20 December 2010

Bridges news roundup

Art, engineering intersect in Carnegie exhibit
Pittsburgh exhibition covers Candela, Maillart, Menn, Isler and Ammann.

Iconic landmark could have been so different
Brunel's dad proposed two -span solution for Clifton Suspension Bridge

I-195 pedestrian bridge design winner selected, will put café at water level
Buro Happold and inForm win Providence pedestrian bridge design contest. The winning entry (pictured) is a composite steel-and-timber bridge incorporating bowstring and triangulated timber trusses.

Bridge over the Sava shall be world attraction
Cable-stayed bridge proposed in Belgrade.

Tay Road Bridge protection work gets green light
£19m to be spend defending Dundee bridge against ship collision.

160-year-old Berwick bridge lit up two weeks late
Snow delays illumination of Stephenson's bridge.

Skateboarding on top of a Valencia bridge
Calatrava's La Peineta (a.k.a. Exhibition Bridge, built in 1995) offers two ways to get across.

Crowd gets look at Scottsdale's Soleri Bridge
Plug-ugly cable-stay bridge wins plenty of praise.

15 December 2010

"Robert Maillart: Builder, Designer and Artist"

There can't be many bridge engineers who don't regard the Swiss engineer Robert Maillart as one of the finest bridge designers in history.

Maillart's lasting reputation beyond his native country was generated in large part by the writings of Max Bill and Siegfried Giedion, both of whom saw him less as an engineer and more of a modern artist. The art world's interest in engineering culminated in Elizabeth Mock's 1949 book, The Architecture of Bridges, published by the Museum of Modern Art (featuring a good dozen of Maillart's designs), and the same institution's Twentieth Century Engineering exhibition in 1964.

Maillart's reputation amongst his fellow professional engineers has been encouraged by the redoubtable efforts of David P Billington, in books such as Robert Maillart's Bridges (1979), The Tower and the Bridge (1983), and Robert Maillart and and Art of Reinforced Concrete (1990). These focus generally on the engineer's built works, rather than the engineer as a person.

I'd read all these some time ago, but only now got around to Billington's Robert Maillart: Builder, Designer and Artist (368pp, Cambridge University Press, ISBN 978-0521057424, 2008, originally published 1998) [amazon.co.uk].

This comprehensive biography is the only one available in English, and the thoroughness of Billington's research is such that it is likely to remain so. He had full access to Maillart's papers and correspondence, and the result is a book that is admirable in its depth but never loses sight of what made Maillart such a special engineer.

Maillart's innovations in bridge design are now well known. He didn't invent the three-hinged concrete arch, but by combining it with the idea of the concrete box girder, he made it his own, most notably at Salginatobel. Nor did he invent the deck-stiffened arch, but again he became its master, with the Schwandbach Bridge being perhaps the best example.

Maillart wasn't notable only as a designer. As Billington's biography makes clear, he was a capable theorist when he needed to be, deriving the principle of the shear centre, and espousing a theory of design that broke with the elastic analysis prevalent amongst his more academic contemporaries, and prefigured the limit state approach now commonplace.

Billington places a considerable emphasis on various disputes and confrontations which arose between Maillart's pragmatic approach to design, and the prevailing orthodoxies of the time. Many of those concerned with the architecture of bridges sought to maintain the monumentality familiar from the age of masonry, and the presence of such people on design competition juries significantly hindered Maillart's career. Similarly, the engineering academics of the time sought to shackle the possibilities of design within the confines of what could be properly theorised, an attitude that Maillart ridiculed.

These tendencies are still with us today. As in Maillart's time, the common understanding of what makes an attractive modern bridge owes only a little to the logic of structural engineering, and much more to preconception, fashion, and the taste for spectacle.

Similarly, engineers remain hamstrung by our design codes. Improved understanding of the engineering science is a helpful tool in the designer's armoury only until new materials or forms are introduced, and then it often becomes a hindrance.

On several occasions, Billington notes the division between the designer and the analyst, and the extent to which it is reinforced by bureaucracy, especially the bureaucracy of engineering education. This too has change little, with a gap between academia and the design world which is difficult to bridge, and which inculcates a narrow-mindedness amongst engineering students.

