Design published for new Polcevera viaduct

Details have been made public for the new Polcevera viaduct in Genoa, Italy, the proposed replacement for the bridge which collapsed in August this year killing 43 people.

The bridge design is credited to architect Renzo Piano, who offered his services free-of-charge in the aftermath of the August disaster. I imagine a structural engineer is involved, but I haven't found out who they are yet.

The €202 million construction contract has been awarded to Pergenova, a joint venture of Italian contractors Salini Impregilo and Fincantieri Infrastructure. They state that the new bridge will be complete in 12 months from the site becoming available, which will be when demolition of the existing structure finishes in March 2019.

The new structure is to be an elegant and, I think, consciously unspectacular steel viaduct, some 1.1km long, comprising twenty spans mostly 50m long, with some 100m long where required to span the railway and river. The reinforced concrete bridge piers are elliptical in section.

The design is sober and straightforward. The only nod towards the site's tragic history comes with the lighting columns positioned above each bridge pier. These extend in height and support a number of solar-powered lanterns, reportedly one for each victim of the bridge collapse.

I do wonder about the width of the piers - in some of the renderings, the deck is shown as much wider than its supports, which for me looks visually unsatisfactory, potentially unbalanced if only one side of the roadway is heavily loaded.

Of course, there will be no issue if the bridge deck is heavy enough, and Genoa is not in one of the more seismically active areas of Italy, classed as having a peak ground acceleration between 0.05g and 0.15g, for a 50-year return period, and from what I can see, at the lower end of this scale even for a longer 475-year period.

The Piano design was chosen against one other competing contractor, Cimolai, who submitted four proposals, three of them designed by Santiago Calatrava. The proposals featured a girder viaduct with a series of 125m spans; a cable-stayed viaduct with 140m spans; and a 550m-span tied arch bridge, all pictured below. You can find more detail in Cimolai's video and in a gallery on the Domus website.

It's no surprise that Cimolai were unsuccessful: which public authority would wish to run the risk of an over-budget fiasco at this particular site, as so often seems to accompany anything Calatrava designs? It's also apparent that any of these solutions would take longer to build than the Pergenova proposal.

Visually, two of the three lack a sense of lateral stability, and all of them are visually over-complicated and inappropriately dramatic. The arch design, with its awkward stiffening truss at deck level, is particularly poor.

Cimolai's non-Calatrava option (no designer appears to be credited), is worse still:

Meanwhile, some 20 people are reportedly under investigation on suspicion of involuntary manslaughter in connection with the original bridge failure.

Collapse of the Polcevera Viaduct

I'm sure most of my readers will have seen yesterday's tragic news that the Polcevera Viaduct in Genoa, Italy, collapsed, with at least 39 fatalities reported. At the time of writing, rescue and recovery efforts are ongoing. A state of emergency has been declared in the local region.

The 1.1km long viaduct carried a major toll road through Genoa, and was a key connection in the route from central and southern Italy to the south coast of France. It was completed in 1967 to a design by the famous Italian engineer Riccardo Morandi, one of a series of innovative concrete stayed bridges that he created starting with the Lake Maracaibo bridge in 1962.

These were distinguished by the use of very simple stay arrangements, using prestressed concrete stays rather than the steel cables already in wide use at the time (e.g. Strömsund Bridge in Sweden, 1955, and the Nordbrücke in Düsseldorf, 1957).

Eduardo Torroja's Tempul Aqueduct, built in 1926, is one of the few predecessors, although Torroja used concrete only as a protective material, it was not prestressed. Concrete-encased stays were also later used on the Prins Willem-Alexanderbrug in the Netherlands (1972) and the Metten Danube Bridge (1981). Morandi's tower arrangement was also used (without the concrete stays) on the Chaco Corrientes Bridge in Argentina (1973).

For more on Morandi's bridges and their relatives, see Walter Podolny's 1973 paper Cable-stayed Bridges of Prestressed Concrete.

Morandi's designs, although highly innovative, were a dead end. They required extensive temporary works to support the bridge deck until the stays were completed.

