Manchester Bridges: 2. Hulme Arch Bridge

Completed in May 1997, the £2m Hulme Arch Bridge is one of two or three structures which made the early reputation of Wilkinson Eyre as bridge design architects. Designed by their predecessor, Chris Wilkinson Architects, in collaboration with Arup, it is one of the key landmark bridges in Britain built in the last two decades, and a good place to start my visit to the bridges of Manchester. It was built by Henry Boot construction, with the steelwork by Watson Steel. As with all my posts, click on any image for a larger version.

It resulted from a design competition held in 1995, with an open stage followed by submission of full entries from six shortlisted teams. The competition (as so often) sought a regeneration centrepiece, which also served to reconnect Stretford Road across the very busy major city artery, Princess Road. It replaced a footbridge which had been on the site since Stretford Road was split by the construction of Princess Road in the 1960s.

I don't have details of the entrants (images appear in the 28 July 1995 issue of Building Design magazine), but the designs were as follows:

• a twin leaning-arch cable-stayed bridge
• a cable-stayed bridge with a centre mast including a water feature
• a two-span bridge with a large central planted area on top
• an asymmetric cable-stayed bridge with a fabric canopy
• a bridge with foot traffic on a separate planted, high-level timber walkway

The Arup/Wilkinson design is for a single steel box girder arch, spanning diagonally across both highways, and from which the bridge deck is suspended by cables, each half of the deck hung from half of the arch. The arch is parabolic in profile, rising 25m above the bridge deck. The deck spans 52m, and the foundations consist of piled bridge deck abutments, and ground bearing concrete blocks resisting the thrust from the arch.

The tight curvature of the arch requires substantial internal stiffening to prevent the steel plates from buckling, and at the arch crown, which is only 0.7m deep, concrete was added internally to provide the necessary stiffness.

Viewed from Princess Road approaching the Manchester city centre, the deck support cables cross each other, looking not unlike a network arch. It's a classic gateway bridge, a landmark in the literal sense (rather than in the sense promoted by so many developers, of any bridge which is sculptural, "iconic" or like a logotype-writ-large).

Looking along Stretford Road, there's a very different view, and walking around the structure or driving through generates a wide range of dynamic and generally interesting elevations. This is quite unlike the more traditional single or twin arch solutions which could have been adopted. Indeed, pretty much the only viewpoint which is unsuccessful is one where the arch is seen head-on, and surely only quizzical pontists with a camera in hand will seek that angle out.

The arch is trapezoidal in cross-section, and this section tapers in both width and depth between springing and crown, adding to the visual interest. However, the bridge deck is less of a visual success, with peculiar cylindrical ribs attached which seem to be there for purely visual effect (see photo below, Wilkinson Eyre describe it as acting like a "nose cone").

All this comes at a cost, of course. The inclination of the hangers generates horizontal forces in the deck, with a tendency for it to twist out of plan which is resisted by the bridge bearings.

The arch itself must carry large unbalanced lateral loads, generating significant lateral bending moments which dominate its design. Much of the bridge material is therefore preoccupied with holding the sculptural form in place, rather than merely holding up the traffic.

Close up, the bridge has not worn especially well, particularly with substantial moss growth (a feature that will be repeated throughout this series of posts), but this is largely invisible from a distance - the bridge isn't really designed for an intimate experience. It would be interesting to know how well the bridge is being maintained - the designer adopted a vapour corrosion inhibitor inside the arch section, which requires regular reapplication, but which is the sort of thing which often never happens in practice.

I wasn't lucky enough to visit on a bright, sunny day (so apologies for the somewhat grey and lifeless nature of the photos throughout this series), but the bridge must be visually striking and instantly recognisable in any conditions, a consequence of the simple and bold basic concept.

Further information:

• Wikipedia
• Google Maps
• Wilkinson Eyre
• Structurae
• The Hulme Arch Bridge, Manchester (paper in ICE Proceedings, 1999)
• A Critical Analysis of the Hulme Arch Bridge, Manchester (PDF)
• Bridging Art and Science by Wilkinson Eyre

Manchester Bridges: 1. Introduction

Earlier this month, I had a day to spare in Manchester, and took the chance to visit quite a few of the city's bridges. Some of them are well known, but several will probably be new to most readers.

I'll cover them all over the course of about a dozen posts, but there may be a few delays along the way, as my paid work is exceptionally busy right now, with some big deadlines coming up in March.

