10 essential bridges books: 8. Robert Maillart: Bridges and Constructions

If you're looking for a book about the great Swiss engineer Robert Maillart, then David Billington is your man. He's the author of Robert Maillart's Bridges (1979), Robert Maillart and the Art of Reinforced Concrete (1991) and the biography Robert Maillart: Builder, Designer, and Artist (2008). Maillart also features prominently in his The Tower and the Bridge (1985) and The Art of Structural Design: A Swiss Legacy (2003). If there's a number one Robert Maillart fan (and expert), it's quite clearly David Billington.

However, none of these is the essential book on the subject. That honour remains with Max Bill's Robert Maillart: Bridges and Constructions (1949; my copy is the 3rd edition, 1969, Pall Mall Press, 184pp).

Bill, an artist, architect and journalist, wasn't the first to recognise that Maillart's talent merited recognition beyond the narrow field of structural engineering. In the mid-20th century, a number of people were gradually recognising that the singular reinforced concrete constructions of the Swiss designer had merits beyond the purely technical. However, in choosing to write at length about Maillart's work, Bill played a key role in the designer's enduring fame.

The bulk of this book is taken up by documentation of many of Robert Maillart's key works: black-and-white photographs and generally short explanations in German, French and English. All his finest bridges are here, plus certain key buildings. There is much for the curious reader to consider, and although the text is generally brief, the overall impression is thorough. In contrast, Billington's books never really provide a comprehensive survey, but are significantly better in their analysis of Maillart's significance and his design methodology.

The reason Bill's book is still worth searching out secondhand is that it features a great deal of primary material. There are plenty of diagrams and design drawings, including reinforcement details, but also a number of Maillart's own writings are reprinted - I believe these are not otherwise available in English (the originals are available online at the Schweizeriche Bauzeitung archive).

Maillart was undoubtedly one of the true geniuses of 20th century bridge design - his reputation may have been hyped in recent decades, but it is certainly deserved. I suspect that he is the first name most bridge engineers will think of if asked to name a famous bridge designer of the period.

Bill's book remains valuable in returning the reader directly to Maillart's own words and designs, prompting the inevitable thought of how might the modern designer have drawn and built the same things, or things of equivalent quality. Reading Maillart's passionate defence of simplicity ahead of "accuracy" in calculation, it becomes clear that even today, few structural designers have such clear vision or self-confidence.

"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.

"Vom Holzsteg zum Weltmonument" by Andreas Kessler

A mere two months after visiting the amazing Salginatobel Bridge itself, I was delighted to get a book on it for Christmas: "Vom Holzsteg zum Weltmonument - Die Geschichte der Salginatobelbrücke" (ISBN 3 9520963 1 8, Verlag AG Buchdruckerei Schiers, 1996) [order from the author]. The title roughly translates as "From the boardwalk to the World Monument - The history of Salginatobel Bridge".

It's not the easiest book to get hold of. The Prättigauerhof hotel in Schiers didn't have any copies when I visited, and the local publisher communicates only in German. My copy was purchased directly from the author - follow the link above to email Mr Kessler. It can be paid for either with international bank transfer (expensive) or simply by posting cash. It's definitely best to email first as postage rates may vary and I don't know how many copies he has left.

If ever there were a labour of love, this 232-page book is it. The village of Schiers has a population of roughly 2500, yet this locally-published book is nonetheless an unexpectedly lavish tribute to one of the world's greatest bridges. To my knowledge, it's the only book devoted entirely to this very singular structure.

The book is in ten chapters, mostly written by Andreas Kessler, but with contributions from Jürg Conzett, Duri Prader (son of the bridge's builder, Florian Prader), and others. There is also an extensive bibliography, several pages of the original bridge design calculations, and three fold-out construction drawings showing the general arrangement of the bridge, the concrete reinforcement, and the timber falsework.

The book explains the difficult site on which Salginatobel Bridge was built - a deep ravine between the small village of Schiers and the tiny hamlet of Schuders. Before the bridge was built at high level, a number of low-level crossings of the Salgina existed, generally of timber, prone to flood damage, and providing access only to a steep path leading up to Schuders.

The book discusses the key figures reponsible for the design and construction of the bridge: Robert Maillart, the structural engineer (pictured); Richard Coray, who designed the falsework; Peter Lorenz, the district engineer; and Florian Prader, the contractor. Jürg Conzett explains the state of the art in arch bridge design at the time, and compares Maillart's various designs both built and unbuilt from the Stauffacher bridge of 1899 to the Lachen bridge of 1940, with the help of an excellent scale drawing showing them all. While the Salginatobel bridge is one of Maillart's most spectacular achievements, he proposed far greater arch bridges at Schaffhausen, Bern and elsewhere.