At Polcevera, temporary prestressing was used in the cantilevering deck sections, only to be removed once the stays had been installed. For his bridge at Wadi el Kuf, in Libya (1972), an array of temporary stay cables was used to support the longer spans, with all these cables then removed, rather than left in place as the permanent support system, which was the more logical and much more widespread solution. The Libyan bridge, incidentally, was recently closed for safety reasons.

Possibly relevant to the Genoa disaster, these designs also lack structural redundancy. The failure of any one key structural member of the bridge can lead to disproportionate collapse.

The Polcevera Viaduct, and its cousins, have been much admired by engineers and architects.

Michel Virlogeux, in his paper Bridges with Multiple Cable-stayed Spans, notes that the Lake Maracaibo Bridge was "much admired by architects who understand the evident flow of forces and who are sensitive to the impression of strength that emanates from the mass and shapes of the structure". Leonardo Fernandez Troyano described the same structure as "one of the great works in the recent history of bridges" in his book Bridge Engineering - A Global Perspective. In a 2010 paper summarising Morandi's work, Luca Sampo claimed that the Polcevera bridge's "technical features may still today be considered unsurpassed".

There is plenty of speculation on the internet regarding the cause of the collapse, which I won't repeat here.

The bridge's brand-new Wikipedia article is a pretty good source of information. There's a paper from 1995 which discusses previous remedial works to the bridge's main stays. Probably the best read is a contemporary article from 1968 with lots of construction drawings and photographs. All the images I've used here are taken from that article.

Outcry over plan to rebuild the Ponte dell'Accademia, Venice

A repeat of the battle between city authorities and traditionalists over Calatrava's bridge in Venice may be in the offing. Proposals have been announced to replace the Ponte dell'Accademia, which spans the southern end of Venice's Grand Canal.

I featured the bridge here last year. It's a steel arch bridge built in 1986, clad in timber to resemble the previous structure, Eugenio Miozzi's 1933 bridge. As I reported then, plans have been brewing for some time to improve disabled access over the structure which, like many Venetian spans, has steep steps difficult for many users to negotiate.

The latest proposal (pictured), from Bolognese architects Schiavina, is for a complete reconstruction. The new bridge would cost about £5m, and depending on which website you read, would be built either in glass, steel and stone, or in glass, steel and timber.

In support of the plans, the city council claim that the cost of maintaining the existing bridge is exorbitant, and therefore reconstruction combines the ambitions of reducing ongoing cost and providing access for all. A recent refurbishment cost about £220k, and the authorities consider the future costs to be unsustainable.

Preservationists are up in arms, with the president of a Venice heritage group crying "The bridge has a certain dignity. Why don't we just restore it?" At present, there's no final commitment to build the new design, which has been considered by the local authorities and sent to the heritage ministry in Rome for approval. Protesters are however probably mindful of the precedent set by the construction of Calatrava's Ponte della Costituzione, where concerns over its contemporary design were ignored (in my view, probably rightly, as it hugely improves accessibility in an area where many visitors first encounter the city).

It's unclear how the work will be funded, with Venice notoriously unwilling to dig into its own coffers to maintain the historically valuable fabric of the city (as seen here). One report has multi-millionaire Renzo Rossi covering the costs.

I don't know whether replacement is the right solution. As an engineer, the question must mainly come down to the sustainability of bridge maintenance: is it really in such a bad condition that it is too expensive to look after? It did look a little on the shoddy side when I visited, but so do many bridges, and I suspect the cost of maintenance is being exaggerated.

Venice argues that its 1986 reconstruction means it is no longer a historic artefact and therefore not deserving of heritage protection. I don't buy that at all. A considerable effort was made to preserve the general appearance of Miozzi's bridge when it was rebuilt - it maintains a continuity of appearance which should not lightly be set aside.

The bridge's context is also highly significant. While I'm no fan of attempting to preserve the environment in aspic, there are few places where the cityscape is of such historic (and economic) importance as in Venice. The bridge is a key gateway to the Grand Canal, and in a place like this there should be a very good reason indeed for changing the bridge's appearance so much.

It is also worth considering the technical difficulties that imperil every bridge in Venice, particularly the inability of the ground to support large foundation loads. The first bridge at this site was a simple metal truss, imparting only vertical load. Miozzi's wooden bridge was relatively lightweight, but at 48m span even that required a large number of both concrete and screwed timber piles. The current bridge is heavier, but a new steel and stone bridge will be heavier still. If the Schiavina proposal proceeds, expect a lengthy political battle followed by difficulties with cost escalation (it is far from apparent whether an engineer is yet involved).

