19 January 2017

French Bridges: 17. Viaduc des Arts, Paris

Okay, three more Paris bridges to go and then I'll stop.

For this one, it's time to move away from the river to find something very different.

The Viaduc des Arts is an old railway viaduct which has been repurposed into a high-level, landscaped walkway. Just like New York's High Line, except that the Viaduc des Arts was completed in 1994, while the High Line first opened in 2009. The Paris greenway was its inspiration.

The 1.5km long viaduct was originally built as part of the Paris-Bastille to Marles-en-Brie railway line in 1859, and was closed to rail traffic in 1969. The refurbished structure now forms part of a longer route normally called the Promenade plantée, which connects the Paris city centre beyond the Boulevard Périphérique ring road to the Bois de Vincennes park.

The viaduct refurbishment was planned by architect Patrick Berger, and quite literally works on two levels. On top of the viaduct there is a walkway with extensive planting, a very pleasant promenade affording view down into and across adjacent streets. The walkway opens out where existing flat spans have been refurbished to create more open spaces with views in all directions. Up here, you are away from the noise of the city streets below. Stairs and lifts provide access at intervals.

Below, the brick arches have been turned into 64 glass-fronted tenanted spaces, primarily occupied by art galleries and craft studios (hence the new name of the viaduct). The structure has also been extensively cleaned. The occupation of the spaces is nothing new: they were occupied when trains still ran here, but what is new is the provision of a consistent theme and identity.

I think this is the key to the viaduct's successful new life. The walkway is pleasant, but the tenanted spaces presumably generate cash to maintain the viaduct as well as contributing more generally to the local economy and community. The Semaest organisation ensures the viaduct and its occupants are managed and promoted, and I think this level of positive, purposeful activity is key.

I can think of plenty of places where old railway arches are occupied by successful businesses but which lack this sense of community, visual identity, and promotional effort. There's a fine article in the Boston Globe (link below) which goes into more detail on this, and the lessons that other cities might learn.

Further information:

17 January 2017

French Bridges: 16. Pont d'Iéna, Paris

All the Paris bridges I've featured so far here have been in the vicinity of the two islands in the Seine, the Île Saint-Louis and the Île de la Cité. The remaining bridges that I'll feature all lie further afield.

The Pont d'Iéna must be one of the most-crossed bridges in the city, lying directly between the Eiffel Tower and the Trocadéro, although it was originally constructed long before the Tower, or the Palais du Trocadéro.

The five-arch bridge was proposed by Napoleon in 1806, and completed in 1814. Napoleon decided on the bridge's name to commemorate his victory that year over the Prussians at the German town of Iéna. The bridge was built as masonry arches supported on timber piles, and sculptures of Imperial eagles installed above each pier.

However, in the same year that the eagles were installed, Napoleon was defeated by the Prussians and their allies, who took control of Paris and sent Napoleon into exile. One of the winning generals was Gebhard von Blücher, who had previously been on the losing side at the battle of Iéna. Blücher wanted to destroy the Pont d'Iéna but was unsuccessful and eventually dissuaded by his allies. Instead, the eagles were removed, and the bridge renamed as the Pont de l'École Militaire.

Napoleon returned in 1815 before being defeated at the Battle of Waterloo. The eagles were subsequently replaced with the letter "L" (in recognition of the restored King Louis XVIII). After 1830, the bridge retook its original name, and the Imperial eagles were restored in 1852.

The bridge as originally built was not particularly wide (roughly 14m). In 1900 it was widened to 19m in total by the additional of steel extensions to each side. In 1937, it was widened again, this time to 35m in total, by the construction of reinforced concrete arch structures to either side, supported on concrete piles. Steel girders support a bridge deck spanning between the new and old arches. The arrangement during construction of the two extensions can be seen in a photograph of a model at the Ponts de France site linked below. The concrete structures support a stone facing on which the eagles were re-installed. If you look closely at my photos, you can see that the facing is cracked over every span, presumably because it is not properly supported.

In more recent times, the bridge was seen to be experiencing significant foundation settlement of as much as 0.36m on the worst-affected pier. Schemes were investigated including micro-piling and pressure grout injection. From 2015 to 2016, the foundations were strengthened by drilling micropiles vertically through the masonry piers (from deck level), with 12 piles installed per affected pier. In addition, steel straps were installed below water level to these piers, and these straps prestressed by tie bars through the masonry. The straps are visible in the photos as well.

Further information:

15 January 2017

French Bridges: 15. Pont d'Arcole, Paris

This bridge is on the site of a long-since-demolished suspension bridge, the Passerelle d'Arcole, designed by the great French engineer Marc Seguin. The two-span footbridge was replaced from 1854-1856 by the present bridge, designed by Nicolas Cadiat and Alphonse Oudry, in order to establish a crossing for heavier vehicular traffic.

The bridge is a single arch span in wrought iron, 80m long. It sagged suddenly in 1888, leading to strengthening, apparently by adding "two additional trusses".

I wonder if this accounts for the peculiar elevations of the bridge. In my photos you can see that there are reasonably stiff arch ribs, connected to the deck by two superimposed truss systems. There is a larger "W" pattern truss, and then additional triangulation is present in the upper half of the panels only. I'm thinking that this additional metalwork was a later addition, which would have served a dual function, of bracing the main diagonals against buckling, and of introducing more frequent points of support to the main deck elements.