The book covers various proposals for a new Salgina crossing from 1914 onwards, including a suspension bridge design proposed by Richard Coray. Eventually, a competition was held to obtain design-and-build proposals, with Prader & Cie's tender (designed by Maillart) proving to be the least-cost design. The construction of the bridge during 1929 and 1930 is documented in detail, including a series of photographs which show the difficult cantilevering construction of the falsework very clearly. In the steep rocky terrain, the timber centering (pictured, in model form!) was a major achievement in its own right.

The book goes on to address the bridge's history since it was opened, including the period in the second world war when plans were made to install explosive charges for the bridge's possible destruction. There is also an extensive chapter discussing the bridge's growing reputation as a work of art or historical importance, citing the writings of architecture critic Siegfried Giedion in the 1930s, Max Bill's book on Maillart in 1949, the substantial writings of David Billington, and many less well known authors.

The bridge's award in 1991 of the status of an International Historic Civil Engineering Landmark by the ASCE (American Society of Civil Engineers) is also covered, including descriptions of the other IHCELs declared up to 1995.

Throughout, the text is supported by excellent black-and-white photographs and diagrams, many unavailable readily in print elsewhere, making this book a very fitting tribute to a marvellous bridge.

The only problem, for me at least, is that it's all in German, which I can't read. So I must apologise that I can't comment in detail on the text at all! Even with this somewhat major handicap, it looks to me to be an excellent book, with high production values and a welcome thoroughness. Expect a fresh review if my German is ever up to it!

Swiss Bridges: 4. Salginatobel Bridge

The first day of the IABSE study tour had been pretty marvellous - great scenery, great bridges, great company. But it was very much run into second place compared to the excitements of the second day.

As with the first day, there was plenty to see from the coach to indicate that the Swiss have engineering accomplishments to be proud of. Driving from Zurich to Schiers, we passed below the 1965 Bircherweid stressed ribbon footbridge designed by René Walther, for example. Driving from Schiers towards Klosters, we stopped at a reinforced concrete vierendeel truss bridge (apparently the only one of its type in the country) for a quick look. And of course the Alps are riddled with spectacular road tunnels, avalanche shelters, funicular railways etc.

The first stop of the day, Robert Maillart's Salginatobel Bridge, was the only structure in the entire trip to be properly signposted, with brown tourist signs (Rossgraben did have some less visible signs, and both it and Schwandbach have tourist information boards). It was also the only bridge that seemed to have an entire cafe dedicated to visiting pontists: the Prättigauerhof in Schiers. You can stay overnight, eat pizza, drink coffee in this 1627 building.

Or just admire the various artefacts on display. Tourists stopping off en route to the bridge can examine detailed construction drawings, a model of the formwork, souvenir lumps of concrete taken from the bridge (presumably during its refurbishment), and inspect an original section of the bridge parapet. Sadly the Prättigauerhof didn't have any copies of Andreas Kessler's locally published book Vom Holzsteg zum Weltmonument - Die Geschichte der Salginatobelbrücke, let alone any souvenir t-shirts. I had to settle for a postcard.

Writing about the Salginatobel Bridge when it was presented with an International Historic Civil Engineering Landmark plaque in August 1991, David Billington said:

"Such structures remind us that in this fragmented world, a highly rational, deeply educated engineer can integrate utility and beauty and bring into being objects to which all engineers must make at least one pilgrimage in their lifetimes." (Structural Engineering International 4/91)

So: a marvellous mecca for engineers, or just a nice lump of concrete neatly set off by the lovely landscape?

It took some time to get around Salginatobel Bridge. For one thing, there were plenty of places to view and photograph it from - on top, underneath, from the road at one end, and from a viewing platform at the other end. But no amount of rushing around could distract from a palpable sense of awe that grew the longer I stayed there. Billington is right: this was an almost religious experience, which caught me quite by surprise.

The design of Salginatobel bridge is undoubtedly excellent (although not quite perfect - see below). And the setting, 90m above the bottom of a deep valley, with forest to one side and gnarled rock to the other, is magnificent. Photos struggle to do justice to its promethean splendour - you have to be there with the mountains to all sides to really understand how great this bridge is. Many photos of the bridge nestling amongst the forested hillsides fail to give any idea of its scale - it's a big bridge in this context, making it even more remarkable how good it looks.

It's a great example of how the introduction of a bridge can transform relatively ordinary scenery. Sure, it's grand scenery, but there are far more spectacular gorges and mountains throughout the rest of Switzerland. Without the bridge, this would just be one of many pretty mountain valleys. Salginatobelbrücke literally makes concrete the pervading spirit, the genius loci, of this particular valley, as if rocks layed down a hundred million years ago had just been waiting patiently for a bridge to one day vault majestically outwards.

