Swiss Bridges: 1. Rossgraben Bridge

Setting out on the first day of the IABSE study tour of bridges in Switzerland, we were told by our Swiss guides that many local civil engineers are unaware of the marvellous historic bridges to be found in their country. I'm not entirely sure I believe this. Sure, young engineers everywhere are often surprisingly ignorant of the achievements of their forebears, but come on - this is Switzerland - we're talking about several of the greatest bridges of the last century here!

Zurich itself held little hint of what was to come: there is an 1899 Maillart bridge here, on Stauffacherstrasse (glimpsed briefly from our coach later on). However, it was neither innovative for its time, nor visually interesting, having had its concrete structure faced with a conventional masonry spandrel wall at the insistence of city architect Gustav Gull. Most of the city's other bridges are similarly unremarkable, although there is an interesting rail station at Stadelhofen by Calatrava, one of his early works.

It was only as we left Zurich and headed along the highway towards Bern that hints of Switzerland's rich engineering heritage began to appear. We passed an unidentifiable building where massive steel arches supported a low-level flat roof. I also spotted Heinz Isler's incredible twin Deitingen shells. Built in 1968, these ultra-slender concrete shells are each supported at only three points, and are amongst Isler's most daring works. They were nearly demolished in 1999 (see John Chilton's book on Isler for details), so it's great to see them still in use.

Our first destination was Robert Maillart's Rossgraben Bridge, in Schwarzenburg. Built in 1932, it's a three-pinned reinforced concrete arch, spanning 82m and very similar to his better known Salginatobel Bridge (which we would be seeing the next day). Salginatobel had been built two years earlier, and there are various features at Rossgraben which are improvements: the centre hinge is made more visible, and the heavy concrete parapet at Salginatobel is replaced with a lightweight steel parapet, making the deck look far more slender.

Maillart's bridge designs are noted for two key types which he developed well beyond what his contemporaries achieved. One type is the deck-stiffened arch, for which we had three examples lined up to visit later in the day. The other, of which Rossgraben is a great example, is the three-hinged arch.

The three-hinged arch was often used in early concrete and metal arch bridges because it simplifies design calculations. It is also less vulnerable to ground settlement than other arch forms. It's rarely used in modern design partly because the hinges themselves are very difficult to design and to maintain. Rossgraben, for example, has a limited live load capacity partly because of corroded reinforcing steel in its hinges, which are of the Freyssinet hinge type.

In Switzerland, bridges in lightly-populated areas with little traffic, such as Rossgraben, are the responsibility of the commune, the smallest level of local government. More than half the communes have a population of under 1,000, and little money available to maintain bridges like Rossgraben, however historically important they may be. It's a tribute to the ongoing ingenuity of Swiss maintenance engineers that these bridges are sufficiently well refurbished to survive.

Where Maillart surpassed his contemporaries with the three-hinged arch was in his shaping of the concrete to very carefully mirror the internal forces. The distinctive near-triangular concrete side walls at Rossgraben and Salginatobel very closely match the shape of the bending moment diagram for a bridge of this type, with the result that there is a very even state of stress throughout the bridge, such that material is used very economically.

Rossgraben was a great start to our bridges tour: it's an excellent structure, totally at home in its environment. There's nothing inessential about it - every part does what it needs to and no more. Its shape isn't structurally optimum, but looks far better than if it were indeed optimum. Maillart reduced the curves on later three-hinged designs (most notably at Garstatt), but the more conventionally arched soffit at Rossgraben seems to soar across the river, quite a feat for hundreds of tons of the lumpy grey stuff.

What I most liked about the bridge is it's rough-hewn concrete physicality. Climbing up the arch towards the box section, you can get up close to the sawn-boarded surface finish, the only trace left from the original timber formwork. Concrete is much disliked for its monolithic grey intransigence, but striding between rocky outcrops it's far more at home in the landscape than an equivalent steel solution.

It has also weathered well. Like several other bridges we saw, lichen growth and staining add subtle colour to the surface, a yellowish hue which matched the autumn leaves when we visited.

It certainly isn't perfect. From certain angles it's apparent that the soffit curve isn't quite smooth. Also, because the arch widens out slightly at its ends, it gives the visual illusion of a reverse curve towards its springings, which looks wrong if seen from a very sharp angle.

However, these are just quibbles. Rossgraben is a mighty structure, beautifully shaped and charmingly textured. Still, we couldn't hang around to admire it for long - time was short and we had to walk to the nearby Schwandbach Bridge.

