Context:
The Tancarville bridge is a suspension bridge crossing the Seine river from Tancarville to Marais-Vernier near the Havre. The bridge was constructed between 1955 and 1959 for a total cost of nine billion francs, with the aim of allowing those living in the Havre to cross the estuary with greater ease, enabling subsistence. The people of the Havre used to cross the estuary through a system of boats that ran at regular intervals, because the nearest bridge was much further into the estuary than was convenient. However, crossing the estuary by boat was extremely impractical because it did not allow for large volumes of produce and/or many vehicles to cross at one time. Whilst there were many propositions made between 1931 and 1951 for how the crossing should be made, including a subterranean rail line, it was finally decided that the best way to link Tancarville to Marais-Vernier was through a suspension bridge.
Structure:
The Tancarville bridge is a suspension bridge. A suspension bridge is one in which a deck is supported by suspender cables. These suspension cables are anchored at the abutments, but importantly, they are not anchored at the top of the pylons over which they pass.
The suspension cables are in tension and relate the downwards force caused by load on the deck to the pylons where the force becomes compressive. The abutments at either end of the bridge counteract the horizontal tension from the suspension cables by anchoring the cables.
The deck is at a height of 50m in order to let boats pass through the estuary and has a span of 608m. The pylons are each 125m from ground level - a world record at the time - however the left pylon has its foundations 28m below ground as oppose to 18m for the right pylon, due to the inconsistency of the marshy riverbed. The abutments are enormous, the larger of which famously has almost the same surface area on the ground than the Arc de Triomphe, at 47 by 47 by 40 metres. The two suspension cables are 1070m long from one abutment to the other and are composed of 60, 72mm diameter steel cables each of which consist of 169 4.7mm diameter steel strings. The strings and cables are twisted together by a machine to create the suspension cables. The process of twisting the cables instead of bundling them together is to add torsional strength to the cables.
Structural behaviour:
The benefits of suspension bridges are that the cables are highly efficient at carrying loads, and the risk of buckling in the deck is extremely low meaning that the dead load is greatly reduced whilst allowing for a greater span. Equally, suspension bridges are not very rigid due to the deck being supported by elements capable of moving quite freely. This makes them ideal for areas susceptible to earthquakes or high winds, the latter of which are very present in the estuary of the Seine.
However whilst suspension bridges are ideal in high wind environments, this means that the deck of suspension bridges is often subject to large torsional forces which risk undermining its structure. To counter this, the deck in the Tancarville bridge has a Truss structure. Truss structures are triangulated structures composed of straight beams or elements, connected at nodes, which allow a more even distribution of loads, and because of the rigidity they offer, mean that the trussed element, acts as one single element as oppose to a collection of individual beams. This means that the trussing of the deck is a very effective way to counter torsional forces it may be subject to.
Construction:
Construction of the Tancarville bridge was made difficult by the geology of the terrain. The Havre area is a very marshy one and this meant that it was necessary to dig very deep in order to reach more stable bedrock and lay foundations. The foundations for the pylons were established by sinking an armed concrete caisson 28m into the ground for the left pylon and 18m into the ground for the right pylon, to create a dry environment from which the pylons could be built.
The abutments, which need to resist large traction forces from the suspension cables, were constructed from prestressed concrete, due to the material’s characteristics. In addition, two access viaducts on either side of the bridge bring the road level to that of the deck in the suspension bridge. These viaducts were one of the first parts of the bridge built because they then enabled the transport of materials necessary for construction to the elevated work site. The access viaducts are composed of concrete voussoirs of 7.25m in length and 24 pillars, which rest on armed concrete foundations.
Whilst the bridge has remained largely the same since construction, the original suspension cables were replaced from 1996 to 1999 by two suspension cables on each side to better support the deck.
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