Billington recounts in detail the intersection of Maillart's personal and professional life, particularly the way in which his radicalism, intransigence, and innovative spirit led directly to a lack of work and financial hardship. Although in many cases Maillart's design concepts won through because they resulted in a structure which was more economic to build, this was not always the case, and the conservative engineering culture which surrounded him often failed to allow time or space for innovation. It's still true to some extent today that innovation in structural engineering is beset with difficulty, and must survive in small pockets where politics, funding, and client receptiveness come together in the right way.

I was also struck with the results of the various bridge design competitions which Maillart entered. Despite becoming the best known and most respected pioneer of reinforced concrete in his native Switzerland, Maillart rarely won competitions. He had most success as part of a design-and-build team where money mattered more than highly subjective judgements of visual quality, and his flare for economy could play a central contribution. There are examples of where Maillart had to go against his own best judgement in order to respond to what he knew were key juror's prejudices. Several instances are also reported where the results of competitions were subject to intense criticism in the Swiss engineering press, often instigated by Maillart's friends, something that's sadly all too absent today.

One area where Billington is perhaps a little reticent is on the actual engineering itself - there is little here which discusses or explains Maillart's designs in any depth, and a general shyness about technical matters that sometimes leaves the reader having to take the author's assertions at face value. To some extent, Billington's other books fill this gap.

It's also often difficult amidst a minefield of detail to see Maillart's achievements in context. He was not the only innovator of his time, and while Eugène Freyssinet is mentioned, others such as Alexandre Sarrasin or Rudolf Dick are absent. Without these points of reference, Maillart's genius is apparent but his place in history is less well understood.

Overall, it's an excellent book, putting a context to Maillart's individual structures which makes adds to their appreciation. I can think of few engineering biographies which even come close to its thoroughness, which is a shame.

12 December 2010

Scottish Bridges: 16. Roxburgh Viaduct Footbridge

The last bridge from this recent visit is probably the least well known, not being such a historic site as at Gattonside or Dryburgh, but it's the most interesting structurally.

Roxburgh Viaduct (also called Teviot Viaduct) was built across the River Teviot by the North British Railway in 1850. It was designed by John Miller, who also designed the spectacular Ballochmyle Viaduct, which features the second longest masonry arch span in the UK. Roxburgh Viaduct, which no longer carries trains, is much less notable than that.

The Viaduct's four river piers are extended to one side to support a low-level footbridge, of a type which while supposedly once common, has very rarely survived.

The footbridge is 54m long, with three simply-supported spans of approximately 15m each. It is thought to date from the same time as the viaduct. Each span consists of a wrought-iron lenticular truss, 1.2m deep at midspan, with bars curving downwards below a very slender deck which curves upwards. Unlike many trusses, it is not fully triangulated, and relies on the V-shaped 'legs' to hold its shape.

Another way of thinking of it is that each span is a self-anchored suspension bridge, with the lower member being the suspension 'cable' (which would be above the deck in a conventional suspension bridge), restrained by the upper member at its ends.

The footbridge seems to have had very little alteration since its construction. The original parapet railings have been extended upwards in timber, presumably to give a greater sense of security, and infilled with a chain-link mesh (which strikes me as completely unnecessary at a rural site such as this). All the structural metalwork appears to be original.

The bridge has certainly suffered a little from age. This structural form has very limited torsional stiffness, and over time, the spans have twisted permanently, perhaps due to slip in the joints. It makes crossing the bridge a slightly unsteady experience, but not unsafe.

It's a type of bridge which is much under-used, although there are a number of very modern examples. Strasky's book on stress ribbon bridges features several, mostly in Japan, and a recent paper by Ruiz-Teran and Aparicio surveys a number of others. There's also the relatively modern "bridge to nowhere" over Biel Water at Belhaven, although I don’t know exactly when that was built.

Older examples include Robert Stevenson's proposal for an under-deck suspension bridge at Cramond from 1821 (although that was planned to be ground-anchored, rather than self-anchored), and the Micklewood Bridge of 1831, described in Charles Drewry's book on suspension bridges. There were also several examples in Europe, much discussed in Tom Peter's excellent book on Guillaume Henri Dufour.

It's hard to believe that a modern designer, constrained by modern design codes, could produce something as lightweight and charming as the Roxburgh footbridge, but if so, that's a shame. I hadn't even been aware of its existence until quite recently, but it's already one of my favourite historic bridges in the UK.

Further information:

10 December 2010

The privatisation of public space

I have a general rule that I won't post anything here that isn't related to bridges, however interesting it might be to me or my readers. However, I couldn't resist this one.