However, this may just be a peculiar feature of the original design. The Planete TP website offers this explanation of the strengthening, which I can't make great sense of:
"In 1889, intermediate arches were added under each footpath and the longitudinal girder anchors were removed from the abutments as they had been responsible for structural defects."
As can be seen from the photos, the bridge looks very good lit up at night (although one panel of balustrade lighting was out of action). It gives a very crisp profile to the ornate balustrades. You might think that a triangulated truss bridge would look less attractive than its neighbours, but I don't think this is the case.

Further information:

12 January 2017

French Bridges: 14. Pont Marie, Paris

I've skipped a couple of bridges, and now we've switched to the right bank channel of the Seine, heading back downstream past the Île Saint-Louis.

Pont Marie is Paris's second oldest surviving bridge, completed in 1635, some twenty-eight years after the oldest surviving bridge, Pont Neuf. It is named for Christophe Marie, who proposed it in 1608; Marie is variously described as a developer and as an engineer. When the bridge was proposed, the island was actually two islands, largely used for pasture, and the bridge was built to facilitate house building. Started in 1614, the same year the islands were joined into one, the bridge took two decades to build.

The structure has five arch spans. There is a niche above each pier's triangular cutwater, intended to house statues, but no statues were ever installed on the bridge. As with many bridges of the period, wooden houses were built on the bridge, eventually being removed in 1788. Again, as with many nearby bridges, the bridge has been repaired and altered on several occasions, including reconstruction of two arches between 1660 and 1670 following collapse caused by a flood, and major works in 1850-51.

As can be seen in my photographs, the largest span is not the central span, but the second span from the left, which is elliptical, while the others are semi-circular. I'm not sure whether this relates to the partial reconstruction of two arches.

Whitney comments on the bridge:
"The ornamentation of the bridge is refined and in good scale. The plain masonry and the proportions of piers and arches give the bridge an appearance of dignity and great strength".
I am happy to agree.

I find it interesting that such an asymmetrical bridge can be regarded so highly - nobody would design a bridge of this type in such a way now without expecting adverse comment. I suspect most observers simply never spot the odd-arch-out.

Further information:

10 January 2017

French Bridges: 13. Pont au Double, Paris

The Pont au Double was originally built in 1634 to connect the Hôtel-Dieu on the Île de la Cité (an early hospital) to an annexe on the left bank of the Seine. The bridge accommodated further hospital buildings along its length, but was also open to the public as a toll bridge, with the fee being a double denier, hence the bridge's name.

This twin-arch bridge was replaced by a single arch structure in 1848 to improve river navigation, and then again in 1883 by the present-day structure, designed by Jules Lax. Today the bridge provides a key link from the left bank of the Seine to the piazza in front of the Notre Dame cathedral.

Various sources describe the bridge as wrought-iron with steel bracing, but this seems self-evidently incorrect. Seen from below, the 11 arch ribs can be seen to be made up of short cast iron segments, although the bracing may well be different in origin.

The two edge arches, and the balustrade above, are coated in copper, a highly unusual feature which makes the bridge gleam in the sunlight. The bridge was extensively refurbished around 2002-2004, and photographs from before this show the copper in its distinctive green weathered colour.

The French edition of Wikipedia refers to a copper galvanising process developed by M. Oudry, which is described in a book on electroplating of metals. The process involves specially varnishing and treating the cast iron before placing it in a bath of copper sulphate, which coats the cast iron object through the normal electrical galvanising process.

I guess the use of this process may explain the short length of the cast iron segments on the bridge, which would have to be short enough to be placed into a galvanising bath.

It's not clear how the coating was re-applied in the 2004 restoration project: there was an article in Bulletin ouvrages métalliques but I don't have a copy, so if any reader can shed more light, please do so!

I'd also be interested in examples of any other bridges where the same treatment has been applied, if there are any. It seems quite unusual, but it's certainly responsible for a key part of the bridge's beauty.


Further information:

08 January 2017

French Bridges: 12. Petit Pont, Paris

I'm continuing with bridges heading upstream along the Seine.

The Petit Pont is at the site of some of the oldest bridges in Paris. Bridges here reportedly date from Roman times, with Emperors Caesar and Julian both writing about bridges spanning across the Seine to the Île de la Cité. A bridge on this site has been destroyed and replaced on numerous occasions.

The present bridge was designed by Alexandre Michal and built by Ernest Gariel from 1852-1853, replacing a previous three-span structure, in order to improve river navigation.

The resulting bridge can at best be described as sturdy, cut from a rough-hewn type of stone which can be seen on other structures in central Paris.

[PS: Readers interested in seeing these Paris bridges, and many others, photographed a few years ago and in daylight, should check out the Bridge of the Week blog - follow the links for April and May 2009.]

Further information:

06 January 2017

Footbridge Awards 2017

The triennial, global Footbridge Awards have returned in time for the Footbridge 2017 conference.

Pedestrian and cycle bridges opened to the public in the years 2014 to 2016 are eligible for submission in one of five categories:
  • Short span: For new footbridges with a maximum clear span of 30m or less
  • Medium span: For new footbridges with a maximum clear span of between 30m and 75m
  • Long span: For new footbridges with a maximum clear span greater than 75m
  • Renovation: For a project to renovate or refurbish an existing footbridge, or to repurpose another type of bridge to be used by pedestrians, cyclists and/or equestrians
  • The Jonathan Speirs Lighting Award: For a lighting scheme on a new or existing footbridge of any size. Created in memory of Jonathan Speirs of Speirs and Major. 
Award entries need to be submitted by 6th March 2017, and the winners will be announced at the Footbridge 2017 conference in Berlin on 6th September 2017.