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Île de Ré Bridge


The Île de Ré bridge - original

Context:


The Île de Ré bridge is a trapezoidal box girder bridge, built from 1986-1988, connecting la Rochelle on mainland France, to the Île de Ré, an island just off the coast. The decision to build a bridge connecting the Île de Ré to the mainland faced much backlash due to its effects on the tourism in the island. The island has 18,000 inhabitants, and before the construction of the bridge welcomed 670,000 tourists a year who would cross the short distance from la Rochelle by ferry. The island now has 3 million tourists a year, marking an almost 450% increase in tourism. The inhabitants of the island and those who had property on the island were frustrated to see so many people crowding their beaches come summer, and transforming the island from the luxurious retreat it once was to one of the most packed holiday destinations in France.


In 2018 during a routine maintenance assessment on the bridge, it was seen that one of the prestressed steel cables in the second viaduct from the Île de Ré which was 440m in length and 15cm in diameter had snapped. This caused great alarm and an immediate survey of the entire bridge was ordered whilst restraints were put on the load and speed with which vehicles could cross the bridge. It was discovered that during construction polyurethane had been injected into the anchor point of that specific prestressed cable which allowed more humidity to reach the cable than concrete would, and this had caused it to corrode and snap. The cable was since replaced and the bridge is now completely uncompromised.



Structure:


Concrete voussoirs - original

The Île de Ré bridge is a trapezoidal box girder bridge, of 2926.5m in length, 16m in width, with a maximum height of 42m and a radius of curvature of 5,000m. The bridge consists of 28 piers, and 23 arches each made of a number trapezoidal box girder voussoirs, of which there are a total of 796. The bridge is separated into 6 viaducts, in between which there are expansion joints, to enable a certain degree of flexibility between segments of the bridge with respect to thermal dilation/contraction and to absorb vibrations from passing vehicles. The trapezoidal box girder voussoirs are made of prestressed concrete, which has been post-tensioned in this case. The girder in this case is a mono-cell in order to allow for the water and sewage supply lines to pass through the bridge, from the mainland to the island.


Road-view of an expansion joint - original

Structural behaviour:


One of the advantages of post-tensioned concrete as oppose to pre-tensioned concrete, in the context of the Île de Ré bridge, is that the steel cables in the concrete corrode less in post-tensioning, due to less exposure to humidity during the fabrication process. This is especially important in a highly saline environment, being on the Atlantic coast, as the steel cables within the concrete corrode more quickly and so it is important that they have not corroded during the fabrication of the prestressed concrete. The use of a trapezoidal box girder instead of a rectangular box girder increases the bridges shear strength, torsional strength and aerodynamics, all of which are essential to a girder used in a bridge with curvature, and especially one which is frequently exposed to high winds due to its location.


Trapezoidal mono-cell girder voussoir

Construction:


The foundations of the piers were lain first, using caissons to establish a dry digging environment, and were dug 12m deep with a diameter of 2m. The piers were then constructed above these, at variable heights, the greatest of which is 42m. A man named, Pierre Richard, designed a new machine to assemble the concrete voussoirs called the projection beam - “poutre de lancement”. At 285m long the projection beam would be fixed to two piers and would place a voussoir first, directly on the third pier, and then would place voussoirs on either side of this first voussoir, in alternation in order to maintain the piers’ balance, before reaching the mid point between two piers and moving the points on which the projection beam was originally fixed to the next pier along. This process allowed for the construction of 20m of bridge a day, greatly reducing the total construction time.


The projection beam in action

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