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.

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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.

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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.

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21 October 2014

London Bridges: 38. Chobham Academy Footbridge

It's time to finish off my round-up of bridges from London's Olympic Park. There are a couple of interesting bridges I've not visited yet, so hopefully I'll follow these posts up on a later occasion.

Chobham Academy is a brand new school built as part of the Olympic "legacy" developments, to take advantage of the infrastructure left behind from the Olympic games. The school is separated from its playing fields by a road, Temple Mills Lane, and a footbridge has been built to save the students from dodging cars on their way to and from the playing fields. It's difficult to escape the notion that a pedestrian crossing would have cost a lot less, but hey, we get a bridge to look at, so never mind.

The bridge has been designed by architect Allford Hall Monagham Morris, with engineers AKT II, and as with many Olympic Park bridges, is built in weathering steel. Like the nearby Bridge 1, it uses the common footbridge form of a half-through arrangement, where the two edge girders form the parapets and the floor plate forms the bottom flange. This minimises the height of approach gradients.

As with the standard Network Rail rural footbridge design, to which this bears a clear family resemblance, stiffening U-frames wrap around the floor and webs, restraining the web and top flange against buckling.

The bridge trough is supported on two main supports, each a tetradactyl arrangement of weathering steel fingers or branches, on which the trough is perched. The "finger piers" are in turn supported on chiselled concrete plinths.

The trough varies in height, being tallest at the middle of the centre span. This is a decidedly odd arrangement, as it makes the bridge strongest at what must be one of the most lightly stressed places: as a continuous bridge, it should be most heavily stressed above the pier supports.

The most significant design feature on the bridge is that the u-frame stiffeners have had their spacing varied, concertina fashion, ostensibly so that the ribs are bunched most closely together in areas of highest shear stress. The effect appears grotesquely exaggerated from most viewpoints, and has certainly been played for effect, as the spacing of the stiffeners becomes much closer together than can provide any real structural benefit. I guess the u-frames are also doing very little to stabilise the top flange, which will be in tension over all or most of the span.

Whatever the rationale, I find the effect highly visually uncomfortable. I guess it has been inspired by the other stiffened-girder Olympic Park bridges, but it has been done far more crudely.

Although the deck is clumsy, I do really like the finger piers. The shaping of the weathering steel branches is excellent, and very well integrated with the support plinths. It makes me think of Constructivist art, or perhaps a skeletal Richard Serra.

It's interesting to see the ageing process on the bridge's weathering steel. The weathering to the two sides is different, presumably because of the wind or sun, or a combination, and the weathering to the underside is very different to the side faces, presumably due to the shelter and lack of moisture running along the underside. Various parts seen close up show "streaking" due to where rainwater has run, although I am sure this will vanish over time.

As with the other weathering steel bridges in the Olympic park area, it will be interesting to visit again in years to come and see how their appearance has changed.

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19 October 2014

London Bridges: 37. Olympic Park Bridge 1

Of the London Olympic Park bridges I've covered so far, this is certainly my favourite. It connects the former Olympic athlete's village (now a residential development) to the neighbourhoods of Leyton and Forest Gate. A road bridge always carried Temple Mills Lane across the railway tracks here, but it is narrow and barely with room for pedestrians. A new pedestrian bridge therefore offered a significant improvement in the quality of the local link.

The new footbridge sits immediately alongside the existing road bridge. It was designed by Knight Architects with Arup, and spans 35m over railway tracks. In form, it is a half-through steel twin-girder bridge, the conventional solution for a footbridge over a railway. The advantage of the arrangement is that it maximises clearance below the bridge, by combining the function of the support girders with the function of an imperforate parapet, a standard requirement of the railway authorities.

There are standard designs of this type, but this design is far from standard. As with many of the Olympic park bridges, it uses weathering rather than painted steel, to avoid the need for future maintenance painting above an electrified railway. This looks attractive from a distance, although it's very noticeable close up quite how uneven the appearance of the steel can be.

The girders are painted on their inner face, so that any graffiti can simply be painted over. Their governing feature is their sheer height, determined by the fact that here the bridge spans not just a conventional railway, but part of a high-speed railway line (the sidings to the train depot, I believe). It's easy to imagine what an oppressive passageway could have resulted, but the designers have worked hard to mitigate the girder height, by inclining the girders outwards, and by using only mesh panels for the highest parts. The mesh is given a varying inclination to set up a geometrically attractive curved intersection between the mesh and its support girder.

Stiffeners in the girder, required mainly to stabilise the structure against buckling, are expressed on both the inside face and the outside face, with a varying pointed edge to the external stiffeners again providing the visual interest.

It's a very attractive bridge, although I have to note that, as with some of the other Olympic park bridges, its most attractive side is a little wasted at present. One side of the railway is private land, from which the public can't see the bridge, and on the other, it is a public park from which views of the bridge are limited. However, it's entirely possible its visibility will change over time.

Although the bridge's mesh panels are a little austere, my overall impression of the bridge is that it is likeable, even friendly. It lacks the "security-facility" aesthetic that some of the other nearby bridges display to their pedestrian users.

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