Like many great bridges, what you can see is only half the true story. Excellence in bridge design is as much about how a bridge will be built as how it will look. Without Richard Coray's audacious timber centering, the 90m span of Salginatobel Bridge could never have been built. In 1930, Maillart won the job because his was the most economic solution, and it's unfortunate that this would no longer be the case. Now, a prestressed concrete structure or welded steel bridge would be much cheaper, and both lead to structural forms suited to factory production or repetitive site assembly, certainly not an arch requiring major temporary works. A bridge like this is unlikely ever to be built again.

So where are its flaws? The masonry abutments certainly detract, and the solid concrete parapets give a heavier appearance than at Rossgraben (but not terribly so). However, I think it would be quite frightening standing on Salginatobel and looking down 90m if the parapets were only of the post-and-rail type (it's okay less disconcerting at Rossgraben because the drop is only about 12m).

Of course, although the arch shape looks like it has been precision-engineered to match the bending moment diagram for a three-hinged arch (see diagrams linked below), it's the perfect shape just for one very specific (and unlikely) arrangement of loads. Robert Maillart realised this and changed the shape of his later three-hinged arch bridges, but the less logical shape at Salginatobel undoubtedly looks more beautiful.

As at Rossgraben, if you look along the arch at an acute angle, there seems to be a reverse curve towards the springings, an illusion created by the way the arch widens at its ends.

The original bridge design also dates from a time before concrete's long-term durability was well understood - there was no waterproofing, minimal cover to reinforcement, poor quality concrete and inadequate drainage. These were all put right with repairs in 1975/76 and a US$1.3m refurbishment completed in 1998, including complete replacement of the parapets (which is why there's a section of parapet outside the Prättigauerhof). The engineers did a remarkable job on the repairs, blasting off and then shotcreting most of the concrete surface. Unusually, formwork boards were then applied to the shotcrete to reinstate the original appearance.

The flaws are pretty irrelevant. It's as much the glorious setting as the bridge itself, but Salginatobel Bridge remains Maillart's masterpiece, a truly singular sculpture in reinforced concrete that must rarely, if ever, have been equalled. We had a busy day ahead and were already running late, but it was difficult to tear ourselves away - I would have been quite happy just to stay there for another hour drinking in the view, or exploring the bridge more closely.

It was lucky the bridges still to be seen would prove to be Salginatobel Bridge's equal, in their own ways.

Further information:

• entry at structurae
• entry at Wikipedia
• location on Google Maps
• Schiers tourist information (including picture of scaffolding)
• Prättigauerhof information (including more pictures of scaffolding)
• photos of refurbishment (in French)
• diagram of bending moments explaining shape of bridge
• The Salginatobel Bridge (student paper, PDF)
• David Billington's b0oks Robert Maillart's Bridges; Robert Maillart and the Art of Reinforced Concrete; and The Art of Structural Design - A Swiss Legacy
• Heinrich Figi's paper Rehabilitation of the Salginatobel Bridge (Structural Engineering International 1/2000)

Swiss Bridges: 3. Traubach Bridge & Bohlbach Bridge

From Bern we headed to Habkern, near Interlaken, home to two more deck-stiffened reinforced concrete arches by Robert Maillart. Habkern is in a very hilly but essentially agricultural area. It has steep-sided valleys but they are somewhat less spectacular than the settings of the two bridges at Schwarzenburg (Rossgraben and Schwandbach).

Built in 1932, Traubach bridge is straight in plan, and much plainer in appearance than Schwandbach. In particular, its solid concrete parapets look very heavy, and despite the slender arch it would be difficult to describe the bridge as elegant. A slight overhang on the outer face of the parapet only draws attention to its monolithic flatness, rather than breaking it up.

Also contributing to the heaviness are the plain-faced concrete abutments and wing walls, which Maillart successfully avoided elsewhere. A couple of service pipes are supported on one side of the bridge, and overall it gives the appearance of something sturdy and practical, rather than aesthetically exceptional.

The arch spans 40m, and the bridge has been recently strengthened to carry heavier vehicle loads. One advantage of the deck-stiffened arch when it comes to strengthening is that most of the live load bending is carried in the deck, which is much easier to strengthen than the arch. At Traubach, the deck slab and parapet beams have had much of their concrete replaced, allowing a prestressing system to be introduced (see photos at link below). A hydrophobic surface coating has also been added to guard against chloride ingress from highway de-icing salts.

Bohlbach bridge is a short walk further along the same road from Traubach, and was built in the same year. Spanning a mere 14.4m, it's another deck-stiffened arch, but curved in plan similar to Schwandbach Bridge. In many ways, it was a dry run for Schwandbach, which would be built one year later.