Finally, I have to ask why only a single architectural firm has presented a design for this site. Schiavina are not noted as bridge designers, but if the case for replacement is ever proven, then surely a site such as this merits the consideration of design proposals from competing designers?

Nervi Bridges: 6. Messina Straits Bridge

I've got one final bridge design by Pier Luigi Nervi to cover in this post and conclude this series.

At the end of the 1960s, Nervi was amongst engineers asked to propose a design for a bridge across the Messina Straits, between Italy and Sicily. The depth of water in the Straits meant that the bridge had to cross about 3km in a single span (although other designers, such as Leonhardt, were still proposing designs with deep-water piers at the same time - see the book by Richard Scott linked below).

Nervi's contemporary and compatriot Sergio Musmeci proposed a peculiar suspension bridge where the suspension cables are hung not directly from towers, but from cable stays which are in turn suspended from super-towers beyond the ends of the main bridge. Musmeci's idea included lateral cables either side of the deck to provide it with transverse stability.

Although this was an odd design, the proposal by Nervi was even stranger.

Nervi sought to achieve lateral stability by inclining the main suspension cables away from the deck, so that the deck hangers are no longer vertical, and the towers supporting the main cables are separated by a considerable distance.

The towers are hyperparaboloid concrete shells capped with enormous steel assemblies. They're restrained by stays to resist the incredible horizontal forces they would have to carry.

It's hard to imagine how Nervi thought this bridge might be erected. The main cables would have to be aerially spun with a conventional vertical sag, and then additional horizontal cables slung between them in order to pull them into the correct inclined alignment, at which point the deck segments could be fixed in place. I can't imagine how it could possibly work on this scale and the design is therefore, at best, fanciful rather than pragmatic.

The deck itself appears to be a trapezoidal concrete box which would be incredibly heavy and attract enormous wind loading.

On the whole, Pier Luigi Nervi was not a great bridge designer. Few of his ideas were taken up by others, largely because his attempts to maximise material efficiency were always at the cost of construction complexity, leading to greater cost overall. They also weren't always successful aesthetically.

In February I made a series of posts on the bridges of Eduardo Torroja, another concrete shell innovator, and whose bridges, like those of Nervi, are far less impressive than his better known designs. Was there something about the dedication required to become a master in one field that inhibited their intuition in a second field? None of the other great shell designers (Candela, Dieste, Isler, Hossdorf) seems to have left behind any great bridges either. If anyone has a counterexample, I'd be interested to hear it!

Further information:

• Messinacity: Il Ponte di Pier Luigi Nervi 1969 (Italian)
• In the wake of Tacoma: Suspension bridges and the quest for aerodynamic stability, Scott, 2001
• Pier Luigi Nervi: bridge designer [PDF], Manuel Cresciani, IASS Symposium 2007
• Pier Luigi Nervi and the Art of Building [PDF], Fausto Giovannardi, 2009

Nervi Bridges: 4. Ponte del Risorgimento, Verona

In 1961, the town of Verona held a design competition for a new bridge across the River Adige, partly to celebrate the centenary of Italian unification. Apparently, the results were uninspiring, and set to one side. Two years later, Pier Luigi Nervi was invited to prepare a design, which went on to be built, opening in 1968. The bridge was built by Edilbeton, who offered a lower tender than Nervi's own firm, Nervi e Bartoli.


The river is 131m wide, and Nervi proposed a three-span bridge with a central span of 62m and two symmetrical side-spans of 34.5m each. This partly echoed the existing upstream bridge, Ponte Catena, which had three roughly equal arch spans. It's a cellular reinforced concrete box girder, with its cross-section varied in response to the bending moment and shear force envelopes.

The soffit is curved to enhance the girder depth at the bridge piers, where the hogging bending moment (and also shear force) is greatest. The bottom flange thickness also varies, increasing at the piers to carry the resulting increased flange compression force. These are both concepts common to bridges of this type.


