20 November 2008

Swiss Bridges: Epilogue

So that was that, the end of the IABSE Study Tour in Switzerland, and very much back down to earth afterwards.

One of the main things I took away from the trip was how much can be achieved by engineers when they are confident in their creativity, skilled in their art, and (relatively) unimpeded by bureaucracy. Few if any of the bridges we saw had the involvement of an architect, or if one was involved, the structural engineering was central to the design process. Much of this is down to the challenges set by the remarkable Swiss landscape rather than anything else: the engineering absolutely has to take precedence.

That most of these bridges are also aesthetically successful is far from a foregone conclusion. Several of Robert Maillart's bridges are visually clumsy (including one that we visited, Traubach Bridge). These bridges are masterpieces because of the deep involvement and care lavished upon them by their designers. I think bridge designers anywhere can learn what can be achieved when they have a strong vision and can minimise the need for compromise.

David Billington has suggested that all engineers should consider making a pilgrimage to the Salginatobel Bridge. Before this trip I'd have dismissed that as daft idealism, but now I'd quite happily go along with it. There's plenty to learn technically from structurally challenging historic bridges such as these, but more important is what they offer in both inspiration and aspiration.

I know I'm already looking forward to the next study tour!

While in Epilogue mode, can I take the opportunity to ask for more feedback from anyone reading this blog? I'd be keen to hear comments on the bridges, opinions, news or anything else; it would be good to know that someone is reading, and I'm particularly open to discussion, debate or even dispute!

19 November 2008

Swiss Bridges: 7. Traversina Footbridge

From the Pùnt da Suransuns, we walked to our final bridge of the day, and our final bridge of the study tour. Built in 2005, the second Traversina Footbridge is another design by Jürg Conzett, and replaced his previous structure on the site (you guessed it, the first Traversina Footbridge), which had been destroyed by a falling boulder.

One thing that's impressive about Conzett is his ability to apply equal levels of ingenuity and imagination to the design of very disparate structural forms: in addition to those at Via Mala, his Coupurebrug in Belgium is another unusual example. While at first sight the second Traversina Footbridge looks like a relatively conventional suspension bridge, it turns out to be far from conventional and probably unique.

Spanning 56m across a 70m deep gorge, the levels of the hiking trail on each side are very different, and as a result the bridge is a staircase which increases in steepness towards one end. The deck and handrail are a mixture of steel and timber elements, hung from two suspension cables with a truss-like arrangement of hangers. The main suspension cables hang from abutments which are at essentially the same height, with the result that the hangers are short at the upper end of the staircase, and progressively longer towards the lower end.

The geometry and cable forces were derived using that quintessentially Swiss technique, graphic statics, specifically a Cremona diagram. Where many engineers would plunge in with the latest non-linear form-finding software, Conzett gets out a pencil and graph paper and harks back to the methods of a century ago. The trussed hangers make the structure far stiffer than a normal suspension bridge, and in practice it barely sways at all in use.

I've given links below to a couple of websites that have photos of the bridge construction, and these are well worth a look. Building a bridge above a gorge where the only access routes are steep, narrow mountain paths, is quite a challenge. As several people on the study tour noted, the people who work out how to build a bridge are often the unsung heroes of any project. All the five Maillart arches we saw relied heavily on the falsework designer to bridge the gap first, and the erection engineering at Sunniberg would have been a major design package in its own right.

To build Traversina, a specialist firm installed a "cable crane", essentially a travelling crane running on cables strung between the trees. This allowed materials and most importantly the concrete for the abutments to be brought up from much further down the hillside. The cable network and deck panels could then be assembled in mid-air using a helicopter and roped-access specialists. It's quite a feat in a place like this.

The bridge that results is magnificent but also highly peculiar. This was pretty much the only bridge we saw where I got vertigo just crossing it, let alone leaning over the side. I think this was a combination of the precipitous location and the fact that it's a staircase - you never really feel you're on a level platform, and it's mildly disorienting.

It was getting quite late by now, and much of the return journey down the hillside was in near darkness. Looking back across the Via Mala gorge, I could only just make out the bridge, a pale grey ghost amongst dark grey shadows. It did seem afterwards like something out of a dream, the genius loci returning to its home in the spirit world.

Further information:

Sunderland gets its iconic bridge. Or does it?

Time for another quick interruption to my ongoing dissection of all things Swiss.

Sunderland City Council have today announced their preferred design for the new River Wear Crossing, a project which has been rumbling on for some time and may finally be heading towards a resolution.

They've gone for the much-discussed iconic design by Stephen Spence and Techniker (pictured). This RIBA competition-winning design was kept secret for three years, before finally being unveiled in September, and then subject to a somewhat ill-informed public consultation.

Sunderland's decision to go along with the weight of public opinion leaves them with the not-so-minor difficulty of finding an extra £30m to pay for it.

