Issue 46

F. Bazzucchi et alii, Frattura ed Integrità Strutturale, 46 (2018) 400-421; DOI: 10.3221/IGF-ESIS.46.37 407 The number of the analyses that have been carried on investigating this collapse was enormous: simulated rain tests to locate the water gateways, chemical tests for the crystallized residuals and molds, and each segment was carefully demolished and each cable was extracted and inspected (Fig. 10). A complete autopsy was conducted on the bridge. When the investigations will be finished, the civil engineers will learn a lot from the final reports of the investigation committees. Figure 11 : overview of the Polcevera bridge collapse (a) ; structural scheme of the tower (b) ; stay cables replacement intervention (1993) (c,d) [4]. Polcevera bridge: design, maintenance and durability On August 14, 2018, the Tower 9 of the Polcevera bridge, abruptly collapsed on the underlying industrial hub, Polcevera river and rail track of the city of Genoa (Fig. 11(a)). This collapse represents a categorical failure for the Italian Structural Engineering community. Whatever the causes of the collapses would be addressed to, object of the undergoing investigations, it is evident how there have been errors in the check procedure. Starting from the design (1960), the structural system (“ cavalletto ” [4]) that realized the tower relied on the balanced compressive force along the lateral columns (Fig. 11(b)) coming from the stays. The assembled framed tower, plus a four-point supported deck, exhibits a complete absence of robustness and structural redundancy, which means that in case of failure of one of the components, there was no control in preventing a catastrophic collapse [5]. Many colleagues affirmed and condemned the designer for the collapse. As written in the introduction, we can affirm that the lack of robustness was normal for the typological structures of that era and consequent to the available tools in the 60’. It is easy today to perform analyses for highly indeterminate structures by using computers, but it was not at that time. The structures were designed by using the simplest possible schemes, in order to have easier calculation to be made. Consequently, no ductility expedient, at any scale, was adopted to exclude fragile collapse [6]. Robustness and ductility are two design control parameters for structural safety in the ultimate state of collapse. Moving forward along the chain, maintenance and monitoring are two control activities that usually go along with an important bridge structure. Because of its audaciousness, Polcevera bridge has been continuously investigated and consolidated over the years. Along the 70s, the designer himself, started with regular visual inspections [7], while over the ‘80s a distributed restoration of the concrete components was carried out [4] starting from the towers and then moving through the deck. In 1993, a detailed inspection through the insertion of an endoscopy inside the stays of the Tower 11, evidenced a severe oxidation of the metallic protective membrane and of the strands themselves. Additionally, significant presence of humidity and absence of grout were reported. Moreover, some of the strands appeared slacked or sheared, sign of a tensile rupture due to steel cross-section reduction [7,8]. This preoccupying scenario advised the entire structure management authorities over the risks of material decay due to micro-cracking in the prestressed concrete. In fact, the concrete section of the stays was designed to be subjected to light but continuous decompression-compression cycles due to live loads. The peculiar cables disposition, core of the M5 patent [9], had the aim to avoid large stress variations in the steel strands, but this was possible only with a magnitude of the tendon force close to the ultimate one. All the described conditions led the structure to a quickened state of material decay and the entire sets of stays of the Tower 11 were replaced with an external prestressing system (Fig. 11(c), Fig. 11(d)). After this massive intervention (12 cables  150 mm with 22 0.6’’ diameter strands for each stay and 4 months of work with reduced