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:

12 October 2014

London Bridges: 34. Olympic Park Footbridge (revisited)


The site of the 2012 London Olympics was massively redeveloped for the event. Alongside the sports stadia and facilities, an extensive path network was built, and a large number of temporary and permanent bridges. During the Olympics itself, many of these bridges were either inaccessible or difficult to reach. Now that the event has long since left town, the Olympic park has been turned into a public park, and it's finally possible to see some of the infrastructure legacy properly.

The centrepiece bridge in the Olympic park was the winner of a design competition. I first visited it in the Olympic year, when it was temporarily configured as a wide plaza-type bridge, barren from above and ugly from below, largely due to the nature of the temporary decking used.

I won't "explain" the bridge here again in any detail, as you can read all that in my previous post. However, the permanent structure, with the temporary decking removed, is a much more successful bridge than before. It is Z-shaped in plan, with two main bridge decks joined by a secondary walkway (for an utterly bizarre explanation for the layout, see YouTube).

The original competition visuals from architect Henneghan Peng showed the area carved out into a lush, green basin, with the bridge as a series of shimmering stainless steel elements perched above. Cost, or pragmatism, has done away with most of the greenery, and the landscaped basin is somewhat barren. This highlights the fact that the bridge spans a canal, and that at the lower level, there is no way across. It is therefore not an especially pleasant space, with the main attraction being the shifting reflections to be seen in the bridge decks above.

Those decks are looking pretty good now. The bridges are steel trusses clad in segmental stainless steel panels. They've been nicely cleaned up since the Olympics, and their segmental nature helps break up the reflections in an interesting way.

The bridge abutments, environmentally friendly gabion banks, are less visually successful, looking cheap and nasty. It would have looked far better if the bridge had emerged direct from the ground rather than from an array of fussy and awkward detailing.

At the upper level, the bridge still lacks any kind of character. The parapets are detailed appropriately enough, but I am really puzzled by the lighting units which stick out at the base of each post. I can't see any reason why they need to intrude so much into the walkway space.

Overall, I was impressed with this bridge on my revisit. The stainless steel works very well, and my only real criticism is that the harsh landscaping provides a rather bleak setting for the structure.





Further information:

08 October 2014

The great debate: Who owns bridge design?

I've been in Bristol and took the opportunity to attend this debate, held at the Arnolfini as part of the Architecture Centre's Bridge150 season, celebrating 150 years since the inauguration of Brunel's Clifton Suspension Bridge.

The debate was set up as architects against engineers, but from the start you could tell there would be a happy ending and both parties would make the case for living happily together ever after.

On the side of the engineers were Ian Firth, of Flint and Neill, and Julia Elton, engineering historian and proprietor of Elton Engineering Books.

Firth has been responsible for a significant number of innovative and architecturally interesting bridges. He introduced most of the key points that would recur throughout the debate. He noted the different involvement of the professions in projects of different scale, with the architect's role diminishing in inverse proportion to the size of the bridge. He was clear that both architects and engineers can be responsible for bad bridges, and that this usually occurs where one party fails to properly challenge their co-designer (River Wear and Glasgowbridge were offered as two examples). He tagged badly run design competitions where the engineering was sidelined or the client badly advised.

Firth identified the cult of the celebrity as one recent phenomenon which worked against engineers receiving proper recognition. Architects tend to be better known, and better at presenting themselves, and therefore are sometimes the only designer credited for a structure, however significant the engineer's role may be.

I was most pleased when he noted that good engineers could design very good bridges without architects involved, citing the work of Schlaich Bergermann und Partner in the present day, and Robert Maillart in the past. However, I was left with the impression that engineers should better aspire to working in collaboration with good architects, and that this was where good design was more likely to arise. Firth noted that the engineering education did not produce design-led, creative types, and that architects could contribute a better-trained imagination.

Elton also focused on education, citing a series of innovative engineers in the past who were perfectly capable of designing excellent structures with only minimal architectural assistance. She decried the lack of awareness amongst engineers about their forebears, suggesting this was why modern-day engineers lacked the confidence and context in which to be creative in their own right. Jean-Rodolphe Peronnet was offered as an example: few in the audience had heard of one of the greatest structural engineers of the 18th century. In contrast, architects understand their history very well, and see themselves as part of a tradition of productive, creative personalities.

I have a lot of sympathy for this point, seeing it in both colleagues and students. I see it as an issue of aspiration: designers need role models to learn from and to set scales of ambition. Many engineers simply fail to understand how great they could be, and along with an education which concentrates on analysis to the almost total exclusion of communication, presentation and creative design, this really does hold them back. I don't personally think we need to develop engineers who are good collaborators with architects - I think we need engineers who can be great designers in their own right, and they will then be successful both alone and in collaboration.

The architectural side of the debate was presented by Jim Eyre, of Wilkinson Eyre, and critic Hugh Pearman. Eyre addressed many of the same points, and presented a few quite ugly examples of what an engineer could produce alone, citing Maillart's Zuoz Bridge as the start of an era of boring engineer-designed bridges. Eyre noted that an architect's training made them far more aware of issues of context, culture, the experiential rather than solely functional side of a bridge, and that engineering training produced people ill-suited to addressing these issues.

He highlighted a period post-Zuoz when the engineer became king, and poor quality visual design resulted: the age of the motorway expansion, where least-cost no longer meant least-material (i.e. elegant), but fastest-to-build, and dull identikit concrete structures resulted.

Eyre also highlighted the key role that the procurement process plays in determining the success of design. Projects are increasingly led from a commercial rather than design perspective, with the contribution that design can make devalued. The increase in contractor-led design, in particular, has led to a focus on value-engineering which often reduces cost while simultaneously diminishing genuine value.

