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

Footbridge proposed over River Trent at Gainsborough

Design proposals have been published for a new pedestrian bridge in Gainsborough, designed by Houchell Studio and TALL Engineers. The 276m long bridge has an estimated construction cost of £6m. That strikes me as a little on the light side, comparing for example the not entirely dissimilar 315m long Derry Peace Bridge which has a price tag of £8.7m.

The bridge is S-shaped in plan, and suspended by cable stays from two symmetrical pylons. The South Quay Footbridge comes to mind as a possible precursor, although that was a swing bridge. What's perhaps most striking about it is the very shallow inclination of the pylons, which seems at least visually to be lower than might normally be expected.

The deck is to be shared by pedestrians and cyclists, and the bridge provides a link in a circular route including riverside paths.

The client, Gainsborough Stakeholder Network, currently has no money to build the bridge, and is now seeking sources of funding.

Tower Bridge engineering tours

Applications are being invited for engineering tours of Tower Bridge in London.

These will take place from 17th to 23rd January 2011. The tour will include access to the parts of the bridge not normally open to the public, including the control room, machinery rooms, and the huge bascule chambers (into which part of the bridge deck rotates when the bridge is opened to shipping).

The cost is £30 per person.

(Image courtesy of Stuart Barr on Flickr).

Bridges news roundup

Cracks appear in Pulteney Bridge as the debate goes on
Bath's iconic inhabited bridge to be propped up on scaffolding.

Atkins acquires Danish bridge company
Gimsing & Madsen join up.

3 teams in running to build SR 520 floating bridge project
New US$900m bridge in Washington state, USA, due for completion in 2014.

Lack of technical knowhow brought CWG bridge down
Contractor blamed in Delhi Commonwealth Games footbridge failure, and UK supplier Macalloy is drawn into the criticism.

Bridge bosses unveil plan for visitor centre
New Forth crossing may create opportunity for long-wanted visitor facility.

Fehmarn Link bridge images and video released

The Fehmarn Belt link is the third super-bridge to be proposed in Denmark (the others being the Øresund Bridge and the Great Belt Link). The bridge proposal is currently being tested against an immersed-tube tunnel alternative, with a decision to be made next year on which will proceed. The three links together connect Denmark to Sweden and Germany across significant stretches of water.

The 17.5km long bridge option is being designed by a COWI / Obermeyer joint venture, with Dissing + Weitling as subconsultant architect (D+W worked on the other two link schemes previously, and COWI worked on the Great Belt). They have just released a set of images (below), and this video:



The bridge has four cable-stayed spans, slung from three V-shaped towers. These support one level of highway deck on truss girders sheltering a lower rail deck.

The V-shaped configuration of the towers is unusual for a bridge on this scale, as the inclined towers are more expensive to construct, and don't help brace each other when complete, as in the more conventional A-frame tower. However, they have been beautifully sculpted by Dissing + Weitling, and look very attractive from most viewpoints.

I'm not entirely convinced about their appearance from the perspective of a moving driver, it looks a little disconcerting, opening attention to the wedge of sky above rather than offering the psychological security of an A-frame or Y-frame solution.

The central tower has double legs, a feature sometimes used on multi-span cable-stay bridges to enhance the stiffness of the pylon and hence the entire system (it was also used on the Rion-Antirion Bridge and Millau Viaduct). It's odd to see it here, as I'd have thought the trussed deck offers sufficient stiffness (as in the design proposals for the Mersey Gateway).

Update 9 November:
I've received a few more images of the design directly from Dissing + Weitling:

Providence River Pedestrian Bridge: Shortlisted designs released

I'm sadly way too busy to give these a thorough review right now, but the eleven shortlisted designs in this contest have been released for public view via Flickr.

From a quick look, there are at least one or two entrants with interesting features, but if you have more time than I do, I invite you to play "guess the designer" for each one (the entries are anonymous but the eleven entrants are known), and report back here!

The winning design is due to be announced at the end of the month.

"Dan Cruickshank's Bridges: The Heroic Designs that Changed the World"

Whenever a book about bridges comes along which is aimed at a broad, general audience, the expectation is that it may be a bit on the shallow side, certainly for the reader with greater specialist knowledge. Dan Cruickshank is an architectural historian best known in the UK for his populist TV documentaries, such as Around the World in 80 Treasures and Adventures in Architecture. His television style is enthusiastic to the point of being gushing, and I worried that his latest book, Dan Cruickshank's Bridges: Heroic Designs that Changed the World (Collins, 384pp, ISBN 978-0007318186, 2010) [amazon.co.uk] might prove somewhat superficial.