When I visited South Quay Footbridge recently, I was particularly struck by the way in which the corporate privatisation of the surrounding public realm had allowed the bridge's original context to be over-run with new buildings. While this spoiled the setting of the bridge, the feeling that you can only see it under the sufferance of security guards was more disturbing.

It's encouraging, therefore, to see that London's planning committee has been tasked to investigate the whole phenomenon of corporatisation of the public realm. A number of examples are cited by BDOnline which are far more depressing than the situation at South Quay.

I'm not sure what, if anything, bridge designers can do, other than to be aware of wider political and social concerns and try not to respond slavishly to the agenda of private bridge promoters.

I recall one case where a client wanted a bridge, but was anxious that it would create a sheltered space which undesirables could inhabit. Their preferred solution was simply to create a caged area below the bridge with security fencing.

The possibility of coming up with a more creative design solution (e.g. opening up the site so that the area was more visible, or consciously occupying it in the way that is frequently done with railway viaduct arches) didn't occur to them. The possibility of engaging with the community to address the wider issue (presumably, a lack of adult-free space where young people could meet and socialise) would have been even more remote.

I wonder whether anyone else has examples of bridge designs where there is a conflict between private and public benefit?

08 December 2010

Scottish Bridges: 15. Dryburgh Abbey Footbridge

The crossing of the river Tweed at Dryburgh is the site of a key development in British bridge history. In August 1817, a footbridge was built here by John Smith, spanning 260 feet (79m). It was cable-stayed in form, rather than adopting the catenary suspension bridge form that had been pioneered elsewhere. There were other stayed bridges in the vicinity at King’s Meadows near Peebles, and in Galashiels, both also built in 1817.

In January 1818, less than six months after it opened, Smith’s Dryburgh bridge blew down in a storm. The form of bridge was blamed by authors like Charles Drewry and Claude-Louis Navier, and cable-stayed designs went on to become a rare eccentricity until their revival in the second half of the twentieth century.

The failed bridge was rebuilt by Smith as a more conventional suspension bridge design by September 1818, and survived for several decades, although it also eventually failed. It's not entirely clear how old the present structure is.

The current structure is a lightweight, unassuming suspension bridge, with metalwork towers. Contrasting it to the stone towers and overweight deck at nearby Gattonside, it’s a much more delicate structure, although its dark green paint counteracts the lightness somewhat.

The main suspension cables are spiral steel rope, arranged in pairs on each side of the bridge. The deck is hung between them from inclined hangers. It's unusual to see inclined hangers on a suspension bridge, as while it increases the lateral stability of the deck, this comes at the cost of greater tranverse forces in both the deck and the towers - it's also less straightforward to build, as the main cables have to be pulled sideways, away from their natural catenary.

The blocks which clamp the hangers to the cables are a bit odd, resembling large blocks of lego, and certainly not original, judging from earlier photographs. Their sheer size detracts from the elegance that should normally be expected from a suspension bridge.

The towers are made from latticed metalwork, braced together overhead. To me, this gives the bridge a twentieth century look, similar to several of the suspension footbridges of David Rowell & Co., although there are also resemblances to the bridges of Louis Harper, which are mostly from the late 19th century. Structurae lists Dryburgh Suspension Bridge as dating from 1872, but RCAHMS cites J R Hume as believing it to be a twentieth century rebuild.

I can't say I find latticework towers particularly attractive, and there's something to be said for the more monumental masonry adopted at the nearby Gattonside footbridge. Metalwork is clearly more economic, though.

It's also interesting to compare the parapets at Dryburgh with the very heavy modern design at Gattonside. The lightest, and cheapest, parapets I've seen on historic suspension footbridges in the UK consist of wire ropes attached directly to the hangers, and used to support a mesh or chain-link infill. The parapets here are clearly a step up from that, with horizontal wires providing the pedestrian containment, while the main metalwork elements serve the primary purpose of stiffening the bridge deck.

They really do show how over-the-top the Gattonside design is. The parapets at Dryburgh are unintrusive (green paintwork aside), nothing more than they need to be, and all the better for it.

Further information:

06 December 2010

Scottish Bridges: 14. Gattonside Bridge

I recently visited the Scottish Borders, and took the chance to see three footbridges in the Kelso / Melrose area. The first two of these are historically interesting, but they are really only curtain-raisers for the third one.