However, Nervi also varied the width of the bottom flange in response to the moments, so that is wider at the piers, where greater compression capacity is required, and narrower at midspan. The top flange is kept at a constant width, so Nervi created a bridge where the slope of the side walls varies continuously.

At the piers, the cross-section is of a trapezium with the bottom flange wider than the top, while at midspan, the reverse is true.


There is a hydraulic as well as structural logic - the side wall of the deck juts out above the piers to form something like a cutwater, helping to divide flood flow and creating a downward thrust under flood. The shaping at midspan may reduce hydraulic drag under flood conditions.

To the modern engineer, the varying girder depth and flange details are sufficient for an economic design - the curved side walls, shaped as hyperbolic paraboloids, require expensive varying formwork, and make detailing of the internal reinforcement more difficult.

Personally, I'm not sure that the visual effect justifies the effort. It's an intriguing design, but not so startlingly attractive as to make the cleverness seem the result of necessity rather than whim.

Further information:

• Google maps / Bing maps
• Wikipedia (Italian)
• Structurae
• Pier Luigi Nervi, bridge designer [PDF], Manuel Cresciani, IASS Symposium 2007
• Pier Luigi Nervi: Architecture as Challenge, Silvana Editoriale, 2010

Nervi Bridges: 2. Corso di Francia Viaduct, Rome

In 1956, Pier Luigi Nervi became involved in a number of structures for the 1960 Rome Olympics. Most notably, this included the two indoor sports arenas, Palazzo dello Sport and Palazzetto dello Sport, and the outdoor stadium at Flaminio. It also included the Via Olimpica viaduct, now called the Corso di Francia Viaduct.

The Corso di Francia runs right through the middle of the Olympic Village. Apparently, it divided the housing for male athletes, to the east, from that for female athletes, to the west, although any notion of propriety was ruined by the use of the viaduct by Italian men to spy into athlete's bedrooms (I don't know how true this story is).

The road had to be elevated to allow free passage of athletes and spectators throughout the site, and the use of T-shaped piers by Nervi may have been an attempt to maximise space at ground level and reduce visual obstruction (see picture left - all colour photos are courtesy of Nicolas Janberg at Structurae).

While the structure is more conventional than the two early bridge designs I featured last time, it still has its share of idiosyncrasy.

The columns have an unusual varying section, from rectangular at the top to a cruciform section at the base. This enhances stability while also adding to the visual interest, and Nervi frequently provided varying columns in his designs. The Palazzetto dello Sport has some, as does his roughly contemporary UNESCO building in Paris.

The beams are 16m long, precast and prestressed, a simple and economical solution for bridge viaducts that seems unusual for Nervi, who was not a great user of prestressing. More unusually, they are V-shaped in cross-section, with a very narrow bottom flange, and with the top flanges placed in contact. The overall cross-section of each viaduct therefore consists of a "corrugated" or V-wave profile, which is essentially similar to the approach Nervi had taken on a number of roof structures, including the Palazzo dello Sport.

Nervi's design for the viaduct is featured in Fritz Leonhardt's Bridges. Leonhardt notes that the bridge did not age well, with water seeping through joints and down the piers (I imagine the deck was discontinuous, as on the Midland Links viaducts in the UK, which have been notoriously plagued with leakage and associated concrete degradation).

Leonhardt also criticises the adoption of column and crosshead piers to support the deck beams. Although this is the most straightforward solution to build, it is visually more congested than the alternative of having the crosshead integral with the deck and within its depth. The latter approach, however, requires temporary propping of the deck beams and construction of a more difficult joint detail.

Further information:

• Google maps / Bing maps
• Structurae
• Pier Luigi Nervi's works for the 1960 Rome Olympics [PDF], Iori & Poretti, Actas del Cuarto Congreso Nacional de Historia de la Construcción, Cádiz, 2005

Nervi Bridges: 1. Early bridges

Pier Luigi Nervi (1891-1979) was one of the greatest Italian structural engineers of the twentieth century. Indeed he was one of the greatest designers anywhere in the world, a pioneer of large-scale reinforced concrete roof structures, highly successful as the engineer, architect and builder of his own works.

His many astonishing designs include St Mary's Cathedral in San Francisco, the Palazzo and Palazzetto del Sport in Rome (the former is pictured, right, courtesy of Jeroen Meijer on Flickr), the Papal Audience Hall in the Vatican, and exhibition buildings in Turin.