I shouldn't have been concerned. Bridges is certainly full to the brim with Cruickshank's trademark passion, but it balances this with a well-informed, wide-ranging and erudite approach to its subject.

Cruickshank presents a potted survey of bridges in ten chapters, each organised partly around a historical period and partly around a related theme. Thus the chapters cover "Empire" (Roman bridges), "Forging the Railway Age", as well as the more oblique "Defining Places" and "Works of Art".

Some context is offered by a lengthy, rambling introduction, which discusses a number of important structures (Zhaozhou Bridge, the Old Bridge at Mostar, Charles Bridge in Prague etc), as well as drawing in the likes of Marcel Duchamp, Kabbalah, Charles Dickens and Canaletto. The introduction manages to establish bridges as artefacts of legend, the physical residue of acts of wilful magic, as well as to delineate the basics of structural form and materials.

The ability and desire to hop rapidly between the realms of folklore, history, technology and wider cultural topics is central to Cruickshank's expansive approach to his subject. Ordinarily, I dislike the kind of book which sees bridges simply as a convenient hook on which to hang the author’s interests in folk history, where the anecdotal and incidental is given precedence over the supposed subject at hand. This book's digressions (of which there are many) kept my interest, largely because they do generally shed light on cultural attitudes to the products and processes of engineering.

For example, there's a thorough discussion on Islamic views of paradise, by way of introduction to the three great bridges of Isfahan, Iran (Si-O-Se Pol, Pole-i-Khaju, and Pol-i-Chubi). While they are architecturally extravagant and impressive, the bridges are not technologically significant, and a knowledge of their place in Shah Abbas's garden city undoubtedly adds to their appreciation.

The book's early chapters return repeatedly to the concept of a bridge as a place of habitation, rather than simply as a means of crossing obstacle. There are bridges supporting homes, tea rooms, travellers rests, and fortifications, and bridges as places to promenade upon, or as the sites for pilgrimage. Cruickshank is generally good on the "whys" as well as the "hows" of bridge building – their roles as military corridors, monuments to kings and gods, religious sites in their own right. Particularly interesting is the resulting way in which bridges are funded, e.g. the English taxing the French to pay for bridges intended to facilitate continued English military occupation in France.

The book is filled with a love of engineering, and the ways in which the functional can also be beautiful, or at least, "just". Writing on Eiffel's Garabit Viaduct, Cruickshanks suggests that "truth is synonymous with beauty and thus [it] is the pinnacle of excellence". He even finds beauty (or at least, splendour) in the colossal latticework of New York's Queensboro Bridge, probably not a structure often admired for its good looks.

Queensboro is one of several bridges where the author's personal touch really brings the book to life. Similarly, his chapter on early railway bridges is viewed largely through the lens of Stephenson's Newcastle High Level Bridge, and it's Cruickshank's account of his own exploration of the structure that stands out beyond the diligent research. The choice of such structures is generally sure-footed, with great but less well known bridges rubbing shoulders with their more famous cousins, the Brooklyns, Vecchios and Salginatobels.

Imagining myself into the shoes of a lay reader, I think it's an excellent book, accessible but informative, seeking to share an enthusiastic understanding of structural behaviour with a wider awareness of the part that bridges play in society.

As a professional bridge designer, I found some of the technical explanations to be inexact, some to the point of being wrong. For example, the book reports that the Millennium Bridge and Basse-Chaîne Bridge suffered from the same type of oscillation, and elsewhere that suspension bridges are destroyed by an increasingly rapid flutter phenomenon (the speed is roughly constant, it's the amplitude of oscillation that increases). It also sometimes misses the significance of technological innovation, as with the Severn Bridge where the inclined hangers are cited as the key design achievement, rather than the adoption of the aerofoil box girder deck.

These are quibbles, irrelevant to the book's intended readership, and they shouldn’t trouble the technically knowledgeable either. There's little in Dan Cruickshank's Bridges that the well-read pontist couldn't find elsewhere, but the love of bridges, both as engineering feats and for their wider role in history and society, shines through every page, and that's what makes it such an enjoyable book.