The suspension footbridge at Gattonside was first erected in 1826. Design has been attributed both to John Smith (who certainly designed the towers, and who had in 1817 designed a cable-stayed footbridge at Dryburgh, only to see it destroyed in a storm the following year), and also to John Stevenson Brown of the contractors Redpath and Brown. Brown also built a stayed footbridge in 1817, at King's Meadows near Peebles (which lasted much better, succumbing to flood damage in 1954). The Dryburgh bridge will be the subject of my next post.

The 1826 bridge was built to link Gattonside village with the town of Melrose, a function it still serves today.

Spanning 91m across the River Tweed, it had iron chains and a timber deck, and went on to become the oldest structure by Redpath and Brown to survive. They refurbished it in 1928 free of charge.

There are various photos of the original bridge online, especially at the RCAHMS website linked below. These show twin iron chains side-by-side, with a slender post-and-wire parapet. The towers are stabilised by lateral stays, which look a little unnecessary.

Unfortunately, the bridge was "refurbished" in 1992, by Travers Morgan (now part of Capita Symonds). In reality, everything except the towers was rebuilt, including replacement of the deck and the cable saddles. The original suspension chains were preserved, but moved wider apart so that a new suspension chain could be inserted between them - this new member is a series of bars with intermittent links, and carries all the load, leaving the original chains essentially as cosmetic. The old chains are, in fact, supported directly upon the new chain (as can be seen in the photo below - click on this or any other image for a full-size version), having had the original hangers removed.

One result of the reconstruction is that the bridge was downgraded from Listed Building Category A to Category B. It's hard to understand why its Listed status didn't offer it better protection. I have a copy of an artist's impression of a refurbishment proposal from 1988, which gives little impression of the very chunky parapets that were actually adopted, although the engineer's reconstruction drawings from the same period are more explicit.

In his 1994 guide to Borders architecture, C A Strang recorded: "Bridge completely reconstructed, 1991, with most unsympathetic new deck; this wouldn't have happened to a Chippendale chair of the same date".

In his paper "Some iron suspension bridges in Scotland", Ted Ruddock was even less sympathetic.

"The bridge's treasures of blacksmith’s empiricism were destroyed in a dreadfully misguided 'refurbishment' of the bridge in 1991-92. Without any increase in the loading, or more than modest and treatable corrosion, the application of modern desk analysis persuaded the local authority’s consulting engineer (or justified his prejudice) that the structure that had served for 170 years should be replaced by a vastly heavier steel structure – a disastrous decision in relation to current conservation principles. All that remains of the original ironwork is the chains, no longer functioning but draped as dead sinews amongst an untidy jumble of new cables and railings".
The Gattonside bridge is one of only three early suspension bridges built in the Scottish borders that still stands (the failures include the bridges at King’s Meadows, Dryburgh, and Galashiels, while the 1820 Union Bridge and 1835 Kalemouth Bridge are the other survivors), so its thoughtless reconstruction is a particular shame.

I’m not entirely sure that Ruddock is being fair. The original Gattonside bridge may have stood for nearly 170 years, but it was not without fault. A notice has been retained at the end of the bridge warning of fines for deliberately "swinging" the bridge, and advising that no more than 8 persons should stand on it at once. It would be a surprise if the bridge could have been shown to carry modern pedestrian design loads, leaving the maintaining authority with the difficult job of balancing the risk of overloading (generally low, but high intensity loading is not unforeseeable) against the consequence of failure (likely to be sudden and calamitous). A history of past performance is little guarantee of future safety, as is readily apparent from any study of bridge failures.

In my own work, I’ve had to advise a client on precisely this issue: a historic suspension footbridge which is self-evidently safe under its normal loading, but which can’t be shown to be safe against crowd loading. In my case, the bridge in question was repaired but not altered.

Other vintage suspension bridges have been strengthened but largely unaltered, as at Union Bridge, where the original suspension chains were supplemented by wire suspension ropes but not rendered redundant, and where a load restriction (one vehicle at a time) was imposed and presumably has been reasonably well obeyed.

The treatment of the suspension chains at Gattonside is poor, but the design of the deck is worse. Transparent railings have been replaced with clunky stiffening girders which double as parapets. These seem to me to be entirely unnecessary – there are several unstiffened suspension footbridge in use throughout the UK dating from both the 19th and 20th centuries. If stiffening was required, it surely didn’t need to be as obtrusive as this, with its thick welded sections which visually merge into a solid wall from many angles. Again, there are plenty of examples of small suspension bridges with stiffening parapets which use much more slender latticework than this.