The council's chief exec David Smith is quoted: "It's something that has never been done before in a shape and size like this. An international audience will be talking about Sunderland's bridge." They certainly will - although not necessarily for the reasons that Sunderland might imagine.

To their great credit, Sunderland now acknowledge that the current design may not survive further feasibility study and design development, and will be embarking on further work to "come up with a working design" and firm up the cost. This may apparently include "toning it down". It's not clear where this leaves Spence and Techniker, who might be somewhat disgruntled at someone else taking forward their competition winning bridge.

So, headline is that Sunderland will be getting its iconic bridge after all, but it's clear that this remains far from certain - the funding isn't in place, and the design could still be radically changed or even ditched.

18 November 2008

Swiss Bridges: 6. Pùnt da Suransuns

Throughout our second day of the bridge study tour, we were accompanied by Jürg Conzett. In the afternoon, we visited two of his bridges, both part of the Via Spluga hiking trail in Via Mala gorge near Thusis.

The gorge itself is absolutely stunning. At several points, you can look 80m down and struggle to see the water below in a dark crevice as little as 1m wide. And looking up there are tall cliffs for another 100m or more. Where it widens out, it's crossed by bridges including Christian Menn's Great Viamala Bridge.

We walked only a relatively short length of the hiking trail, under the Menn bridge to first see the Pùnt da Suransuns, a remarkable stressed ribbon footbridge spanning 40m across the river. I'm not entirely a fan of the stressed ribbon bridge - while they can be beautifully slender structures, the way they droop often looks unhappy - see the Maldonado Bridge in Uruguay for an example of this. They rarely meet the engineering or administrative constraints of most sites - the sag leads to slopes greater than are desirable for many users, and for the same reason they don't work where there is limited headroom below (which is the case for most footbridges). They are also almost never the most economic solution, with the costs of expensive foundation anchorages far outweighing the material savings to be made in the main bridge deck.

Suransuns, however, is a perfect example of just how a stressed ribbon bridge can work well. The river valley sides have plenty of stable rock, required for an efficient anchorage design, and the setting demands as minimal an intervention as possible. And Suransuns must be amongst the most minimal of stressed ribbon bridges you could get.

The bridge comprises granite planks, 60mm thick, 250mm wide, and 1100mm long, which sit directly on stainless steel strips only 15mm thick, and are held apart by 3mm thick aluminium inserts. That's basically the entire structural system of the bridge, and it is beautifully complemented by ultra-minimal steel handrails supported on 16mm diameter vertical rods.

Clearly, the location allowed Conzett to break many of the rules which normally bind footbridge design: minimum widths; maximum gradients; strength of parapets; susceptibility to vibration. However, it's the way he responded to some of the challenges of stressed ribbon design which was particularly impressive. Bending of the deck slab at its support abutments is a key consideration in design, which Conzett dealt with using a seemingly simple "leaf-spring" arrangement, just using more of the main stainless steel strips locally.

The elegance of the bridge's minimal silhouette is matched by the simplicity of the engineering, and for me, this was one of the best bridges we saw. Conzett, Bronzini and Gartmann are designing a very similar bridge (albeit multi-span) at Gemeinde Windisch, and it will be interesting to see how it compares.

Further information:

17 November 2008

Swiss Bridges: 5. Sunniberg Bridge

From Salginatobel, we drove south towards Klosters and by far the biggest bridge on the study tour. Christian Menn's Sunniberg Bridge is, to my mind, his finest work, and since its completion in 1998, it has been a major landmark in the Landquart River valley.

The bridge is part of the Klosters bypass scheme, but at CHF 17m, its cost is dwarfed by the CHF 345m cost of the 4km long Gotschna Tunnel, which the highway enters at the south end of the bridge. Nonetheless, the bridge design was (rightly) chosen in place of a cheaper box-girder option because of the environmental sensitivity of the landscape.

Bizarrely, the bridge was opened seven years after completion, in 2005, by Prince Charles, that notorious foe of modern architecture. Prior to opening, the bridge had provided the construction access for the tunnel works. I find it hard to imagine the opening ceremony: "Yes, one is proud to open this monstrous carbuncle if it makes it ten minutes quicker to get to one's skiing holiday."

Approaching the bridge from the south, we drove down towards it from higher in the valley. From here, the bridge looked small, minimal, barely present in the landscape. It was only once we approached it from ground level that its scale became apparent. 526m long, 12m wide, with a longest span of 140m, and pylons up to 77m above ground level, it's a bridge on the heroic scale.

Menn's adoption of the extradosed bridge form allows the deck to be more slender than in a conventional box girder bridge (because it's supported by cables from above), while the overall bridge doesn't compete unduly with the mountain landscape (because the cables are at a much shallower angle than a conventional cable-stayed bridge). The extradosed bridge is really a special case of a post-tensioned bridge, where the cables are lifted out of the deck to allow the overall bridge deck to carry higher bending moments and shear forces at pier positions. Essentially, it's more complex and expensive than a post-tensioned bridge, but less efficient than a normal cable-stayed bridge. But when done well, it can certainly look very impressive indeed.

