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.

Further information:

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.

Further information:

16 October 2014

London Bridges: 36. Olympic Park Bridge 20

Right next to London's Olympic Park Bridge 14, you find Bridge 20. Don't ask me how they numbered these things!

Bridge 20 is an 85m span highway bridge, which carries the roadway across two levels of railway tracks, the Channel Tunnel Rail Link and the Docklands Light Railway. Like Bridge 14, it was designed by Arup and Knight Architects, and built by Morgan Sindall.

The steel arch is 18m tall, and sits in between two single-lane carriageways. The tied arch is a fabricated steel box, which varies in cross-section from base to crown in an unusual manner. A weathering steel spine box girder sits below the deck and is suspended from the arch by cables. The deck is supported on either side of the spine girder with weathering steel cantilevers.

The lower end of the hanger cables is protected from vehicular impact by a concrete upstand beam. The edges of the bridge are protected by concrete parapets. Both parapets and upstand beam contain low-level lighting units, eliminating the need for conventional streetlighting.

The strength of the bridge's design is in its simplicity, particularly the use of the single arch, which renders it an immediately legible landmark. The arch is nicely shaped in elevation, being wider at its base than at its crown, but its width is a little disconcerting - it is wider at the crown than at the base. From some angles this gives it a top-heavy appearance, and it doesn't seem to make structural sense to me, as resistance to lateral buckling of the arch is to a large extent determined by the transverse fixity at the ends.

The cantilevers supporting the deck from below are another detail which is not entirely happy. They make use of the full depth of the spine beam, meaning they are stronger and stiffer than the shallower cantilevers more normally seen. This allows large parts of their material to be cut away, but also has the effect of making these essentially secondary elements visually quite prominent.

Perhaps this doesn't matter, as the underside of the bridge cannot be seen from most public vantage points. Although my photos show a small road passing below, this is a private access road behind a secure fence. Perhaps future developments in the area may render this part of the bridge more visible.

The issue of perspective is significant. To photograph the underside of the bridge I had to walk down a dead-end road and peer through a security fence. To photograph the bridge in elevation, I had to peer over a bridge parapet which clearly was not meant as a vantage point - there was no footway in front of it!

Along with the other interesting bridges to emerge from the Olympic park, Bridge 20 can't disguise the essential issue with the whole area, which is that it is criss-crossed with waterways and railway lines, both of which render significant parcels of land unreachable. The bridges are essential to make connections across this landscape, but the focus on their external appearance belies the fact that they are principally viewed by the people actually crossing each bridge. With this in mind, both Bridge 20 and Bridge 14 present a somewhat sterile, industrial appearance to their users, almost brutalist in their treatment of the streetscape.

Further information:

14 October 2014

London Bridges: 35. Olympic Park Bridge 14

I recently visited a handful of bridges built in or near the London Olympic Park. I've covered the park's central bridge last time, and will cover the remainder over the next few posts.

I thought I would be able to say little about Bridge 14, as for such an architecturally interesting bridge, it is almost invisible. It is a pedestrian and cycle bridge which sits alongside a highway bridge and spans over a railway line.

The facts are fairly straightforward: it's 57m long, 6.7m wide, and weighs 203 tonnes. It was designed by Arup and Knight Architects, and built by Morgan Sindall (the contractor only credits their own designer Benaim, and neglects entirely to mention the architect on their website, which is rather poor form).

The weathering steel footbridge girders are 1.8m tall, and double as the parapets. The height, and the solidity of the parapet, are standard requirements of Network Rail for an electrified railway line.

The bridge's most interesting feature is the architectural patterning to the web stiffeners on the external girder. The profile of the stiffeners is varied along the length of the bridge to create a "wave" effect. Anti-climb plates have been included unobtrusively.

Every third stiffener is required structurally to stabilise the girder against buckling, while the others are slightly thinner and entirely architectural, The wave pattern has no structural rationale to it, which I think is a shame, as the web and flange buckling effects are not constant over the span and therefore could perhaps have been used to inspire the stiffener geometry.

Unfortunately, this fascinating girder faces onto the railway, and areas of land inaccessible to the public. It can be seen from a distance, as in my zoomed-in photograph here, but not from close at hand. If you want to see what it looks like, try the Knight Architects website linked below. I am unclear whether surrounding developments will leave this highly sculpted facade permanently invisible to the public!

On the side where the bridge abuts the presumably pre-existing highway bridge, it has the same sculpted girder, except here the girder is hidden within a cage, perhaps to ensure its sheer visual awesomeness doesn't escape?

As I couldn't get a decent photograph of the bridge from its most interesting side, I've been reduced to offering close-ups of the inside face of the girders, in the hope that there are some keen students of weathering steel out there. The inside faces are protected by stainless steel mesh covers, with lighting accommodated behind. However, the upper part of the girder, a "steeple coping" intended to prevent delinquents walking along its length, has already suffered from graffiti, which will be tricky to clean off from this material. The upper girder weld also looks pretty awful.

This poor bridge has been horribly compromised by the realities of its location, and worse, by the way it has been accommodated in the site. Ugly guardrails and palisace fencing at its end add to the impression that the designers have provided the concept for the bridge but been offered no involvement in how it integrates with its approaches.

For a structure with such interesting visual potential, it has been horribly mistreated.

Further information: