Bridge competition debris part 16: River Soar

Back in February, Buro Happold and Explorations Architecture were announced as the winner of RIBA's design competition for a new £1.5m footbridge over the River Soar in Leicester. But what of the other five competition entries, how did they compare?

I've tracked down the entries, and as usual, they're shown below. As always, click on an image for a larger version, and links are provided where a competitor has more details on their own website. Further information on three of the designs can be found at Europaconcorsi, from where I've shamelessly poached most of these images.

So, would I have chosen a different winner to the judging panel? The risk-averse engineer within me would have been nervous about the large foundations required for the Happold design, and the consequential risk of higher than expected geotechnical costs. The same is true to a lesser extent of the Gifford bridge, which must resist large horizontal forces at some point due to both its assymketry and suspension design (large only in the context of a small footbridge, of course).

It's interesting that only one of the designs includes a mast element - this is in line with the current landmark footbridge trend which is away from the Calatrava-esque (all poles and cables) and towards a more intimate experience. Without knowing the context of the scheme it's hard to judge whether a cable-supported bridge is the right solution - it certainly offers the advantage that the deck and parapets can be physically and visually much lighter. It's also nice to see the suspension bridge option chosen for a footbridge - I'm a big fan of historic examples like the bridges of David Rowell and Louis Harper.

The Price and Myers design has an attractively rusty brutalism to it, and the McDowell & Benedetti entry borrows from the modernist boardwalk feel of their excellent Castleford footbridge. I very much like the form of the Moxon arch bridge but it looks a little too enclosed for this site.

My favourite is the Ramboll Whitbybird entry, an interesting play on the traditional lattice truss bridge which in its creation of visual interference patterns offers a nice nod towards the nearby Leicester Science Park and National Space Centre.

Price and Myers / Allies and Morrison





Gifford / Knight Architects





Arup / McDowell & Benedetti





Arup / Moxon Architects






Ramboll Whitbybird

Weave Bridge woven

Back in October, I reported briefly on a new footbridge at the University of Pennsylvania, called the Weave Bridge. At the time, it was nearing completion on site. Well, the US$2.4m bridge was opened to the public on 19th January, and an update on the finished article must be long overdue.

The bridge was designed by Arup, specifically their Advanced Geometry Unit (founded by Cecil Balmond, who is a professor at UPenn), although essentially they acted as concept engineer or even as architect - the detailed engineering design was prepared by Ammann and Whitney. This seems to be an increasingly common arrangement - the boutique engineer standing in for the architect while leaving someone else to do the "hard sums". Marc Mimram and Santiago Calatrava are others who take the same approach.

When I last posted, I noted:

"Balmond seems to have little sympathy for the conventions of bridge design, taking standard concepts like the arch or truss and transforming them into stained-glass geometric puzzles ... Balmond's design seems to arise, however, mainly from playing with geometry on a computer, rather than from the purely structural imperatives which drove James Warren's solution."

The image on the left is a design visualisation from before construction. It suggests a colorful rattlesnake structure, which I mistakenly likened to Warren's truss bridge design from the 19th century (thanks to Paul Kassabian for putting me right). In reality, it largely dispenses with the top chord of a truss, instead carrying the axial forces by means of the "X" shaped elements in the roof. This is like taking a pair of opened scissors and pushing them against a metal plate - the force tends to open them about the central hinge.

The result is a design with poor structural efficiency. The scissor action is resisted mainly because the "hinge" is in fact a solid welded connection, and partly by the restraining action of the side trusses. The connections must therefore be considerably over-designed compared to a conventional truss bridge, as must all the other steel elements. Quite how inefficient it is will depend on the extent to which the bridge is simply supported (with the roof "bracing" all in compression) as opposed to continuous over pier supports (with the roof "bracing" varying from tension to a lower compression). Some images suggest the latter is the case, but I can't tell for sure.

Praising the bridge, architecture critic Inga Saffron wrote:

"Unlike Frank Gehry, who is happy to drape his bloblike forms onto any old framework that can hold them up, Balmond sees structure and design as one and the same. There are no vertical supports holding up the Weave Bridge because its twisting stainless-steel strips carry the load, in the way that cables and trusses do on conventional bridges."

Nonetheless, the compairison with Gehry isn't entirely unfair - both are designers who put abstract geometry to the forefront, often at the expense of functionality. For the post-modernists entranced by their piles of crumpled paper or the digital fantasies of algorithmic design, the modernist notion that form follows function is long dead. Instead, form determines function - the ability to generate a particular complex geometry decides the structural form of the bridge, not the exigencies of cost or buildability that preoccupy most engineers.

The image on the right (courtesy UPenn website) shows the bridge under construction. It seems less organic than in the visualisation, but much of that may just be because it's a close-up view. The tricky connections between the truss webs and the "roof bracing" look a little unattractive - you would visually expect the flanges on the web members to continue at the same angle on the roof rather than stand upright, but of course they have to do so. This visual oddness could have been reduced by chamfering the rectangular cross-section of the bridge, at the expense of yet more engineering complexity.

The image on the left (courtesy of Shana Lee on flickr) shows the bridge in its nearly finished state. I'm struck by how colourless it is compared to the early renderings, which is a shame because Balmond is interested in colour elsewhere. I'm also left wondering how much trouble they'll have cleaning the outside face of the glazing above the railway lines in the future - how well will this bridge weather?

The main difficulty is of course that of expectation, particularly for the engineering observer. For the lay person, I think it will look unusual but otherwise fine. For a bridge engineer, the absent top chord on the trusses defies what you expect to see. The question is whether the bridge is visually better or more interesting, and I think there's no real improvement over a conventional Warren truss structure, especially since there's a slender bottom chord member (presumably to support the floor) left in.

The bridge seems really to be designed to be viewed from within. The visualisation on the right suggested a contrast between translucent blue glazing, marked with lines to echo the timber flooring, and returning across the roof to mirror the floor panels. It's geometrically interesting if perhaps trying a little too hard.

The photo on the left (courtesy Shana Lee again) again suggests much of the colour has sadly bleached out. The design seems clever, but not a delight. Little compromises have been made which detract, especially the loss of the glazed roof sections (presumably because they'd be too difficult to clean). I'm also not sure I like the interplay of the shadows and the floor "bracing" - they're too similar in colour and I'd prefer to see the bridge structure stand out rather than dematerialise.

As ever, it's almost impossible to review a bridge without visiting it in real life, and I've no plans to visit this one soon. Overall, I don't like it - the departure from conventional structural logic isn't justified by the creation of something sufficiently magical, and Balmond's Coimbra Footbridge in Portugal seems to me to be more successful. But I'd welcome other comments, especially from anyone who has visited it or was involved in the design.

"Traversinersteg" by Wilfried Dechau

One of the great footbridges of modern times was Jürg Conzett's first Traversina Footbridge, a delightful filigree timber-and-cable design that was built in Switzerland in 1996 and then sadly destroyed by a rockfall in March 1999. Luckily for pontists everywhere, it was replaced in 2005 by an equally impressive structure, also designed by Conzett. The second Traversina Footbridge is a highly unusual timber-and-steel suspension bridge, which, like its predecessor, perches high above a precipitous gorge.

I visited the bridge last November, and noted that the span was a tribute not just to its remarkable designer but also to the people who built it in such a difficult setting. The bridge's dedicated website includes many excellent photos of the construction process, and the people involved, as well as descriptions of the bridge in German (there are more construction photos online elsewhere).

I only recently discovered that there's an entire book devoted to this bridge, "Traversinersteg" (ISBN 3-8030-0662-7, Ernst Wasmuth Verlag, 2006) [Amazon UK], by the photographer Wilfried Dechau. He was also the photographer for Mike Schlaich and Ursula Baus's excellent volume, "Footbridges", which I reviewed here last year. And having just received a copy of the "Traversinersteg" book, I have to say that it's well worth getting hold of.

The photos on the website are nice, but nowhere near as attractive as when reproduced at larger scale in this gorgeously designed, large-format (27.8cm by 38.2cm), slipcased tome (Amazon has images of some of the pages).

The establishing images of the spectacular location in the Via Mala valley set the scene. The building of this bridge was at once acrobatic and heroic, and also intimate and craftsmanlike. Construction of the foundations required concrete and materials to be lifted up the hillside on an aerial ropeway suspended from trees, and the bridge span itself involved a team of specialist climbers.

The photos clearly show this range of emphasis - construction workers suspended in harnesses over the abyss, as well as close-ups of hands tightening bolted assemblies. There are several portrait images, showing a range of people involved in the scheme, not only the designers but also the workers who are so seldom credited in the creation of landmark structures. I believe one of Dechau's main concerns was to depart from the conventions of architectural photography, which fetishise the object and largely discount human involvement. If so, he has succeeded admirably.

Many of the photos are reproduced at full page size (or to fill a two-page spread), which really does them justice. Use of colour is sparing but effective. My favourites tend to be the photos that are most vertiginous, that capture what is unique about the bridge and its setting.

The book also includes short texts by Jörg Schlaich, Ursula Baus and others. Schlaich makes the point that "watching [the bridge] being built is often more interesting than admiring the finished structure", but I'd suggest the Traversinersteg is a rare case where the bridge is as impressive as its making. Schlaich and Baus both note that the bridge was built without scaffolding, that the materials required to build it mostly form part of the final structure, even to the extent of using timber from trees cut down to make space for the aerial ropeway.

There are relatively few bridges which merit a lavish book to themselves, but this is undoubtedly one. It's also a rare book where the design and imagery is as impressive as the structure it decribes. It's not going to be casual purchase for anyone, but I'm delighted with it.

Happy 1st birthday!

I feel a little surprised at myself. It's 12 months today since I started this blog, embarked on this gephyrophilic voyage of discovery.

In that time, I've made over 125 posts altogether, including several on key topics such as bridge criticism, Swiss bridges, bridge competition debris and tensegrity bridges. I've written less than I had originally intended on new publications relevant to bridge design (books, standards etc), but there's only so much time available and I need to go with the topics that don't seem too much like hard work! I've also largely failed in my wish to visit and review more individual bridges.

So, what's coming in the next year?

I have a couple of bridge competition debris posts already lined up (for the London Millennium Bridge and River Soar Footbridge). There are a number of interesting bridges nearing completion (including the spectacular Kurilpa Bridge), and one or two designs yet to be made public (most notably Calatrava's design in Calgary). These will undoubtedly merit some further comment. I'll also be revisiting a post I made last year about the success rate of RIBA bridge design competitions - one year on, have they resolved their problems, are they a good way to get a bridge built yet?

However, the main thing I'd like is more discussion, either to suggest news to cover, or matters for future posts (email me at happypontist at googlemail dot com), or to debate any of the bridges or other issues I feature. So next time you read a post that you have an opinion on, please hit that "comments" link at the bottom of the post! More dialogue is what will keep this blog healthy for another 12 months.

Bridges news roundup

Mill Lane footbridge wins engineering award
Steel arch span in Bracknell, designed by Jacobs


So does Aylesbury footbridge
Bourg Walk bridge, also designed by Jacobs

Tabikappa blog
I've recently added a new link to this Japanese blog which posts regular photos of bridges. If you read The Happy Pontist through an RSS newsreader rather than visiting directly, please visit the home page occasionally, as it links (in the right hand margin) to the latest updates of several other blogs which may be of interest. What especially attracted me to Tabikappa was this terrifying bridge.

Safety concerns raised about Brisbane's Kurilpa Bridge
Vulnerable to impact from freakishly tall vehicles, apparently (isn't this true of any bridge over a highway?) Mayor gets in a tizzy, consultants seem to have a dislike of steelwork, politicians see passing bandwagon and chase after it.


New fibre reinforced composite footbridge installed in six hours
Ultra-low maintenance bridge is no beauty.

Soaring or boring?
Decision on controversial Portland light rail bridge moves closer. Journalist opposes elegant, efficient design (pictured below). Blogger offers insight into the decision-making process.

Bridge competition debris part 15: Foryd Harbour Bridge

I recently posted the winner in the Foryd Harbour Bridge competition, a design-and-build contest for a £4m opening footbridge at Rhyl in North wales. It was an unusual twin-leaf bascule bridge, with a central mast resembling that of a ship, to be designed by Gifford and built by Dawnus.

I've been sent information and images for three of the four other entries, and rather than wait for the promoter, Denbighshire, to make them all public, I thought I'd just get right on and show them here. As ever, click on any image for a larger version (links are given only where I've found relevant information on a firm's website).

First, a few thoughts on the engineering aspects. The swing bridge (from Knight Architects & Parsons Brinckerhoff) is at a disadvantage on the reliability front, as swing bridges require interlock or jacking mechanisms at the ends in order to properly secure the bridge in its closed position. This isn't necessary for a bascule bridge, which can secure itself in place by virtue of its dead weight. As a result, a bascule solution has one less mechanism to cause problems.

The Knight / PB design also shares a disadvantage with the winning Gifford design, of having a central pier, and therefore no fixed link from shore to the operating mechanism. This means that electrical and hydraulic supplies have to be buried below the river bed, adding to the environmental impact of construction, and also that access for maintenance is rendered more difficult if the bridge mechanism were to fail while in the open position.

