IABSE Symposium Venice 2010

This was my second time at an IABSE Symposium, and the first where I had time to sit back properly and take in as much of the conference as possible. The event was held in the Casino building on Venice Lido, an interesting venue with considerable historic charm, some of the rooms featuring unusual lighting, a copper ceiling, and wall mosaics.

The papers presented were widely varied in both quality and content. There were one or two where the engineering principles seemed surprisingly lightweight. Many of the papers were also quite esoteric, by academics for academics, and very few tackled the Symposium’s ostensible theme ("Large structures and infrastructures for environmentally constrained and urbanised areas").

The guest and keynote presentations were generally of high quality, but the conference format, which gave each speaker about 8-9 minutes for their presentation, actively discourages discussion and debate. Most of the best discussion took place over coffee or meals.

There were, however, moments when a greater level of interaction broke through. The design for the new Bundek Bridge in Zagreb (pictured, left) was presented, and it was difficult if not impossible to see how it actually stood up. It seemed to comprise an organic, curved gridshell frame suspending a highway deck, something that might work well as a building roof supported on all edges, but made little sense for a bridge spanning end-to-end. When this was queried (by the famous Swiss engineer Jürg Conzett, who wondered where the top flange of the lattice truss was), the speaker had no real answer, and I can’t find any in the full paper either. Conzett’s own Traversina Footbridge was presented in the same session, and offered the perfect counter-example, true structural art rather than a fantasy.

The Zagreb structure wasn’t the only example of a design where any engineering rationale had been submerged (drowned, even) by the architectural vision. One voice which clearly articulated the opposing view came in a paper "on the development of structural criticism through case studies", a topic dear to the heart of the Happy Pontist. The author, Ignacio Payá-Zaforteza, compared two large cable-stayed bridges in Spain, Calatrava’s Serreria Bridge (image courtesy of e.phelt on flickr), and Troyano’s Bridge over the Lérez River. The presenter took as his critical frame the criteria of David Billington: economy, efficiency and elegance, and unsurprisingly came out very much against the Calatrava design, which has as its major sin the use of a 155m main span to cross a redundant river bed, something more readily achieved with a multi-span viaduct.

While I’m all in favour of more open criticism of structural design, this paper veered too close to the puritan orthodoxy for my taste, citing with approval Schlaich and Menn’s contentions that an appropriate price premium for a more attractive bridge is of the order of 5-20%, and absolutely never more than 50%. This sort of dogma ignores the fact that bridges (and even giant sculptures disguised as bridges) have a value too, not just a cost.

Among the many other papers, one that stood out was Yoshiaki Kubota’s "Systematization of structures and forms of truss systems", which presented an elegant framework showing how five basic transformations can be used to generate the geometry of most or all known truss systems, as well as many more types not already known (just one of the transformation diagrams is pictured here). This formed part of a wider framework emphasising the continuity between the traditionally separate bridge forms of beam, truss, arch and suspension bridge.

With a total of 378 papers included in the conference proceedings, it's impossible here to do more than scratch the surface. As with any conference, there was as much value in the opportunity to "network" than in the technical material presented. In Venice, that also mean the opportunity to explore and enjoy the city itself, including both the bridges already featured here and the Pier Luigi Nervi exhibition previously mentioned.

The next IABSE Symposium is in London, in September 2011, which should be particularly interesting as it is being jointly organized with the International Association for Shell and Spatial Structures (IASS).

Venice Bridges: 5. Miscellaneous

Okay, I'm nearly done with reporting on my trip to Venice. In this post, I've got a few photos of the more ordinary bridges encountered there, generally without comment. These give some idea of the range of structural forms used on the smaller spans in the city. Click on any image for a larger version. In the next post, I'll say a few words about the IABSE Symposium that prompted my trip.



In addition to the four bridges previously blogged, Venice's most famous span is the Ponte dei Sospiri ("Bridge of Sighs"). This is a covered bridge originally used to transfer prisoners to and from interrogation rooms in the Doge's Palace. It's not a particularly interesting structure, only spanning 11m, but given its fame I was surprised to find it almost hidden behind huge advertising hoardings.

And I'm not the only one, with Lord Foster and others protesting against this bizarre defacement of a UNESCO World Heritage site. The adverts help to pay for the building restoration work, but it's a little like sticking a giant MacDonalds logo across the Taj Mahal, I think.

Venice Bridges: 4. Ponte della Costituzione

The Ponte della Costituzione ("Constitution Bridge") is the most recent bridge to be built across the Grand Canal, and may seem to be the most controversial due to a number of complaints that I’ll come to in a moment. However, it should be noted that the other crossings were also controversial when first built. Antonio da Ponte’s Ponte di Rialto was the source of criticism from rival designers who felt its single span was over-ambitious. Alfred Neville’s iron truss Ponte dell’Accademia was seen as an affront to the traditions of Venetian architecture.