The bridge was designed without any expansion joints, which means that under temperature variations, the deck "breathes" in plan i.e. sways sideways. The piers are designed with relatively slender bases to be flexible enough to withstand this movement, and along with the need to maintain highway headrooms, this leads to their distinctive and elegant Y-shape. Everything about the piers and pylons is well thought out - their gentle curves, echoing the tall trees nearby; the way the bridge deck nestles between their arms; their continuity (many cable-stayed bridge have very different pylon forms above and below deck); the clever way the massive steel cable anchorages are hidden within them; and their delicate but robust proportioning.

Unfortunately, we spent so much time tramping around admiring the bridge from below, that we had no time to stop at deck level. However, a pretty good idea of the appearance can be found from this advertising poster, which I spotted when arriving at Zurich airport. Good to see the Swiss pride in this monument to engineering heroism.

Later in the day, we'd see one of Menn's concrete arch bridges at Viamala gorge, but it wasn't really flattered by the viewpoint, and certainly didn't compare well to Sunniberg. Sunniberg Bridge is a proper engineer's bridge - all the key elements of the design have a sound technical rationale, but the combination of choices made and the way they are each worked out is absolutely exquisite.

Further information:

Frowning at the Dubai Smile

Another quick interruption to the ongoing Swiss bridge reports - I just couldn't let this one pass!

It's another "iconic" bridge. And in Dubai, home to the world's greatest Competitive Extravagance Championships. Care to guess whether it's a structurally sound idea or not?

I have to admit, I'm always in two minds about some of the crazier bridge schemes which seem to be in vogue at present. On one hand, everything in my sober engineer's analytical heart yearns for designs which, like the Swiss masterpieces I'm currently covering, achieve aesthetic excellence by exploring an imaginative engineering concept with integrity and courage. On the other hand, I understand intellectually that in the age of post-modernist excess, there are other yardsticks by which a structure can be judged, particularly if (as in this case) it's just an excuse to erect a giant logo-as-sculpture, Sheikh Maktoum's equivalent of the McDonalds golden arches.

So, the Dubai smile. Dubai has just announced its chosen design for a seventh crossing of the Dubai Creek, following competitive proposals submitted by a variety of "specialised global companies" (none of whom are named - not even the winner - so if anyone knows who they are, please share). The new bridge crosses a 400m wide waterway, and according to one story it replaces the existing Floating Bridge, quadrupling the current traffic capacity. I hope that story has it wrong, because the Floating Bridge was only built last year!

The bridge deck is suspended from a 100m tall inverted arch (see illustration, although there's a better image here), apparently for the sole reason of creating a welcoming smile for visitors to admire. It certainly doesn't make any structural sense, as the enormous back cantilevers have to carry the weight of 12 traffic lanes in unassisted bending. Essentially, the bridge is a bit like a conventional suspension bridge (the easier way to achieve a smiling appearance) except with the support towers in completely the wrong place.

It has more in common with cable-stayed bridges with the backstay missing (e.g. Alamillo Bridge or the unbuilt River Wear Bridge), except it seems to have been consciously arranged to be even more structurally inefficient. What's especially disappointing is that it could have been made much more efficient fairly easily by stringing a few more cables horizontally between the twin cantilevers.

At 61m wide and (at a guess) about 500m long, the bridge is intended to cost 810 million dirams, which is about £147m. That works out at £4,800 per square metre of deck, which is not inappropriate for a "normal" landmark bridge but is well below the going rate for a structurally inefficient design. I don't know whether the availability of a large pool of poorly treated immigrant labour from India and Pakistan helps get the costs down or not.

Returning to my dilemma, I think it's fairly easy to take a view on this one. I can't find anything to smile about in the Dubai Smile. There's no structural rationale whatsoever, and it will be monumentally difficult to build as a result. It may be a topsy-turvy arch to fit into Dubai's topsy-turvy Alice-in-wonderland world, but essentially it's just plain old kitsch. I can see the puritan faction amongst bridge designers starting a Campaign for Real Bridges right here.

16 November 2008

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:

14 November 2008

Bridges news roundup

Time for a quick interlude before moving on to day two of the Swiss bridges tour. Just three quick links to bridge stories I've spotted recently:

10 Most Amazing Bridges

Proposal for new 900m bridge costing EUR 125m in Helsinki
Architectural competition to be held next year for bridge carrying trams and light traffic

Steven Holl wins Copenhagen gateway competition
Cable-stayed footbridges cantilever in twin-tower "handshake" (pictured below)

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:

13 November 2008

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:

12 November 2008

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:

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 ...