Further information:

• entry at structurae


• location on Google Maps


• UNL has images showing the original construction and bridge drawings


• David Billington's book Robert Maillart and the Art of Reinforced Concrete

Swiss Bridges: Prologue

In early November, the IABSE British Group organised a trip to Switzerland to make a study tour of various classic bridges, both historic and modern. These included five bridges by Robert Maillart (pictured), one by Christian Menn, and two by Jürg Conzett, as well as several passed in between. I felt very privileged to take part on this trip, and although I anticipated it would be enjoyable, it far surpassed whatever expectations I had for it.

It's going to take several posts to report on this trip properly. Some bridges may get an entire post to themselves, others may be grouped according to location (several bridges were often in very close proximity). Although I'll focus on the bridges themselves, much of the pleasure of the tour came from the company of fellow bridge enthusiasts: engineers, academics, architects, and our very knowledgeable and helpful Swiss hosts, without whom such a well-organised trip would have been almost impossible.

The organisers had helpfully provided background reading on the various structures, although many of us had brought along relevant books as well. But much of what we learnt about the bridges came from anecdotes related by the locals, and also from conversation with each other.

I won't say too much more about what I got from this trip just yet - but suffice to say, it was a thoroughly amazing experience, and one that I would recommend to anyone who enjoys bridges. I'm sure the reasons why will become very clear over the next few posts ...

River Soar Footbridge competition announced

RIBA have announced a new competition for the design of a £1.5m foot and cycle bridge in Leicester, connecting the National Space Centre to a new residential development on Wolsey Island. They are seeking expressions of interest from suitable designers with a view to shortlisting six teams to produce entries, each of whom will receive a £6,000 honorarium.

The shortlisted firms should be announced in December, with a winner known by March 2009.

The jury panel includes experienced bridge architect Jim Eyre, with an engineering adviser yet to be confirmed (although I hear that it is likely to be someone with similar experience, suggesting along with the £6k payments that RIBA is gradually improving the way they run bridge design competitions).

River Douglas Bridge competition winner announced

The title says it all: Arup & JDA Architects have won the River Douglas footbridge competition. As predicted here in September, the winning design is a stressed-ribbon footbridge supported on an arch.

The bridge has a structural steel arch, with the deck made from precast concrete segments hung on steel tendons. The competition entry [PDF] suggested that struts between the ends of the ribbon and the arch springings will provide self-anchoring and balance internal load effects, but in this type of design there are still substantial vertical forces at the ends which have to be anchored into the ground.

The £3m bridge's next challenge is for the promoters to actually find the money to build it.

The Curious Case of the Invisible Back-stays

There are two new iconic bridge proposals in the news this week, both unusual variants on the classic cable-stayed bridge form, one an innovation which makes structural sense, and one a carbon copy of a better-known design which makes very little structural sense.

A planning application has been submitted for a new swing footbridge over the Manchester Ship Canal (shown left). It connects the Imperial War Museum North on the south bank of the canal to the Media City development on the north side, which is to become home to several departments of the BBC in a few years time. The Canal is navigable, and therefore the bridge has to be moveable, in line with the Lowry Footbridge and Centenary Bridge nearby, both of which are lifting bridges.

The planning drawings are available online from Trafford Council's planning website (search for application H/70299), and make interesting reading, as they include a fairly thorough explanation of why this particular design was developed by Gifford and Wilkinson Eyre. The bridge is an asymmetric harp-style cable-stay bridge, swinging on a pivot located below the mast. The back-span is weighted to counter-balance the fore-span.

The bridge's details (e.g. the parapets and decking) borrow heavily from Wilkinson Eyre's previous work elsewhere, such as the Gateshead Millennium Bridge and Swansea Sail Bridge. However, the bridge's unusual feature is without doubt its striking central mast, which comprises eight circular poles fanning out from a concrete base. The geometry of these has been generated from two constraints: the harp layout of the cables (which means each pole must be taller than its neighbour), and a desire for the deck cable anchorages to be tangential to the deck's curve in plan (which means the points of support must be spread out horizontally at height). Personally, I think the mast looks unecessarily fiddly, and wonder to what extent it may come to be compared to a giant skeletal hand (particularly when painted grey-white, as intended).

The other new bridge design I've noticed this week is in marked contrast to the Manchester footbridge. The Narrow Water Bridge (shown left) is to be a new highway bridge linking Ireland to Northern Ireland across Carlingford Lough. 280m long, it comprises a large cable-stayed span, with a shorter rolling bascule span opening to permit river navigation.