The fourth bridge was built to replace an existing ferry, or traghetti, which linked Venice’s Piazza Romale bus terminal to its Santa Lucia rail station. At this point, the Canal is considerably wider than is the case further south along its length. The design contract was awarded to Santiago Calatrava in November 1999, an audacious choice for a city presumably afraid to tamper with its heritage for fear of alienating the visitors who provide much of its revenue. The bridge was built by Cignoni, and opened in September 2008.

Traditional Venetian arch bridges never venture beyond a span to rise ratio of about 7:1, as beyond this the horizontal thrusts to be carried by the foundations become very difficult to accommodate, and even below this it can be a problem. Shallower arches have been built elsewhere, but generally on firmer ground.

Calatrava’s proposal was for a steel arch footbridge spanning 80.8m, and with a total rise of 4.76m, a ratio of 16:1. The rationale would seem to be entirely straightforward: it avoids supports within the Canal, with their consequent disruption to navigation. It achieves the required navigation clearance while minimising the slopes that users have to climb, something of a relief compared to its steeper forebears.

The bridge is carried by a ribbed steel spine, consisting of five steel boxes linked by ribs in a configuration which is star-shaped in section. Within the vertical plane, the ribs form a Vierendeel truss, with the truss web members set off-vertical in a radial arrangement. The steelwork is painted red rather than Calatrava’s characteristic white. The bridge deck (which varies from 6.5m wide at the abutments to 9m wide at midspan) is constructed in reinforced concrete, with some glass panels at the edges. The parapets are in structural glass, with bronze handrails.

I think that in overall form it is very much an excellent response to its context. The low-profile arch is a great complement to the other Venetian spans. The arch springings are very beautifully shaped, and there are nice little details like the ornamentation at the ends of the handrails, which nicely subverts Calatrava’s traditional purity of form.

Seen in person and up close, it is however peculiarly disappointing. I think much of this is due to the fact that it is already rather dirty, with the red steelwork taking on a distinctly faded appearance. While this brings it in line with the dilapidation seemingly normal to Venetian bridges, it reduces the bold visual impact considerably.

I think the parapets are great, but the bridge deck less so. This is mainly because of the steps, which vary in tread length significantly as the bridge is climbed. Traditional Venetian arch bridges manage to avoid this by not having the steps follow the arch profile too closely. On the Ponte della Costituzione, the effect is quite disconcerting, and it’s unsurprising that a number of legal claims have been made by bridge users tripping on the steps. The problem is enhanced by the lack of any kind of visibility measure on the step edges (such as the yellow border often used to make them clearer to the visually impaired). I don’t think this would have greatly upset Calatrava’s design vision, as colour contrast can be used as effectively as very bright colours.

The glass decking has also been alleged to be slippery when wet (not for the first time on a Calatrava bridge). However, the biggest criticism of the bridge’s accessibility is simply that the use of steps is inappropriate in the modern age, as making access very difficult for the mobility impaired. As a result, a disabled access lift is currently being added to the north face of the bridge (the face probably least often viewed or photographed), and criticism relates to the fact that it is apparently an afterthought and didn’t form part of the original project.

However, I have some sympathy with Calatrava, as few if any of the other Venetian bridges I saw are accessible in the modern sense, and his shallow slopes make the bridge far less strenuous to cross than the others. Why should disabled access be such a big issue here but not at Rialto or Scalzi? This is a Venetian problem, not a Calatrava problem.

The bridge has also been criticised for its structural behaviour. The star-shaped cross-section can only be stiff torsionally if diagonally braced, which it isn’t, with the result that twisting behaviour was a major design concern. The huge horizontal thrusts generated by such a shallow arch (roughly seven times the vertical thrust, according to the authors of a paper listed below), also require huge foundations, and it is essentially impossible to prevent the foundations from moving over time, such that the bridge’s arch behaviour would reduce, and it would behave more like a beam (as it already does to some extent). To address this, Calatrava has provided jacking facilities at the end(s) of the bridge, such that as the foundations move over time, the bridge can be jacked back to its original profile.

Many of these criticisms are hinted at in the paper on the bridge’s engineering, and while I pretty much agree about the torsion, I think the arch thrust issue is simply an engineering challenge which had to be met if the bridge’s architectural conception was to be respected. So long as the foundation movements don’t affect adjacent structures, the jacking solution seems a reasonable one.