Setting aside these issues, it’s worth asking whether the current bridge is successful in its own terms. The conflict between the historic masonry towers and the modern deck is uneasy, but not awful. The bridge is not entirely unattractive, indeed it’s probably quite a challenge to make an ugly suspension bridge. The deck would definitely look better with a slight upwards camber in place of its dour flatness (I think there was such a camber on the original bridge).

Further information:

05 December 2010

Decrepit suspension bridges

I'd just like to draw your attention to several recent posts at the Japanese bridges blog, Tabikappa, which are excellent examples of old, unloved, but still lovely vernacular suspension footbridges. Great pictures for anyone who enjoys seeing examples of bridges where "mend-and-make-do" prevails over computer analysis and high technology, especially this one and this one.

03 December 2010

Bridges news roundup

Border Bridge Reflects Dilemma
Plans for $250m bridge to open up impoverished North Korea to trade with China remain on hold. If ever a single bridge could have a huge impact on regional politics, this is one.

Kurilpa Bridge now a work of genius
Brisbane's cat's cradle footbridge collects an Australian Engineering Excellence Award. It had already received Project Infrastructure Excellence and Innovation Awards. As with structures like the Birds Nest Stadium in Beijing, all this tells me is that awards committees think "excellence" is simply the successful realisation of the spectacular. Wasn't there once a time when "innovation" meant achieving something worthwhile rather than simply something new?

Fishy foot-bridge contest in Providence
Columnist suspects design competition was intended to flush away traditional design options in favour of modernist orthodoxy: "All of the bridge designs evoke a narrow, cramped definition of creativity, one that values novelty over continuity."

Thoughtful process for footbridge
Irritated Providence contest jury member replies: "We’ve committed our time and energy to ensure that this competition is a fair, inclusive process that engages the public and results in a bridge that improves the image and identity of the city."

02 December 2010

London Bridges: 7. St Saviour's Dock Bridge

The last two bridges were quite different from each other, but both had in common a basic simplicity of structural form that is characteristic of many contemporary footbridge designs.

The tiny footbridge at St Saviour’s Dock, not far from Tower Bridge, is quite different. Designed by Nicholas Lacey Partners with Whitby and Bird (now part of Ramboll), it spans the entrance to a small dock basin just off the River Thames. It can swing open to allow boats to pass through, although I suspect it doesn’t have to do so very often.

It's a small bridge, only about 30m long, with a 15m main span. It cost £750,000 to build in 1996, and was fabricated by Littlehampton Welding on behalf of Christiani & Nielsen.

It’s a difficult bridge to photograph, as there is only public access to its riverside face, but hopefully its form can be understood from my pictures. It’s an asymmetric cable-stayed bridge, with a short backspan balancing the longer main span.

One mast sits either side of the deck, with the cables arranged in a harp-type layout on each side. The deck is timber, but pretty much everything else is stainless steel, and as a result it has worn well, not having changed at all since I first visited it many years ago.

Apart from the pivot, which is enormous and completely out of scale with the rest of the bridge, all the other structural members, including the masts, are as slender as possible. This comes at a significant cost: almost every part of the bridge is extensively braced with stainless steel cables. The deck, for example, has to be pulled down with a network of cables because otherwise it would simply be too light to place the main cables into reliable tension – the cables are therefore presumably stressed against each other.

The result resembles a complex piano-wire cat’s cradle assembled by a severe sufferer from OCD. It’s fussy, confusing, and way over-the-top for such a small bridge. There is a certain attraction to its geometric intricacy, but for me this doesn’t outweigh the problems. Even the short side-span at its west end seems over-detailed (not to mention quite different in style to the main deck).

By far the worst feature is the presence of massive security gates at the western end, which hugely detract from the structure and give it the appearance of an ultra-modern security facility.

Whitbybird produced many better bridges after this, and it must rank as a very interesting but ultimately unsuccessful experiment.

There are a number of other interesting bridges in London's Docklands, so hopefully I'll get a chance for another visit to cover some more another time.

Further information:

01 December 2010

London Bridges: 6. West India Quay Footbridge

South Quay Footbridge lies a short distance south of One Canada Square, the pyramid-topped tower at the heart of Canary Wharf in London’s Docklands. Walking north from it, you’ll pass the spectacular man-made cavern that is the Canary Wharf underground station, then One Canada Square itself. The Docklands Light Railway threads itself awkwardly between, around and even through the various office buildings.