A hallmark of his approach was the use of ribbing to stiffen thin-shell construction, leading to geometrically intricate vaulting reminiscent of the Gothic architecture that Nervi undoubtedly admired.

Unlike his contemporary and compatriot Riccardo Morandi, Nervi is less well known for his bridge designs. Nonetheless, he produced enough of interest that I'm going to cover them in a short series of posts here.

Nervi designed several reinforced concrete bridges early in his career. I've found references to:

• Bridge over River Cecina, Pomarance, Pisa (1920-22, demolished 2001)
• Bridge over Pescia Creek, Pistoia (1922-23)
• Widening of Bisenzio Bridge, Prato, Florence (1923-32)
• Fosso Biedano, Rome (1934, unbuilt)
• Bridge at Arno (1945, unbuilt)
• Reno, near Sasso Marconi, Nologna (1951, unbuilt)
• River Tenza, Salerno (1955, unbuilt)

The proposed bridge at the Fosso Biedano was a reinforced concrete trestle viaduct. The A-frame legs stiffen the curved deck against centrifugal forces, while the three levels of bracing serve to reduce the buckling length of the frame columns.

I'm not aware of many concrete trestle bridges which were built: this was a design essentially harking back to the great timber trestle viaducts of the 19th century.


Two decades later, Nervi's plan for a bridge over the river Tenza was equally unconventional. According to The Works of Pier Luigi Nervi, the designer's main challenge was to reduce the amount of scaffolding required for a viaduct which was some 50m above the valley floor.

The 319m long viaduct had trestle supports at a maximum 50m interval, four with identical geometry and hence the option to reuse the falsework and formwork. These trestles support main concrete cantilever beams, which vary in width, having a thicker web above their support positions, where the shear is highest.

The cantilever beams in turn support precast span beams, lifted into position from ground level. These span only 16m, sitting on roller bearings on their half-joints.

In 1947, Nervi designed the roof structure for the Conte Trossi Wharf in Genoa, which if not a bridge, was certainly bridge-like. The relationship of the reinforced arch to the "deck" slab above is more than a little reminiscent of Robert Maillart's bridge design at Tavanasa.

In 1959, Nervi published a critique of five bridges (Cinque ponti), discussing the Salginatobel Bridge, Rodenkirchen Bridge, Golden Gate Bridge, Elbe River Bridge, and a bridge project by Paolo Soleri. Nervi used his critique to contrast two possible methodologies for structural design:

1. "designers, following the statical, constructive and economical needs, and within the margins of freedom in defining structural and formal details, which they always and anyway concede, try to express their own aesthetic sensibility."


2. "designers start from a purely formal conception, very close to what sculptors could have, and then begin to improve it, with the sub-conscious thought that the 'calculator' will manage to make it stand and the constructor will be able to build it."

Evidently, Nervi supported only the first of these choices.

Further information:

• Wikipedia (Italy) (Ponte sul fiume Cecina)
• The Works of Pier Luigi Nervi, Pier Luigi Nervi, The Architectural Press, 1957 (Biedano, Conte Trossi Wharf, Tenza)
• Pier Luigi Nervi, Ada Huxtable, Mayflower, 1960 (Conte Trossi Wharf, Tenza)
• Pier Luigi Nervi, bridge designer [PDF], Manuel Cresciani, IASS Symposium 2007

Tensegrity bridges: 2. Passerella TorVergata, Rome

So, with an introduction to tensegrity out of the way, let's see who would actually be crazy enough to try and design such a bridge.

First up is a proposal for a 32m span footbridge at the Tor Vergata University in Rome (pictured, left). I first read about the proposed design in a paper by Andrea Micheletti (and others) presented at the Footbridge 2005 conference (abstract available online at "The tensegrity footbridge at TorVergata University in Rome" [PDF]).

The bridge is planned to cross a highway near to the University's Faculty of Engineering, and the design has been developed by the tensegrity systems group there. They chose a tensegrity design for its "sense of transparency and lightness", and their design takes a modular approach, connecting together a row of five "expanded octahedra", each of which is an internally self-stable tensegrity structure (pictured, right).