Calatrava himself is on record as saying "This is my most beautiful bridge", and blaming the bridge's problems not on his design, but on the contractors.

The bridge received perhaps its greatest criticism for its cost, which escalated considerably as the project progressed. The final cost is reported as being €7.3m for the bridge contract, or €11.3m including other costs such as surveys (up from €6.7m when estimated in 2002). A formal opening ceremony was cancelled for fear that it would be disrupted by protesters.

Overall, it’s a fine bridge, far better than the criticism would suggest, which would benefit greatly from being kept clean, bright, and properly illuminated at night.

Further information:

• Google maps / Bing maps
• Structurae
• Wikipedia
• City of Venice project website (Italian, with excellent construction photos and videos)
• Calatrava Bridges (Tzonis & Caso Donadei, 2005)
• Masterpieces: Bridge Architecture + Design (Van Uffelen, 2010)
• Information on computer modelling and erection engineering
• Venetian Red [PDF] (details on the glass specifications)
• The Fourth Bridge over the Grand Canal in Venice: From Idea to Analysis and Construction [abstract] (Zordan, Briseghella & Siviero, Structural Engineering International, 2010)
• Erection design and testing of the IVth bridge over the Grand Canal, Venice (Briseghella, Siviero & Zordan, Footbridge 2008 conference)

Venice Bridges: 3. Ponte degli Scalzi

The third bridge to be built over the Grand Canal in Venice was an iron truss structure built near the Venice railway station in 1857. As with the truss structure of the same vintage at Ponte dell'Accademia, it was replaced in the early 1930s with a new arched bridge, the Ponte degli Scalzi ("bridge of the barefoot").

The new structure was built between 1932 and 1934 to a design by Eugenio Miozzi. It is a stone arch span of 40m, rising 6.75m. The bridge is only 0.8m thick at its crown, which is remarkable slender for the type and age of the bridge. Stone was chosen instead of reinforced concrete to avoid future corrosion problems. The construction cost 2.55m lira.

Miozzi adopted an unusual technique in construction of the arch, which he called the "compensatory systematic lesion" method. This involved leaving open joints at the crown and springing, which would close as the bridge's formwork was removed. This ensured that in the dead load condition, the arch was subject to minimal bending effects, greatly reducing the adverse stresses in the stonework.

As with all Venetian arches, containment of the arch thrusts is the key engineering problem, and the bridge is supported on 0.3m diameter concrete piles, both vertical and inclined, along with timber piles. These were tested with horizontal loads prior to construction of the arch.

Today, the bridge may appear at first sight the least distinguished of the four bridges over the Grand Canal. There are, after all, roughly 300 arch bridges in Venice, most of them in stone. However, its attraction is in its simple elegance, especially its slenderness at midspan. It's both beautiful and durable, an illustration that older technologies are often still of great value.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges in Venice - Architectural and Structural Engineering aspects [PDF] (de Miranda, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)

Venice Bridges: 2. Ponte de Rialto

Along with the Bridge of Sighs (on which, more later), the Rialto Bridge is one of the most famous tourist sights in Venice. For over five centuries, it was the only bridge across the Grand Canal, until the first Ponte dell'Accademia was built in 1854.

The canal was first bridged at this site by a pontoon bridge in 1181, replaced by a wooden bridge in 1255, incorporating a drawbridge opening section (as depicted by the painter Vittore Carpaccio). This, and several successors, were to collapse and be rebuilt until in 1503 proposals were made to build a more permanent stone structure.

The timber bridges already incorporated the shops that were to become one of the Rialto Bridge's most distinguishing features. They provided a rental income which helped to cover the bridge's maintenance costs.

In the late 16th century, a bridge design competition took place, with submissions from Michelangelo, Giacomo Barozzi da Vignola, Guglielmo Marastoni, Gugliemo di Grande, Jacopo Sansovino, Andrea Palladio, and others (several of these can be seen online). Many of the designs, including Palladio's, were for three span structures, but the Canal is relatively narrow at this point and the authorities preferred a bolder single span solution to minimise the impact on navigation (and possibly because the cost of foundations was lower overall).

One of the three-span designs, by Vincenzo Scamozzi, was adopted in 1588 as a template for the single-span bridge which would be built over the next three years. The builder, Antonio da Ponte, adopted the proposed layout of shops and stairways, as well as the balustrades from Scamozzi's design. Scamozzi himself severely criticised the attempt to build a single span, which he thought likely to collapse.

