Context:
The Pont des Arts, originally a cast iron arch bridge, known more often as the “Love Bridge,” crosses the Seine river in Paris, connecting the Louvre to the Institut de Paris, formerly the Collèges des quatre Nations. The bridge which stands today is not the original but an altered version constructed in the 1980s. The first bridge was commissioned by Napoléon and built between 1801 - 1804 to show the British - who were world leaders in the use of cast iron - that the French were able to rival their prowess in using the material.
The Pont des Arts was a very light-weight and efficient structure, differing from previous cast iron bridges in the UK such as the Iron Bridge crossing the river Severn, or Warmouth Bridge, which were both significantly more cumbersome. The Pont des Arts was the first cast iron bridge in France and it paved the way for the increasing use of cast iron in construction.
The bridge underwent a reconstruction in 1980-1984 due to several barges having collided with the piers throughout the 1970s and completely undermining the structure. The bridge earned the name the “Love Bridge,” due to the romantic tradition of placing padlocks on the bridge gates to symbolise eternal love. Due to the immense weight of the padlocks, - 45 tons - the bridge gate collapsed in 2014, and the bridge gates have since been replaced and covered by glass panes to avoid a reoccurrence of such events, however the bridge still retains its name of the “Love bridge”.
Structure:
The original bridge had 9 arches, was 155m in length and 11m wide and was made using cast iron. The number of arches was then reduced to 8 in 1952 due to the amalgamation of the two left-most arches. However, when the bridge was reconstructed between 1980 and 1984, the 8 arches were replaced by 7 to allow the bridge to fit in more seamlessly with the neighbouring bridges, and allow for barges to navigate more easily, with a total length of 155m and a width of 11m and using steel. The arch is composed of five parallel steel arch ribs, with cross-bracing and the piers are of reinforced concrete and have a stone coverage.
The use of steel in the rebuild is due to the ductility that steel has as oppose to very brittle cast iron. Cast iron has a carbon content of between 2%-4% whereas steel has one of 0.1% - 0.5%. This meant that in the early 19th century, cast iron was much easier to machine than cast steel because: it has a lower melting temperature - 1260ºC v.s 1427ºC for cast steel - and it does not shrink as much when cast. The shrinking of steel upon casting was a huge issue in construction in the early 19th century because there was no way yet to successfully ensure that the shrinking would not cause large amounts of internal tension within the steel, making its use in construction dangerous. In addition to this, cast iron has much better vibration dampening qualities due to the graphite structures within it, has great compressive strength, resistance to corrosion and was much cheaper than cast steel. However the pitfall of cast iron is its brittleness, which reduces its resistance to large impact forces. This was shown in the way that the bridge sustained serious damage, making it completely unsafe to the public, after several barges collided with it in the 1970s.
Both the original and current bridges were/are highly efficient in their usage of material, only using materials where essential to the structure. The quasi transparency of the bridge is testament to that, and in fact the only part of the bridge which is non-ergonomic, is the stone cladding around the piers. This meant that the original bridge, at the time, was a one of a kind in Europe, becoming a staple of the more utilitarian style of construction.
Structural behaviour:
The use of the arch structure puts the steel into compression throughout the arch, and the use of a second, smaller arch in the spandrel, along with a truss which is in tension, aids in redistributing the downward force from the load on the deck directly above the pier, to the steel arch, which then puts the reinforced concrete piers into compression, to reduce the risk of buckling. The cross-bracing between the parallel ribs of the arch is in order to increase the torsional strength of the deck, to better resist to wind gusts, although the risk posed by wind in Paris is relatively limited due to the sheltering from buildings surrounding the Seine.