The Happy Pontist

12 November 2014

French Bridges: 6. Carpentras Aqueduct

I leave Avignon behind now, for two final bridges from the south of France.

The aqueduct at Carpentras is a magnificent structure. It was built over a 14 year period, beginning in 1720, to the designs of local architect Antoine d'Allemande with Jean de Clapiès, to carry water from the north into the city. De Clapiès seems to have been an interesting fellow, a Director of Public Works but also an astronomer and professor of mathematics.

The structure has 48 masonry arches, 729m long in total, and up to 23m tall. At its top, it is only 1.75m wide, and carries a channel a mere 0.25m in width.

The aqueduct no longer supplies water, but survives as a historic monument. The main line of arches is impressive but also quite unusual, with an enormous taper to the piers, well beyond what is required for stability.

The most interesting features of the viaduct are at its southern end, where the aqueduct changes direction and is combined with a lower-level structure, suitable only to carry foot traffic. This part of the viaduct spans the River Auzon, and the spans are larger and made architecturally more prominent. The footway spans step out around the pier legs for the taller aqueduct, carried in part on little squinch arches.

I love this bridge both for its magnificence and its idiosyncrasy.

Further information:

10 November 2014

French Bridges: 5. Rhone Railway Bridge, Avignon

I can tell you almost nothing about this bridge. It's just a big viaduct which carries the railway across the River Rhone at Avignon.

It lies relatively close to the water, and so gives some idea of the river's scale. At this point, the river is about 450m wide, and the bridge totals about 560m long, as it also spans highways and flood defences on both banks. Each individual span is about 75m.

The bridge is brilliant in its single-mindedness, comprising braced steel lattice truss girders unvarying in height, supported on masonry piers within the river. There's often something exquisitely beautiful about a bridge which appears to make no concession to beauty.

The most visually interesting feature of the bridge is surely one which was unintentional. The latticework on the trusses uses large diagonal elements which are formed out of steel angles. These inevitably create a pattern of light and shadow on the face of the girders. The arrangement has been alternated between spans, so that on a sunny day, each span of the bridge appears to alternative between a dark and light colour (third photo below).

Further information:

08 November 2014

French Bridges: 4. TGV Viaduct, Avignon

There are many impressive bridges in this part of France, but the twin TGV viaducts spanning the River Rhone are particularly spectacular.

They were built between 1996 and 1999, to a design conceived by Michel Virlogeux with Jean-François Blassel, and developed by RFR. Built as part of a TGV route extension, they were reportedly the first viaducts of their type to be built by the French railway body, SNCF.

The viaducts are each 1500m long, comprising a number of 100m main spans and 50m approach spans. They are post-tensioned concrete box girders, built in precast segments and erected using giant launching gantries. Both internal and external prestressing are incorporated.

A special white concrete was used in recognition of the visual sensitivity of the site, and taking into account distant views of the highly historic Palais des Papes in Avignon.

The most interesting feature of the bridge, visually, is the relationship between the concrete girders and the support piers. Tapering circular piers are topped with chiselled caps, widening out sufficiently to carry the bridge bearings. The lower face of the concrete girders dips down towards these points, while also widening out. This looks okay from some angles, but decidedly odd from others. In some views, it creates a pronounced and unwelcome visual kink.

As Britain develops plans for its second high-speed rail line, HS2, it will be interesting to see whether designers can match the high quality of structures such as the Avignon TGV viaduct. British designers are being challenged to develop repetitive, modular designs which can be largely pre-fabricated off site, and there will be a tension between the need to provide a consistent visual identity for the project against the need to consider the context of different sites through which it passes.

Further information:

31 October 2014

French Bridges: 3. Rognonas Suspension Bridge, Avignon

After the second world war, several European countries commenced a programme of reconstruction and development. In France, this led to a series of modern suspension bridges, of which the first was the Rognonas Bridge at Avignon.

This bridge spans 255m across the Durance River to the south of Avignon. Completed in 1950, it is very similar to the 238m Ancenis Bridge built three years later. Both bridges have truss-stiffened decks, with the riveted steel truss partly above and partly below roadway level, and both have concrete portal towers with concrete latticework at their top.

Structurae credits the construction of both bridges to Baudin-Châteauneuf, and lists Jean Courbon as designer of the Ancenis span. Boussiron is credited with involvement in building the Rognonas bridge.