Just to the north of this, the streetscape becomes slightly less oppressive. At West India Quay, some of the original dockside buildings have been preserved, and they are considerably more human in scale (and pleasing in appearance) than their more recent commercial neighbours. This change in pace is also signalled by a footbridge which is as modern as the South Quay structure, but significantly less ostentatious.

West India Quay Footbridge was designed by Anthony Hunt Associates (now part of SKM) with Future Systems (now Amanda Levete Architects), joint winners of a design competition. Opened in 1996 at a cost of £1.7m, it was fabricated by Littlehampton Welding.

The bridge is 94m long, and the main structure is a U-shaped steel spine beam, 750mm deep, with edge beams supported on cantilevering outriggers which form a triangular bracing in plan. These support stainless steel balustrades (with lighting in the handrails) and aluminium decking (with uplighting in the deck). The spine beam provides the main resistance to torsion, and in his book 30 Bridges, Matthew Wells suggests that the architect refused the engineer's request to add edge cables which would enhance the torsional resistance.

The bridge legs sit on 2800mm diameter floats, more than a little reminiscent of a flying boat.

It has an opening section in the middle comprising two bascule leaves, although as at South Quay I suspect it has opened only infrequently. It has often been likened to an insect skimming across the surface of the water, perhaps a pond-skater, with its slender deck supported on quadrapod frames. The effect is essentially modest, even peculiarly innocuous.

The bridge’s most unusual feature is that it is a floating structure, capable in theory of being untethered from the quayside and moved elsewhere to suit changing development. In practice, it has not moved since being installed, and the floating tanks which carry its legs are anchored to the dock basin’s floor by tension piles.

Future Systems, now defunct, were famous (or perhaps notorious), for their liking for blobitechture, organic in form, the realisation in steel and concrete of forms better suited to amoeboid protoplasm. This, and their interest in idiosyncratic colour choices, gave many of their designs a distinctly science-fiction feel. Apart from its puzzling lime-green colour, West India Quay Footbridge is largely immune from this tendency, with a relatively straightforward structural form (for the opposite extreme in their bridge design, see their unbuilt Glasgowbridge design, featured here previously).

I like the West India Quay bridge. It’s unpretentious, light in appearance, and generally well-detailed (see for example the simple counterweights to the opening spans). The fact that it floats may appear with hindsight to be an unnecessary contrivance, but I think it’s an essential element in the bridge’s charm.

Further information:

30 November 2010

I-70 Wildlife Bridge Finalists Announced

Five finalists have been announced in the ongoing contest to design a wildlife bridge over the I-70 Highway in Colorado. The original shortlist featured 36 entrants.

I've provided a short note below each entry summarising the structual form, but there is much more information on each design at the competition website.

Balmori Associates (New York)
with StudioMDA, Knippers Helbig Inc., David Skelly, CITA, Bluegreen, John A. Martin & Associates, and David Langdon

This design involves a series of transversely laminated timber "plates" which are sculpted to give an organic form underneath. It's clearly the contribution of Knippers Helbig as there's a family resemblance to their Margaretengürtel design.

The Olin Studio (Philadelphia)
with Explorations Architecture, Buro Happold and Applied Ecological Services

The Happolds influence is apparent in the structural form, which is a weathering steel diagrid (Happolds being known for their expertise in gridshell roofs). Overall, it's shaped as a hyperbolic paraboloid, or hypar (also known as the "pringle"), which provides stiffness through curvature rather than depth.

Janet Rosenberg & Associates (Toronto)
with Blackwell Bowick Partnership, Dougan & Associates, and Ecokare International

Structurally, this is the least interesting design, featuring a timber and plastic composite deck used to support fibreglass composite screens. Some of the images make the decking look a little like a scaffold platform.

Michael Van Valkenburgh & Associates / HNTB Engineering (New York)
with Applied Ecological Services, Inc.

Hypars are clearly flavour of the month, because HNTB have also used them in this design, although in a very different way to the Happold entry. The bridge has a series of precast concrete ribs, each of which is made up of a thin concrete shell in hypar form. L-shaped ribs are propped against each other to form three-hinged arches, which would be both lightweight and stiff. The ease of erection, and adaptability to different spans and geometries, make this one my favourite, although architecturally it's less spectacular.