Da Ponte's bridge spans 29m, the shortest of the Grand Canal Bridges, with the arch rising 6.4m. The arch thrusts are substantial, especially for the weak Venetian soils, and as with many such bridges the horizontal component is reduced by adopting a slender arch section (1.25m thick) at the crown, with more weight above the abutments. Part of the arch thrust is stabilised by the loads from adjacent buildings.

The bridge as built was painted many times by Canaletto, who also painted a vision of what Palladio's proposal would have looked like had it been adopted.

Today, it remains a fine bridge, although its attraction is as much as a vantage point from which to survey the Canal as for its appearance. Many inhabited bridges have a deep, stocky appearance, but the dark shopfronts allow the upper arcade to appear lighter than it really is. Because the arcades are set back, the arch can still be seen as a relatively slender element in its own right.

Closer at hand, however, the bridge is remarkably run-down, with considerable grime and graffiti adorning it. Generally, a certain level of dilapidation provides much of the charm in Venice's buildings, and if they were all cleaned up and restored to their original appearance, a certain Disneyland feel would probably be the result.

Nonetheless, I find it surprising that Venice allows a major historic landmark such as the Rialto Bridge to be left in this condition.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Bridges in Venice - Architectural and Structural Engineering aspects [PDF] (de Miranda, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)
• Living Bridges (Murray & Stevens (eds.), 1996)
• Bridges (Judith Dupré, 1998)

Venice Bridges: 1. Ponte dell'Accademia

While in Venice, I visited all four of the bridges over the Grand Canal. I started at the north end, with the most modern bridge, Santiago Calatrava's Ponte della Costituzione, and worked my way south, but here I'm going to cover the four spans in reverse order, which is closer to the history of when they were built.

The Ponte dell'Accademia ("Bridge of the Academy") spans the Grand Canal fairly close to its southern end, running broadly between the Gallerie dell'Accademia and a large public square, Campo Santo Stefano.

The first bridge on this site was completed in 1854. Designed by Alfred Neville, it spanned 50m, comprising a flat iron truss girder. A paper presented at the Venice IABSE conference in September describes it as a "reticular girder" or a "Neville beam", but these aren't terms I've seen elsewhere.

The girder solution is a sensible approach in Venice from one perspective, although only about 1 in 8 Venetian bridges are beams rather than arches. Girder bridges have the advantage of only transmitting vertical reactions, which is helpful, as Venetian soils are of poor quality and prone to long-term settlement, not a great characteristic when subjected to arch thrusts.

However, the beam bridge requires much higher approach ramps, especially if an appropriate navigation clearance is to be maintained. With a headroom of only 4.2m, the first Accademia bridge was a constant obstacle to navigation, and when it became increasingly dilapidated in the early 1930s, it was overdue for replacement.

The new structure was a 48m timber arch span, designed by Eugenio Miozzi, and opened in 1933. This required both concrete piles and screw-tipped wooden piles to withstand the arch loads. Miozzi's bridge lasted even less well than Neville's design, with the introduction of substantial steel arch components in 1948, and further stiffening added between 1963 and 1965.

The present bridge is yet a further evolution, as the bridge was rebuilt in 1986 as a steel arch structure, clad extensively in wood to preserve the general appearance of Miozzi's design. You can see in my photographs that there is a lattice steel arch lurking with embarrassment behind the wooden facade, and this mismatch becomes even more apparent from below, where the steel flanges are clearly visible (don't forget you can click on any photo for a larger version).

As with most of the Grand Canal bridges, this is a steep bridge to cross if carrying heavy baggage, and must be particularly awkward for anyone who is mobility impaired or has a child in a pushchair. The Venetian City Council announced in 2009 plans to modify the bridge to provide disabled access, but they are unwilling to stump up any part of the estimated €5m cost. Instead, they're hoping potential designers and contractors will fund the work via some form of sponsorship.

According to a report in The Art Newspaper, similar use of sponsorship elsewhere in Venice has attracted considerable yet unsurprising criticism (I'll come back to that in a later post). I suspect they have had few takers for this astoundingly cheapskate proposal, which is particularly ridiculous given the amount of money available in Venice for mega-projects such as the anti-flooding MOSE scheme.

I like the Accademia bridge - from a distance. The shape is attractive and well-proportioned, and the relatively open timberwork has a light appearance. From close at hand, however, it looks somewhat shoddy, and the conjunction of the poorly conserved timber shell with a duplicitous steelwork core is very uncomfortable.

Further information:

• Google maps / Bing maps
• Wikipedia
• Structurae
• Brief considerations on the Accademia Bridge in Venice [PDF] (Barbisan, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)
• Bridges in Venice - Architectural and Structural Engineering aspects [PDF] (de Miranda, Pogacnik & Skansi, IABSE Symposium, Venice, 2010)