## 4 Discussion about the Causes of the Viaduct Collapse
### The Destruction Stages of the Viaduct  
The viaduct collapse began with the rupture of the diagonal stay cable. The mechanism of changes in the mechanical properties of the stay cable was as follows:  
1. The value of the pre-stressing force in the strained cables was reduced as a result of pre-stressing losses.
2. Due to the large dead weight of the cable, its shape changed from straight to curved and this led to a decrease of the compressive force, causing tension in concrete at the bottom surface of the cable.
3. The decreasing value of the compressive force in the cable as a result of pre-stressing losses and changes in the shape of the diagonal stay cable caused cracking of the concrete, starting from the bottom surface of the diagonal stay cable.
4. Cracking of concrete resulted in a decrease of the area of the compressed concrete cross-section that transferred the tension forces.
5. The cracked concrete ceased to provide adequate anti-corrosion protection of strands in the tendons and allowed for steel corrosion.
6. The strands not protected with concrete cover corroded and successively cracked, causing a decrease in the steel-concrete cross-sectional area of cable transferring the tension forces.
7. The decrease of the cross-sectional area of the concreted cable under constant forces resulted in increased stresses in the corroded tendons.
8. With such a decrease in the value of the pre-stressing force in the cables that concrete was not compressed in no place of the cable's cross-section, the tension forces were transferred only by the corroded tendons.
9. When the load is transferred only by the corroded internal tendons, the elongation of the cable would be greater than 20 cm.
10. The successive ruptures of the corroded strands in the steel tendons under constant force would cause an increase of stresses in other unbroken strands and their excessive elongation, until the rupture of the last strand in the tendon.  
Figure 11 shows the various stages of the destruction of the viaduct. The mechanism of viaduct destruction was as follows:  
1. The viaduct superstructure allowed for direct loading of the cantilever with three trucks-at the time of the viaduct collapse, presumably three trucks were on the cantilever.
2. The cantilever was structurally connected with the diagonal stay cables, with poor technical condition that could not transfer such a heavy live load.
3. The overloaded cantilever resulted in a rupture of one of the two cables, loaded more in the upper part due to the dead load of the cable than in the lower part (the weight of 1 m of the cable was ca. 3 t (1.1956 \(\times\) 2.5).
4. The north-western cable was broken at support No. 9 (Figure 11a).
5. The broken cable with a weight of about 280 t (92.2 \(\times\) 1.1956 \(\times\) 2.5) fell on the cantilever, which resulted in the destruction of its structure and the rupture of the south-western cable (Figure 11b) located on the other side of the carriageway (the force should be determined taking into account the dynamic impact factor, e.g., equal to 1.5).
6. The destruction of the cantilever resulted in a collapse of the suspended span supported on the cantilever (Figure 11c).
7. The imbalance in the load of the pylon on both sides caused its destruction, and this led to the collapse of the cables, the cantilever, and the span at the eastern side of the pylon (Figure 11d-f).  
The study on post-collapse analysis of [PERSON]'s Polcevera Viaduct in Genoa Italy was presented by others [11]. Their study employed the existing knowledge in estimating the remaining life of the bridge, including: the existing design information about the bridge; post-collapse investigation report of the ministry of infrastructure and transport [12]; and archival truck traffic and axle load data.