These are fair complaints, but appeared to attack the symptoms rather than the cause. Why do clients not value design? I think that both fee-paying clients and end-users, the public, share a lack of understanding of quality, and hence of value. Put simply, they do not know what a good bridge is, particularly where "good" is defined by engineering attributes. This even extends to visual aspects, where a widespread lack of visual literacy leads to some really spectacularly ugly architect-led designs being lauded as if they were actually good. The internet is full of such designs, and design competitons attract them like super-magnets. Instead, clients and the public often fall back on more straightforward yardsticks such as least-cost, or false signifiers of value, such as celebrity (the "it's Zaha Hadid so it must be good" syndrome).

Pearman did draw attention to the role of the public, who would not normally care about who "owns" bridge design. Architects were simply better at articulating their vision, at explaining design, but even they remain for the most part un-named and unknown to the general public.

I think this issue of "credit" for design is, or should be, a red herring. It seems to exercise many engineers who feel their contribution is often ignored, and while this is true, I think back to the motorway era. Although Jim Eyre suggested this period was the nadir of low-visual-quality, engineer-led design, I think there were also some very creative engineers at work during this period, highly innovative, and visually aware. I've recently posted a series of examples from the Sheffield-to-Leeds and Leeds-to-Manchester motorways on this blog: remarkable, sometimes beautiful structures, where no architect was involved, and where the engineers were not seeking any special recognition, but content to serve the public with humility.

I think it's vital that we train better-rounded engineers, who aspire to emulate the great engineers of the past. It's vital that we train people who can explain and articulate their designs. We should aim to encourage great designers who can transcend their training, from whichever background they come. But the relative roles of different designers only serve as the means towards an end, which is producing great bridges. More effort is require to explain to clients and end-users what "great" actually is, and why it can mean structures without flash, bling or spectacle. Only then can the people who use and pay for our bridges be in a position to "own" bridge design, and allow the designers to slip happily back into the shadows.

07 October 2014

Welsh Bridges: 3. Pont y Ddraig, Rhyl


Back in June 2009 I reported on the winning entry in Rhyl's Foryd Harbour Bridge design competition (and also on some of the losing entries). I predicted at the time that it would either go over its £4m budget, be delivered late, or be unreliable in service, the last largely because of a proposed lifting mechanism which would have been unique (and which was eventually dropped from the design).

The bridge was forecast to be complete by mid-2011, but eventually opened in October 2013, with a reported cost of £4.3m, which is pretty good for such an innovative structure, if correct. A report in November 2011 stated that the local council had to find an extra £0.5m to cover unspecified cost increases on the scheme. Council minutes from the time indicate that this was only 50% of the over-run, with the rest to be funded by other bodies, so that indicates a total £1m budget over-run. It's not possible to see if this is just down to the bridge, as it formed part of a wider harbour development scheme. Sustrans quote the bridge cost as £6.5m. I guess we'll never know what it really cost.

The bridge was designed by Gifford (now part of Ramboll) and built by Dawnus. Fabrication of the two bascule decks was by A.M. Structures.

This is, by any standard, an extremely innovative structure. For a start, it's a drawbridge, with the two bascules lifted by cables which pass over sheaves and down the central mast. The bascules can be operated independently or together.

I can't think of another drawbridge that has been built in modern times (examples are welcome), this is an essentially medaeval idea in contemporary form. I'm not entirely sure why they are now so rare, and would welcome comments. I'm guessing that wear to the cables is one factor, especially where the sheaves are very small and the bending stresses in the cables great, as is the case here. Most lifting bridges use much larger sheaves.

There are hydraulic pistons below the bascules, but I understand they are there to hold down the decks rather than to lift them. Without them, wind loads in the raised position could cause problems.

The bascule decks themselves are the most innovative element in the bridge. They are fabricated out of glass-fibre reinforced plastic, moulded like the hull of a boat. Four carbon-fibre plates are incorporated into each deck to enhance the stiffness. I wonder whether this is the future of fibre-reinforced plastic (FRP) bridges. Earlier bridges in this country, as those at Aberfeldy, Bradkirk and Dawlish, have been copies of steel bridge typologies. Moulding is significantly more expensive, but may allow the material to be used more imaginatively and effectively.

I also wonder how much of the bridge's cost is in the presence of two bascule decks, with two independent lift mechanisms. The original Foryd Harbour design contest specified quite clearly that only a single navigable river channel was to be bridged, as was recognised by two of the competition entries, both single-leaf bascules. Twin bascules may provide more navigational flexibility, but seems to have been introduced initially as an unnecessary architectural whim.

I say "architectural", but interestingly for such a landmark structure, there seems to have been no architect involved. Some of this shows, most notably in the clumsy relationship of superstructure and substructure, but also, I think, in the lack of legibility to the mast and cable arrangement. The three elements, central pier, decks, and mast, seem to have been conceived separately rather than together.

This is, without doubt, a remarkable bridge, and a credit to the client, designer and contractor. But is it an excellent bridge? I think not. The engineering innovation hasn't been matched by a coherent architectural vision, and it may in years to come be seen as more of a curiosity than a classic.








Further information:
  • Google maps / Bing maps
  • Structurae
  • Foryd Harbour Lifting Bridge (Hobbs, FRP Bridges Conference, 2012)
  • Blazing a Trail (Aylward, Composites Manufacturing, 2014)
  • Design and Fabrication of Two 30m Long Moulded FRP Decks for the Pont y Ddraig Lifting Footbridge (Hobbs, FRP Bridges Conference, 2014)
  • Foryd Harbour Pedestrian and Cycling Bridge (Marginson, Footbridge 2014 Conference)