The image released to the press shows a design drawing heavily on Santiago Calatrava's Alamillo Bridge, although with paired masts instead of one. Like Alamillo, the masts are inclined away from the deck, so that the weight of the mast to some extent counterbalances the tension in the cables. Like Alamillo, there are no back-stays, the far more obvious and economic way of balancing out these tension forces. Because the live load on the highway is variable, the balance can never be perfect, with the result that the mast must resist enormous bending forces and is therefore both massive and expensive. Its other major disadvantage is that it needs substantial temporary propping during construction (or to be built incrementally simultaneously with the deck, as Alamillo was), which again adds considerably to the cost compared to the more conventional vertical mast.

This case of the invisible (or just plain missing) back-stays is curious because it indicates a trend which is difficult to explain. While Calatrava has had several cable-stay bridges without back-stays built (Alamillo, Sundial Bridge, Puente de la Mujer, Chords Bridge), there are very few other examples (Puente Atirantado, Mariansky Bridge, the unbuilt River Wear Bridge). This is largely because however sculpturally striking the concept may be, few schemes come without the need to match expenditure to value. As the Narrow Water scheme is still at an early stage, it's entirely possible that an entirely different design may eventually be built, but the River Wear case shows how readily public enthusiasm for an "iconic", aspirational statement can build. It's great that the Irish authorities have ambitions beyond the humdrum, but it will be interesting to see whether they can raise the budget that would be required for their current design.

Bridges news roundup

Steel? Psshaw. Concrete is best for iconic bridges

The new chairman of the Concrete Bridge Development Group, David Ball, reckons concrete is under-utilised in British landmark structures, pointing to the material's now much improved durability. Maybe so, but steel remains popular for landmark bridges because it's far easier to erect, and in many cases more compatible with lightweight and geometrically unusual structural forms.

Glasgow bridge in trouble. Not quite

It seems there's always a bridge in Glasgow in trouble of some kind. And now the virus is spreading, as the completion of the Forthside Footbridge in nearby Stirling will apparently be delayed until next year. The Sunday Herald speculates idly on possible common factors, noting the presence of cable supplier Macalloy and main contractor Nuttall both on Forthside and on the unlucky Clyde Arc bridge. (Fortunately for Nuttall, the Herald hasn't recalled that they are also building Glasgow's Squiggly Bridge, which like Forthside has also suffered delays.) Nuttall state that stressing the Forthside Footbridge's stays is proving difficult, something which was acknowledged in the designers' paper at the Footbridge 2008 conference this year (see PDF).

Personally, I think it's entirely to Nuttall's credit that they have taken on several ambitious bridge schemes. Difficulties and set backs aren't entirely unexpected on innovative designs, and perhaps engineers should make more of the opportunity to explain this to their clients, and the public in general.

Weave Bridge to open in November

Cecil Balmond's colourful Weave Bridge is nearing completion at the University of Pennsylvania. 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. In his use of colour and advanced computer geometry, he's an engineer with a distinctive style, a refreshingly eccentric contrast to normal engineering methods.

The Weave Bridge looks from the pictures to be a Warren truss with overhead bracing - a design that's essentially about 160 years old. 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. Similarly, Balmond's Coimbra footbridge looks in elevation like an arch, but in reality it's a twin cantilever bridge whose main purpose is to offer a platform for Balmond's complex, startling glazed parapets.

Bridge competition debris part 4: River Douglas

This is the fourth (and, for now, final) post in a series collecting together entries to recent RIBA footbridge design competitions. The other posts have covered New Islington, Leeds-Liverpool Canal, and Sheffield Parkway.

At the time of writing, the winner of the River Douglas competition hasn't yet been chosen (although the announcement is due imminently). Seven entries have been shortlisted from a total of 110. I've discussed the shortlisted entries before, so the present post will only cover entries from designers who didn't make the shortlist.

One thing noticeable about the unsuccessful entries is that they have a less clear idea of structural form than those that were shortlisted. Perhaps this has something to do with the presence of respected bridge engineer Roger Buckby on the jury panel.

As ever, click the image for a larger version, and the name of the designer for a link to any relevant details on their website.

PS: I'm away for a week so won't be posting or moderating comments until I return.

Not shortlisted

A+J Burridge


Architects in Residence


Peter Barber Architects


Designer unknown (image from Realise3d)


Shuichiro Yoshida


Added 22 December 2008
Amenity Space

Burd Haward

Added 27 January 2009
Feix and Merlin

Added 5 May 2009
Frederic Schwartz Architects