In favour of the swing bridge, and the Mott MacDonald bascule variant, is that the centre of gravity and centre of rotation of the bridge can be located at the same point. This balances the bridge such that the mechnical power required to operate it is much lower - the weight of the bridge is balanced in all positions. Essentially, the mechanical plant only needs to overcome wind loads, and again the swing bridge is best for this, with perhaps the lowest operating cost of any of these designs. The Flint and Neill / Moxon bascule cannot possibly have the two centres coexistent, so is in an unbalanced position throughout most of its opening cycle i.e. the machinery has to push the bridge's weight around as well as overcome wind loads.

From a construction point of view, I'd imagine the PB / Knight design causes most disruption to the estuary simply due to the sheer size of what must be assembled and the possible need for temporary propping in an unbalanced construction state. However, the differences will be small.

The Flint / Moxon design will clearly have structural issues to be addressed by virtue of being supported on one edge only - it must be very carefully balanced, and even then design against twisting due to wind when open might be a challenge.

However, the most unusual of these designs is the Mott MacDonald bridge. I understand that the "ring" feature rotates about its virtual centre on roller bogies , which would mean it is similar in design to the Bellmouth bascule bridge, and the Falkirk Wheel. There's a clear visual resemblance to the latter, although I think the Wheel's fins are architectural, rather than forming a counterweight as on the Motts design. I do wonder how the Motts bridge behaves laterally - the ring doesn't look wide enough to be stable under transverse wind loads.

Visually, my favourites are the Flint / Moxon and Motts designs. The PB / Knight bridge is just too "samey", with echoes of bridges like the South Quay Footbridge and Swansea Sail Bridge. It is very nicely detailed, especially in avoiding the expense of a torsionally stiff box girder.

I very much admire the simplicity of the Flint bascule, and while the Motts bridge has plenty of "logo potential" the ring and its fin look a little small and out-of-scale to the whole bridge on some of the images. Nonetheless, I prefer all three of these to the actual winner.

Alun Griffiths / Parsons Brinckerhoff / Knight Architects / Davy Markham






Kier Construction / Mott MacDonald / Atkins Bennett






Morrison Construction / Flint and Neill / Moxon Architects / Atkins Bennett

Tensegrity bridges: 5. Miscellaneous

So, tensegrity bridges. The Rome, Washington and Brisbane bridges are the major examples I've found, but there are one or two others to briefly mop up.

Bankside Bridge
This was a proposal for a crossing of the Thames as part of the Millennium Bridge design competition. The engineer was Mott MacDonald, I've been unable to confirm the architect. I have no information other than these two images:




St Petersburg
As part of a redevelopment proposal for the Apraksin Dvor area of St Petersburg, Wilkinson Eyre have proposed both a tensegrity roof, and a footbridge over the River Fotanka supported by a tensegrity "cloud". Again, no real details other than this picture:



Deployable bridge
This forms part of a thesis under preparation by Landolf-Rhode Barbarigos, who has a blog devoted to tensegrity structures. It incorporates "active" members (e.g. pulleys or telescopic bars) which help it find the right shape as it is deployed.




Bamboo Bridge
Designed by architect Michael McDonough, this is an unbuilt proposal for a 133m span bamboo truss bridge in Mendocino, California, described as using tensegrity principles. From the pictures, it's far from clear that it's a tensegrity bridge at all, looking more like a modularised cable-truss bridge incorporating some additional cable elements for stiffening.






Tube Bridge
Andreas Kirchsteiger has come up with a concept for a tubular bridge comprising rings connected by woven fabric (reminiscent of Cecil Balmond's Weave Bridge, perhaps). The support struts connect to the rings, but otherwise it's closer to the pure tensegrity idea than some designs. I can't really see how it would work - how are the rings stressed together, and how is overall bending actually carried?

Footbridge using "Simplex" modules
This was proposed in a thesis by Valentín Gómez Jáuregui, and is intended for lightweight short-span modular footbridges.




So, that's it for this series of posts on tensegrity bridges. How can I summarise? Firstly, it seems likely that a structure complying strictly with tensegrity principles (no two touching struts) is unlikely ever to be suitable for a real footbridge. Secondly, that tensegrity can be taken either to inspire a fairly orderly and visually conventional cable truss structure, or used as the jumping off point for chaotic visual complexity. This shows how central is the designer's choice in adapting any structural system.

Over half-a-century after it was first conceived, tensegrity still throws down a huge challenge to the structural designer - conventional habits of design, both in terms of structural analysis and visual order, simply aren't sufficient.