At Bohlbach, the bridge abutments are almost invisible, and the absence of wing walls means the solid concrete parapet is less obtrusive. I quite liked it: the setting is charming, with a waterfall and fallen tree across the stream to one side; it's at a scale where monolithic flat grey concrete works without being overpowering.

However, it's easy to see how Maillart improved the design by the time of Schwandbach. By then, the solid concrete parapets would be gone, replaced with lightweight metal railings, making the bridge far more elegant. Also, at both Traubach and Bohlbach, the arch and deck merge together for a considerable length of the span. At Schwandbach, they only just kiss, giving a much more light and open aspect.

Much of the pleasure of our trip was in the Alpine scenery, and the vernacular wooden buildings to be found everywhere we went. Nearby to Traubach we found a lovely farm hut, festooned with pots, pans and sledges ready for winter, as well as a delightful shepherd's barn.

There was also a simple king post truss covered wooden bridge across a stream, which attracted almost as much attention as the Maillart bridges! This had clearly been recently renovated, with half the roof re-shingled, and other protective boarding replaced. This was very much a craft structure, with traditional timber jointing and marking in evidence. It's hard to imagine that much in the way of analytical calculation was required for its design.

But in many ways the Maillart bridges are also craft structures - plenty of evidence of their making (in formwork boarding marks) is present, and Maillart himself avoided calculations whenever possible (relying on engineers such as his assistant Ernst Stettler for this), to the extent that Swiss academics derided his tanzboden statik ("dance-floor statics").

We left Habkern tired but very much a group of happy pontists after a great day tramping around the countryside in search of classic bridges. I think we had little idea quite how different the following day would be.

Further information:

• entries at structurae: Traubach, Bohlbach
• locations on Google Maps: Traubach, Bohlbach
• photos of refurbishment works to both bridges (German)
• David Billington's book Robert Maillart and the Art of Reinforced Concrete

Swiss Bridges: 2. Schwandbach Bridge

Robert Maillart's Schwandbach Bridge, built a year after its Rossgraben neighbour in 1933, is only a short walk further up the same road. It's regarded (at least by serial Maillart-booster David Billington) as one of Maillart's masterpieces. It takes a design he had developed at Bohlbach in the previous year, and perfects it, much in the same way as Rossgraben built upon what he had learned from Salginatobel Bridge. We were due to visit Bohlbach later in the day, so I'll leave any comparisons for a later post.

Spanning 38m, with an arch only 200mm thick, it's no surprise that Schwandbach Bridge is seen as a classic of minimal, elegant design. It's an example of a deck-stiffened arch, a form which Maillart didn't invent but did pursue more vigorously than others. Essentially, the stiffer the bridge arch is, the more it attracts bending moments - if it can be made very slender, the stiffer deck will then carry most (nearly all) of the bending - allowing the arch itself to be very slender.

The aesthetic merits of this approach are conflicting - sure, the arch looks nice, but the deck above can look very heavy indeed. It works well on relatively narrow bridges, where the parapets can double as deck beams and provide the necessary stiffness. On wider bridges, the deck slab itself must be made stiffer, resulting in a very ungainly structure.

The real advantage is in the cost of construction of the bridge. Because the arch is thin, it can be built using relatively lightweight and hence less expensive formwork. The arch itself is then used as the support while crosswalls and then the deck are constructed.

What makes Schwandbach such a work of genius is not the admirable slender arch, but how the bridge is arranged in plan. Carrying a curved roadway across a deep valley, the arch is curved on its inside edge, but straight on its outside edge.

The inside edge lies directly below the curved edge of the deck, but the outside edge is offset more from the deck as it gets closer to the arch springings. This allows the crosswalls to be splayed out, carrying the thrust from centrifugal and eccentric forces in the deck down into the arch - and as the arch is wider at its supports, it is much more stable against the same loads.

It's the sort of thing that seems amazingly simple in its use of geometry to control the load effects in the bridge, but is usually very difficult to develop into such a consistent and confident solution.

Like Rossgraben, the grey concrete works well in Schwandbach's setting, especially where it's stained with moss and lichen. The artificial geometry of the highway is made to seem like a natural feature, as much an integral part of the setting as the rocky valley sides.

As at Rossgraben, we didn't get to spend all day admiring Schwandbach Bridge, but had to head onwards, to lunch in the Alps en route to two more Maillart bridges near Interlaken.

Further information:

• entry at structurae
• entry at Wikipedia
• location on Google Maps
• sketch of the bridge by Santiago Calatrava
• diagrams and equations explaining behaviour of deck-stiffened arch (in French)
• David Billington's books Robert Maillart's Bridges and Robert Maillart and the Art of Reinforced Concrete

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