Five of Virlogeux's bridges
The Seyssel Bridge over the River Rhone
Designed by Michel Virlogeux, this bridge takes advantage of a small island in the river. A cable-stayed bridge design was chosen with the tower on the island. He chose to place intermediate piers on the shallow side of the river, which many objected to, however he said removing them would very much change the shape of the bridge. These intermediate piers very much stiffen the bridge, and the stay-cables very efficiently maintain the tower which, as a result, can be extremely slender.
The Morbihan Bridge over the River Vilaine
An arch is a structure which resists extremely heavy loads well, as long as the arch is symmetrical. When the arch is built, if you launch your heavy structure (deck) on the arch you will have a very important bending moment in the arch. The advantage of launching a steel structure is that it is very light, and afterwards you can cast the slabs symmetrically in such a way that you control the bending moment in the arch, and so you can have a rather slender arch. When this bridge was being designed it was imperative to give pedestrian access to each bank of the river where locals come to jog and walk. Virlogeux thought it stupid to put joggers in with the smoke of the cars, fortunately he had been inspired by an 18th Century Japanese bridge which allows people to walk on the arches of the bridge, which is what they’ve done with the Morbihan Bridge.
The Burgundy Bridge at Chalon sur Sa˘ne
The shape of the tower has been improved with the input of an architect, but the design was chosen for purely technical reasons. The deck has to pass between the two legs of the towers. He wanted to have vertical stay-cables to ease the anchorage of them, and for this reason, the legs had to be closer at the top, widen to allow the deck and close underneath to limit the size of the bases. You can very clearly see the flow of forces from top to bottom.
The Antrenas overpass on the A75 Motorway
The architect originally designed this bridge with a concrete arch, but thought it too reminiscent of the 20s and 30s so went to Virlogeux for his input. He suggested the arch be made of tubes with a prestressed concrete slab on top. The tube is not circular, but segmental because between the different nodes where the loads come from the slab there is practically no load, so it is much more efficient to have a segmented arch. If you compare the bending moment between a circular arch and a segmental one, they are much more smaller.
The Languedoc Bridge at Gignac
A new bridge had to be build near a historically important old one, thought of as one of the two most beautiful bridges in South France (seen behind). So the first decision was to build the new bridge at some distance from the old to avoid conflict between the two images and in addition, Virlogeux chose an arch design to complement the old bridge. Because of the likelihood of flooding of the river, it was necessary to have some spans on the left bank. And it was very difficult to arrange the system between the arch over the river and the approaches on the left bank. Using a vertical pier on the left bank would have been very ugly so they chose instead to balance the main arch of the bridge with a counter arch on the other side.
Contractor Vol.33 No.6 July 2009
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