The posts will all be headed "Manchester bridges", so I'll apologise in advance to the local protectionists who will insist that some of the spans are actually in neighbouring city Salford, with one being outside the Manchester ring motorway entirely. They're all bridges in "Greater Manchester", I just wanted to keep the titles of each post short!

There were also one or two interesting bridges which I didn't get the chance to see, so perhaps this series will expand if I ever get the chance for a return trip.

Prestressed granite bridges

Granite is not a material much associated with modern bridges, other than as a paving or cladding material. It's mainly seen as a historic material, used in masonry arches or retaining walls.


It was therefore a surprise when I recently discovered it being used as the basis of a whole range of prestressed bridges in Germany, particularly so in light of the incredible slenderness achieved. I can only think of one other well-known bridge built from granite for which such wafer-like minimalism is the major feature (the Pùnt da Suransuns).

These bridges are the creation of Kusser Aicha Granitwerke, who have also been employing granite to make sculptures, paving, and water features for about ten years now. They rely on the high compressive strength of granite (at about 200MPa, it's about four or five times as strong as conventional concrete), combined with conventional prestressing to achieve span-to-depth ratios as high as 50 or even 75 for the shortest spans (a range of 20 to 30 being typical in normal bridge construction). Examples quoted are a 300mm slab spanning 15m, or, my favourite, a 40mm slab spanning 3m, for the typical pedestrian live load of 5kPa.

Advantages suggested in favour of the solution are that the structural material also serves directly as the wearing surface, and granite's considerable resistance to weathering and abrasion. The prestressing tendons are placed in ducts packed with grease to provide corrosion protection, although as with any prestressed bridge, these are the weak link in the system, and failure would potentially be sudden and without warning. I would think this is doubly true for the more slender slabs, where any significant deflection will lead to non-linear geometrical effects of a sort which can normally be ignored in conventional beam design.

The bridges can be factory prefabricated, and load tested, and a PDF on Kusser's website shows examples of load testing carried out. As a designer, the short proven history of these bridges naturally fills me with anxiety, and I'd be looking for detailed technical information.

The attraction, for me, is primarily visual, the brutally simple minimalism inherent to the current design. I don't know whether it's a concept that will see wide use, but it's encouraging to see people willing to take traditional materials in innovative directions.

St Patrick's Bridge - final designs

Calgary's Muncipal Land Corporation recently published the three final designs for their St Patrick's Bridge design competition. The three finalists were shortlisted in November, and each given CAN$50,000 to further develop their designs. A round of final public consultation is now underway before the winner is chosen.

The cable-stayed design from Buckland and Taylor with Kitchell Architecure seems to have changed little, although their submission indicates they have spent their time on the engineering analysis, detailed cost estimating, and closer consideration of the pedestrian experience. This is the only one of the three remaining entries which is unafraid to create a strong landmark.

Arup and Falko Schmitt Architects have also made no major change to their design, which comprises two separate bridges each with a stressed ribbon deck supported by a steel arch. The deck elements use stainless-steel fibre reinforced concrete to reduce their thickness to the minimum possible. As I noted previously, the bridge is essentially a rehash of their competition-winning design at River Douglas, with the same engineer Ozan Yalniz responsible for both.

When I first discussed the entry from RFR and Halsall, I suggested it was "far too skinny", albeit mainly with respect to resistance to ice forces. It's no surprise that this is the design which has been modified the most. The main span arches have been raised in height considerably, and tilted further inwards so they can be braced against each other, rather than separate as originally shown. The initial visuals showed the deck suspended on a small number of near-vertical hangers, which have now been ditched in favour of a set of intercrossing diagonal hangers, a network arch form which is far stiffer than the original concept. Taken together, these changes lead to a bridge which is both stiffer (hence requiring less material to resist deflection) and less prone to buckling than would have been the original concept.

All three bridges remain fine designs with a strong focus on the engineering, and an unwillingness to be seduced by inappropriate flamboyance. I'd be happy to see any of them win, although I have to say that the sheer visual quality of the RFR design reports submitted has swayed me somewhat in its direction.

Torroja Bridges: 6. Pedrido & Martin Gil Viaducts

To round off this series of posts on the bridges of Eduardo Torroja, I've got two more concrete arch bridges built in the early 1940s. They weren't Torroja's last bridges, but they are the last for which I have sufficient detail to present here.

