“On very long spans, the major issue is wind stability. The designers had the idea of making two parallel decks in boxes 9 meters apart, instead of just one, and connecting them using spacers. It is a world first which has made it possible to obtain both an extremely rigid structure and a very thin aerodynamic profile”notes Olivier Flamand, expert on the subject at the research center of the Scientific and Technical Center for Building, in the Paris region.
The establishment, which specializes in the analysis of behavior in the wind tunnel, had studied a competing project for the Dardanelles, with Michel Virlogeux. While the previous world record, the Akashi suspension bridge inaugurated in 1998 in Japan (1,991 meters), has a classic truss deck, the Dardanelles uses the same principle, but horizontally. Like a ladder laid flat, 82 meters above the sea.
At the Dardanelles, could a cable-stayed bridge have done the job? No, because this category of work can barely exceed 1,000 meters in range and between the Aegean Sea and the Sea of Marmara, the depth of the water reaches about sixty meters, which makes the erection several very expensive bridge piers. In the cable-stayed model, the loads from the deck rise by cables inclined towards each pylon, in a symmetrical manner. Horizontally, they balance each other. Vertically, they are transmitted to the ground by the foundation of each pylon.
“In the suspended model, they go up by vertical hangers to the two main cables, which are deviated to the right of the pylons and then transit to the shores where they are anchored in the ground almost horizontally, by means of two imposing massive concrete essential to the balance of the structure. Deviation saddles arranged at their entrance dispatch the two cables into elementary strands which are anchored by means of prestressing cables », indicates Julien Erdogan, director of Central and Eastern Europe of Freyssinet, a subsidiary of the Vinci group which intervened on this sensitive part, in the Dardanelles. The foundations of the two pylons are another subject. Here, the pylons are simply placed on the seabed, so that they can move in a horizontal plane in the event of earthquakes, which are frequent in the region.