Issue 46

F. Bazzucchi et alii, Frattura ed Integrità Strutturale, 46 (2018) 400-421; DOI: 10.3221/IGF-ESIS.46.37 405 Ancona overpass: movement operations This collapse occurred on 9 March 2017, near Ancona, along the A14 Italian Highway. The bridge (167 at km 235+800) was constituted by a steel-concrete composite deck 16.5 m wide and 36 m long with a simply supported beam scheme (Fig. 6). When collapsed, the deck was uplifted for maintenance operation and it was supported by a temporary structure. The deck fell on the underlying highway, causing the death of 2 people that were transiting underneath. The case of the collapse of the overpass on the A14 motorway near Ancona is quite different from the others and was caused only by a resounding error during the execution phase. The works involved the adjustment of the altimetric height of the overpasses, consisting of raising the decks, creating an extension of the support structures and the re-profiling of the associated ramps. The lifting took place through the use of hydraulic jacks with few centimeters steps, after positioning of steel plates for distribution. The appointed subcontractor to perform this specific process decided to replace the supporting steel plates with similar plates in neoprene, much lighter to move. Numerical analyses conducted during investigations by prof. Tarantino highlighted how a difference between the temporary supports of a few millimeters generates horizontal forces not compatible with the shear deformability of the numerous neoprene plates stacked on top of each other. This generated an unexpected bridge horizontal displacement and the collapse occurred. Further details are under investigation. The lesson that can be obtained from this incident reinforces the conviction that in the execution phase often non- specialized and often improvised workers are used. Figure 7 : Overview of the collapsed Fossano bridge (a) and structural scheme (b) . Fossano bridge: construction and design The collapsed overpass is one of the 108 that constitutes the Fossano (Cuneo) bypass road. This road has several viaducts, and the one involved in the failure is one of the ramps of La Reale Viaduct, where it crosses the S.P. 165. The collapse occurred on April 18, 2017, and luckily (2.45 p.m.) no vehicles were going across or underneath the structure. The sudden collapse took place without out-of-plane displacements (Fig. 7(a)). A modest slip ahead in one of the bearings has been identified in the collapsed configuration. The structure was a simply supported beam, 30.80 m long and 8.90 m wide, constituted by a multiple-box post-tensioned beam and by a in-situ casted slab. The section was realized by connecting two concrete precast U-elements by a shear-key casted in situ. In the longitudinal direction, the bridge was built in 3 segment, the central one 11.50 m long and the lateral ones 9.35 m long. The connection between the segments was realized by an in-situ casted concrete joint 0.5 m wide (Fig. 7(b)). Post-tensioning system constituted in 8 parabolic cables, each of them made by 19 0.8’’ strands. Total weight of the structure was about 400 t. The collapsed structure evidenced an intact joint and a collapsed one, together with an evident shear failure where the bridge impacted the ground. For this event, several information can not be given at the moment due to secrecy related to the running investigations. Visual inspections consented to affirm that the collapse mechanism was triggered by a shear failure in one of the joint due to the absence of the equilibrating action of the prestressing (Fig. 8). This means that in some part between the support and the intact joint, the cables were damaged. Visual inspections immediately also demonstrated that the cables did not have sufficient grout protection in the collapsed joint area (Fig. 9(a)). The damage was not compatible with a 20 years old