The Pedrido Bridge was built in 1940 over the mouth of the Betanzos River near La Coruña in north-west Spain. It has a 75m main span bowstring arch, approached by a series of below-deck arch spans (photographs of Pedrido Bridge courtesy of Treboada via flickr).

While this layout strikes me as visually unsatisfactory, it is less awkward than some of the detailing on the bridge.

The modernist outlines of the arches, both below and above decks, conflict with the more traditional sculpting on and above the main bridge piers. The parapets also change type between the main span and the approach spans, for no reason which is readily apparent. The approach span arches also come a little too close to the deck for comfort, clumsily breaking the extrados line.

While this sort of tension between the functional and the decorative was maybe not uncommon on bridges of the early twentieth century, several engineers had already moved beyond it (most notably in connection with reinforced concrete arch bridges, Robert Maillart), and it seems at odds with the more modernist work completed by Torroja elsewhere (e.g. the Algeciras Market Hall or the Zarzuela Hippodrome).

The Martín Gil Viaduct, completed in 1942, is a larger, more spectacular structure, although again not without its flaws visually (photo courtesy of El Ojo Sayagues via flickr).

The main arch spans 210m, with a rise of 65m (various sources give the span as 192m, I believe that is the clear span at reservoir level, while 210m is the theoretical span to centres of springings). This rail bridge had the longest span for a concrete arch bridge when built, beating the 188m spans of Freyssinet's Plougastel Bridge. The Martin Gil Viaduct was only the largest for a short time, beaten by the 264m Sandö Bridge in 1943.

Plougastel was built using timber centering, perhaps the largest timber arch centering ever built. The Martín Gil Viaduct had begun construction in 1934, before the Spanish Civil War, and its main arch was also to be built on timber centering, which had already been erected when the war disrupted progress. The approach spans had also been completed. By the time construction restarted, the original designer Francisco Martín Gil was dead, and the centering had been too badly damaged by the weather to be used.

Torroja took on the task of completing the project without altering Martín Gil's basic arch design.

The challenge was considerable, although fortunately the construction of arches by first erecting an internal centering (one which would be embedded within the concrete) was now being developed, and Torroja took up this method.

Torroja adopted a centering frame comprising two parallel braced trusses (pictured right - all black-and-white images are taken from The Structures of Eduardo Torroja), erected by hanging temporarily from a suspension cable. The finished frame had the form of a three-hinged arch, and it was stabilised against wind and buckling by temporary lateral stays.

The arch concrete was then placed in a series of strips and segments (see diagram, left, for sequence in cross-section), with each strip completed before the next begun. Within each strip, the concrete was poured in segments with gaps at intervals, allowing most of the shrinkage to take place before the gaps were infilled weeks later.

Parts of the top chords were concreted first, then parts of the bottom chord. The composite steel and concrete frame then created was strong enough to support progressively larger concrete pours.

Hydraulic jacks were used (see diagram, right) to convert the span into a fixed arch and also to relieve loads in the upper chord. Jacking at the crown was also employed to compensate for the deflections caused by shrinkage and creep, prior to erection of the spandrel columns and deck.

Torroja's use of embedded scaffold was repeated on several other bridges, including in 1997 to build what remains the world's largest concrete arch span, the Wanxian Bridge's 420m main arch.

The Martín Gil Viaduct is perhaps less successful visually, due to the lack of any formal continuity between the main span and the much shorter approach arches, with their sturdy masonry piers in marked contrast to the very slender spandrel columns in the main span. However, it was a tremendous engineering accomplishment.

Related links:

• Pedrido Bridge at Structurae
• Pedrido Bridge on Google Maps

• Martín Gil Viaduct at Structurae
• Martín Gil Viaduct on Google Maps
• Martín Gil Viaduct at Wikipedia (Spanish)
• Martín Gil Viaduct at Todotrenes.com (Spanish, lots of information on the planning of the scheme prior to Torroja's involvement)
• Viaducto Martín Gil parts 1, 2, 3, 4, 5 (Spanish, PDF, articles published in the public works journal in late 1942 and early 1943, with excellent photos of construction, technical drawings etc - essential documents)

Bridges news roundup

I'm nearing the end of my series of posts on the bridges of Eduardo Torroja, but while I prepare the final one, here are some quick links to stories elsewhere:

