominated process and damage is more spatially distributed. Experiments have evidenced how, avoiding friction, there is no noticeable deviations in concrete ultimate strength due to specimen dimension, whereas strong variations are observed for dissipated energy in compression, with a catastrophic nature for “big” scales [26]. The retrofitting intervention proposed for the Petrulla viaduct (Fig. 15) had also the purpose of increasing the robustness of the structure. Local plates and the external prestressing made recover structural continuity to the beams. Furthermore, the external cables are easy to inspect and check for integrity. This opportunity represents also a form of robustness because of the redundancy of controls. On the contrary, structural arrangement like the one of the Fossano Bridge requires to retriever firstly the structural coherence. The joints represent a discontinuity towards the fragile shear mechanism. This means that, if the cables are unbonded across the joint area, there is no possibility for the vertical loads to reach the supports once the cables are broken. This could be easily testified by looking at the intact joint that resisted the impact on the ground when bridge failed (Fig. 7(a)). Moreover, inspections in the web and epoxy void injections seems unfeasible due to the section shape. In the same way, external prestressing is rather impassable, since it is unknown the adequate force and steel to apply. A promising solution is represented by resurfacing the concrete beam with a steel carter that works both in shear and flexural regime (Fig. 19). The mandatory condition for this repair intervention is the absolute adherence between the steel sheathing and the concrete, in regard to congruent strains and humidity proofing [27]. I NNOVATIVE TECHNOLOGIES AND INVESTIGATION METHODS ealing with existing bridges implicates a first step in building knowledge on the actual conditions and properties of the structure. Scientific community groups all the investigation methods in two categories: Structural Health Monitoring (SHM) and System Identification (SID). SHM is a field of study that has the aim to ascertain the haleness, defined as a function of desirable structural properties as variables, once referred to a state base of safety. To do so, structure is periodically sampled by installed sensors, and after a signal processing phase, statistical analyses determines the health function [28]. SID, on the opposite, can be performed once and has the objective to quantify the actual features of the artifact. Considering both SHM and SID, the procedures panorama is extremely fragmented and it lacks an organic interpretation scheme. The vast set of methods varies among dynamic to static regime, passive to active stimulation, and remote to field sensing. To mention some techniques and systems, we have Acoustic Emission, Fiber Optic Network, Piezoelectric Active Sensors, Accelerometers Modal Correlation, Guided Wave Sensing, Tomography, LIDAR Displacement Detection and so on. As recently criticized by several authors [29,30], even each available method has a huge number of different patented technologies behind. Moreover, accuracy is strictly dependent on the prior knowledge of the monitored structure. Using a medical analogy, it is like having a disorganized number of specialized D