In combining the modules (see image left, showing two modules added together), much of the simplicity of the basic element has to be eliminated. If you compare the two pictures closely, you'll see that several additional cables have been added to provide adequate rigidity in all directions, and also that where the modules join, two struts come together at a common node. A strict tensegritarian, if there were such a thing, would regard this as a no-no, but it's a common and perhaps unavoidable fudge in tensegrity bridge designs, acknowledging that the basic principle really isn't well suited to this application. More information on the design development is available online in "Una passerella pedonale tensintegra per il campus di TorVergata" [Powerpoint].

Form finding techniques have been used to optimise the geometry of the final design (see image on the right). Preliminary engineering studies have shown the design not to be susceptible to vibration, and to weigh about the same as a conventional footbridge design for the same span.

The bridge is proposed to be built by assembling the modules off-site, folding them down for transportation, assembling and prestressing on site, and finally erection by mobile crane. This is not in itself significantly more onerous than a conventional bridge, with the exception of the need for prestressing, which has the potential to be complex.

Visually, the adoption of a modular approach eliminates much of the tangled chaos that is common in tensegrity structures - the bridge seems well-ordered, visually comprehensible, and not entirely unfamiliar. That makes me wonder why you would opt for tensegrity at all, if a similar truss bridge structure could be provided more economically by conventional means, while offering greater freedom to be shaped by the designer. The question in this instance is whether the real transparency attained justifies the effort.

While this bridge seems very unlikely to be built, it's not entirely impractical, and particularly nice to see academia seeking to bring the abstruse and exotic into the real world.

Further information:

• "Modular Tensegrity Systems: The TorVergata Footbridge", Andrea Micheletti [PDF]
• "Una Passerella Tensintegra nel Campus di TorVergata", conference presentation [PPT]
• "Una Passerella Tensintegra nel Campus di TorVergata", Livio Ponzi [thesis in Italian, PDF]

Bridges news roundup

Trips to Venice end in trips in Venice
Apparently Santiago Calatrava's controversial new Constitution Bridge in Venice is injuring tourists. If that makes you think of a steel-and-glass monster rearing up and lashing out at wide-eyed visitors, Godzilla-style, the truth is sadly more prosaic - inconsistent glass stair treads creating a trip hazard. I especially liked Venice's daft solution:

"We'll intervene with some sort of signalling system for distracted tourists, perhaps with stickers on the ground," Salvatore Vento, Venice's head of public works, told Corriere.

Even Calatrava himself, who is notorious for fighting against proposed alterations to his bridges, has suggested some of the steps could be modified.

Helical bridges are the new black
The Harthill Footbridge was lifted into place over the M8 motorway in Scotland at the beginning of October (albeit late and over-budget). The design is a lattice truss comprising intertwining helical loops. It must be intended to be eyecatching, because it's certainly not structurally optimum. It is glazed inside the truss which will make repainting difficult - there's also no evidence of how the outside of the glass will be cleaned (e.g. a maintenance walkway). It seems to be part of a trend: similar bridges include the nearly identical Greenside Place Link Bridge in Edinburgh; and the Marina Bay Bridge in Singapore (inspired, like the Amgen Bridge in Seattle, by the shape of the DNA double helix).

To infinity and beyond
The iconic North Shore Footbridge, designed by Expedition Engineering, has had its two arches installed (using the same crane as at Harthill, incidentally) and is now named the Infinity Bridge. The bridge occasioned some controversy last year with a war of words between Chris Wise and Stephen Spence (also of River Wear bridge fame) as to who actually came up with the design.

Bridges news roundup

A few more links to relevant news that I don't really want to address in detail (more original content coming soon, I promise!):


Viaduct price soars - The competition-winning Te Wero bridge (pictured above) in New Zealand has increased in budget from NZ$35m to NZ$51m. Some people remain keen, others note there are cheaper alternatives. Plenty of details at Auckland's project website including copies of the winning and runner-up submissions.

What are bridges for these days?

Venice cancels opening ceremony for hated Calatrava Bridge


Steel delay latest setback for Trinity River Bridge - another Calatrava bridge with cost concerns (pictured above)

Wear bridge designer's gloomy verdict - public consultation on secret competition-winning bridge due to start in September