Zwarts & Jantsma Architects (Amsterdam)
with OKRA Landscape Architects, IV-infra and Planecologie

Finally, here's another hypar, this time in the form of an in-situ concrete shell. Thin shells of this type are rarely used in bridge construction, partly because they perform well when subject to evenly distributed loads, and less well against concentrated loads. A wildlife bridge, with its plentiful earth fill, is a rare case where the geometry makes sense. However, in-situ concrete is problematic, and the designer has had to illustrate a complex temporary cable net, suspended on stays, to support the formwork. This might add an unwelcome cost as well as cause greater highway disruption than some of the alternatives.

28 November 2010

London Bridges: 5. South Quay Footbridge

On a recent trip to London, I had time to visit three footbridges. All have in common the fact that they are moveable structures, opening to waterborne traffic; that their design had the strong involvement of an architect; and that they are a few years old now. These are definitely not the latest designs, but they all remain interesting in one way or another.

The first, South Quay Footbridge, was the collaborative design of Chris Wilkinson (now Wilkinson Eyre) with Jan Bobrowski & Partners (now Capita Bobrowski), who won their commission in a design competition run by London Docklands Development Corporation. It was a slightly unusual contest by modern standards: a shortlist of six engineers was invited, and given a list of possible architects to team up with.

In its original form when it opened in 1997, the bridge connected the quay walls of a dock basin at Canary Wharf in east London, providing an S-shaped walkway, each half of the 'S' suspended from an inclined mast. The two masts inclined in opposite directions, and the south half of the bridge could pivot open when water vessels needed to pass through (which was not often).

It cost £2.5m, and was 180m long. The masts were 30m tall, and along with the cables carried a deck supported on one side only by a steel tube. The bridge decking was of oak planks (now long since replaced with metal grating due to the oak's slipperiness). The contractor was Christiani & Nielsen.

It was a masterpiece of landmark footbridge design, a brilliant concept which perfectly met the requirements of the site. I very much enjoyed the first time I visited it, its combination of simplicity and dynamism.

It was always intended that further construction work would take place to narrow the dock, by building a new quay wall on the north side (to the right of this photo), and removing half of the bridge. It was planned that the section removed could be used elsewhere, and this was one of the interesting features of the original design. Indeed, the bridge in its original form lasted barely three years before its span had to be reduced.

The reality, however, is that the bridge was emasculated, and the unfortunate peculiarities of Docklands land development have made its context more problematic.

It’s still a striking structure, with the harp-strung mast thrusting priapically into the sky. The mast is circular at its base but narrows at its tip, a forerunner of the more refined mast shaping adopted by Wilkinson Eyre at the much more recent Swansea Sail Bridge.

It seems inconceivable that the bridge's pivot support could be large enough for the deck to remain balanced when it opens.

The deck support girder is a 914mm tube, from which the deck cantilevers. The cantilever members are hidden below deck, but their presence is indicated on the tube by a series of "collars", which I think are one of the bridge's less successful details.

The adoption of two very different styles of balustrade is a feature that has appeared on many Wilkinson Eyre designs. The emphasis is on the walkway’s role as an outward-facing promenade, although at the cost of a somewhat obscured view on the opposite side, where the perforated plate screen is too high for comfort. Wilkinson Eyre described it as a "man-made hedge" i.e. a wind-shield.

The bridge originally sloped upwards to provide clearance over the middle of the dock water, but now it lands at a level higher than the rest of the dock quay – the slope has no visual rationale any more, and it lands awkwardly onto a set of steps which prevent disabled access, and, more importantly, place a barrier right across the pedestrian desire line. Approaching from the north, it’s uninviting and intrusive.

Presumably there was no space for a ramp, because immediately to the north, the footpath is now blocked by a massive glazed atrium, connecting two offices and sheltering the way down to what appears to be an underground shopping mall. Again, coming from the north, this obscures views of the bridge, negating its role as a landmark, and making the pedestrian experience at best unfriendly, and at worst intimidating.

This part of the Docklands has always been a prime example of the harm done to the public realm by permitting its existence only within the terms of privatised land, and this atrium is emblematic of the negative legacy of the way Docklands development was promoted. Where there should be generous and artful public space, instead there are barriers, security guards, and the resulting consciousness that everyone is an intruder, your presence and passage resented.

This may seem something of a digression, but what was once a splendid bridge has been greatly diminished by the increased corporate colonisation of its context.

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