Richard Scott comments on the design:

"Its heavy half-through truss contrasted with a finely etched diagonal grillage in the top portal strut of its concrete towers that, for all its elegance, was taxing to build".

Looking back through my photos, there are a few aspects of the bridge which seem particularly notable.

It's not just the concrete lattice between the towers that is visually attractive - the towers are generally well finished, with some pleasing contrast between rough and smooth surfaces. They are also nicely proportioned, widening only fractionally at the top to accommodate the main cable saddles.

The bridge trusses are discontinuous at the towers. That's not a common arrangement in 20th century suspension bridges, and I wonder how many examples there are.

The main cable arrangements are unusual only if compared to larger spans, being assembled from several smaller cables clamped together, rather than having been aerially spun.

This is a fine bridge where simple, straightforward engineering has been married to a minimalist and highly attractive aesthetic.

Further information:

28 October 2014

French Bridges: 2. Pont Édouard Daladier, Avignon

The River Rhone divides Avignon from Villeneuve-lès-Avignon, and is itself divided by an island, the Île de la Barthelasse. The Pont Saint-Bénézet previously connected all three, but was progressively destroyed by floods, becoming unusable before the beginning of the 18th century.

The river was then crossed only by ferry, until a new timber trestle bridge was completed in 1812. This did not last long, partially collapsing in 1821, and then experiencing further damage over the next two decades. It was replaced in 1843 (on the Avignon channel only) by a suspension bridge, designed by the Seguin brothers, which was in turn replaced in 1888 by a hybrid cable-stayed / suspension bridge, designed by Ferdinand Arnodin.

The present bridge replaced Arnodin's span 1972, and is named after a French government minister who became the Mayor of Avignon. The Avignon branch of the river is bridged with two spans. Each of these spans consists of two parallel concrete box girders. The pier in the centre of the river is extended both upstream and downstream with cutwaters, presumably essential given the Rhone's history of flooding. These have an odd, tacked-on appearance.

I find the approach spans more interesting. These are plain reinforced concrete arches, with their internal structure seemingly made apparent on their elevations. The western approach is a classic of robust, no-nonsense design, while the eastern approach is rendered attractive by a curtain of vegetation. There's a sense of something both enduring and on the verge of ruin, which I think pays suitable hommage to the historic Pont d'Avignon a short distance upstream.

Further information:

25 October 2014

French Bridges: 1. Pont Saint-Bénézet, Avignon

Earlier this year I spent some time in the south of France, and while in the Avignon area I had time to visit a few bridges. I'll cover six or seven of them over this and the next few posts.

The Pont d'Avignon is one of the most famous historic bridges in France (perhaps in Europe), forming part of a World Heritage Site. I won't recount its history here (Wikipedia offers an excellent summary), but it's interesting to query quite why it is such a famous bridge.

Built in the 12th century, it was not especially structurally innovative. It may have been considered beautiful, but it has not been sympathetically treated over the years. Originally, it was a lengthy viaduct of 22 arches, spanning the entire width of the Rhone (which is split at Avignon into two channels with a central island). Today, only 4 arches survive at the eastern end, the others having long since been destroyed by floods.

The stone arches are elliptical, or possibly three-centered, in profile, and are built in four parallel sections. Perhaps this was inspired by the nearby Roman Pont du Gard, the lower section of which was built in the same manner. Today, it seems an odd decision, sacrificing integrity for what I can only guess was short term economy (the ability to reduce the cost of the timber centering by reusing smaller sections).

The arches sit on massive stone cutwaters, but these evidently did not suffice to resist the river's might. The largest surviving pier supports a bridge chapel, with chambers both above and below the main bridge level. The walkway surface is now flat and does not follow the rise in the bridge arches, evidence of unsympathetic restoration works undertaken in the 19th century.

A gatehouse survives at the extreme eastern end of the bridge, with a twin-section timber drawbridge used to bar access. The drawbridge is still raised every evening and lowered every morning, but the only users of the bridge now are tourists who have paid an entrance fee. The ruined bridge goes nowhere - it is simply a pier, where visitors pay for a sense of contact with antiquity.

Perhaps there is something of the "romance of ruins" at work. Certainly, the Pont-Saint-Esprit, a quite similar bridge from the 13th century, which spans the Rhone further north, is nowhere near as well known, and that bridge remains intact (albeit heavily modified). Perhaps the bridge's fame is simply because it is the subject of a well-known song, but surely the song became popular because the bridge is famous, rather than vice versa.

I took many photographs of this bridge.