The Continental Starts When the Calatrava's Done. Spring 2011, Says the City. OK, Then.
Highway bridge in Dallas to be transformed into pedestrian bridge (pictured) - also see the Q&A with the project manager

Guerrilla bridge-makers save New Yorkers from scum river
Proving that small is beautiful (spotted thanks to tallbridgeguy.com)

New design for Molineux bridge revealed
"We've got four lattice arches, Wembley's only got one", Wolverhampton bridge designer fails to say

Tech Know: The dreams that bricks are made of
Forth Rail bridge built in Lego

Meet Shanghai's Mr Bridge
Interview with Lin Yuanpei

Federal help available for Johnson Street Bridge repair
Refurbishment of Canadian bascule bridge looking like a more positive option

Protestors lose fight as Bradford bridge plan is voted through
Cable-stayed bridge with weathering steel pylon gains planning consent in Bradford on Avon. It's quite an interesting design, drawings and documents are available on Wiltshire's planning website

Torroja Bridges: 5. Tordera, La Muga & Posadas Bridges

The pre-war years were clearly a busy time for Eduardo Torroja. At about the same time as he was completing the brilliant Alloz Aqueduct in prestressed concrete (1939), he also designed two steel composite structures, Tordera Bridge and La Muga Bridge. These bridges were very radical departures for Torroja, who is largely recognised for his work in concrete, not steel.

While composite bridges had been built as early as the 1920s, they did not become widespread until after the second world war, and at the time of Torroja's designs, the detailing had yet to become codified.

The Tordera Bridge, completed near Barcelona in 1947, replaced an existing highway viaduct which had been destroyed in the Spanish Civil War. The new structure was 8.5m wide, and makes use of the previous bridge's river piers. The centre span is 54.9m, and the two side spans are each 45.7m. The bridge was strengthened and widened to 11.5m in 1994. All black-and-white images are taken from The Structures of Eduardo Torroja (a good colour photo of the Tordera Bridge can be found on flickr).

The bridge is unusual in comprising inverted bowstring trusses, although bridges of similar design had been built previously (e.g. Biesenbach in 1890, and Little Hell Gate in 1917), and the concept can be dated at least as far back as Robert Stevenson's River Almond bridge proposed but not built in 1821. While this is theoretically an efficient solution, with the deck slab acting in compression and a slender tension member slung below, I suspect it is rarely adopted because the majority of bridges have headroom constraints beneath their spans. It also lacks the benefits of span-to-span continuity that can be provided in a more conventional truss or girder arrangement.

The Tordera design adopts an elliptical arc tension chord, which although slightly less efficient overall than a parabolic form allows the most heavily loaded end diagonals to be shorter and hence less prone to buckling. The diagonals are welded steel box sections, and the tension chord is a flat plate with a small upstand stiffener sufficient to resist bending between the connecting nodes.

Torroja's calculations took account of the effects of concrete shrinkage and differential temperature, which can be significant in composite bridge decks. However, his insistence on the arced chord as the most efficient shape to resist bending made the construction more complex than is now the norm.

As can be seen in the photo, significant temporary support steelwork was required to allow the bowstring truss to be rolled and then dropped into its final position. This would not be seen as efficient today, another blow against the underslung bowstring form, and indeed Torroja specifically tackled this problem in his design for a bridge over the Muga river.

Torroja designed a similar bridge at Posadas in Cordoba, the Puente de Hierro, albeit with more spans and battened rather than solid struts (photo courtesy of RBolance at flickr). Finished in 1951, this carries a highway across the Guadalquivir River with eight underslung bowstring spans. Like its cousin at Tordera, it was strengthened in 1995, with the involvement of Torroja's son, José Antonio Torroja Cavanillas.

The Posadas bridge isn't featured in Torroja's own books, nor is it included at Structurae, so I have little else to add.

Torroja's bridge over the Rio Muga circumvents the difficult construction methods required at Tordera and Posadas by the simple expedient of adopting a flat rather than a bowstring truss. To modern eyes, conditioned by over-familiarity with composite plate-girder bridges, this seems an obvious choice. It allows the bridge to be launched from one end, minimising work in the river, and its simplicity offsets the fact that material is used less efficiently.

The Muga bridge didn't quite take that approach however, being launched as a continuous structure before being disconnected at each support position to form a series of simply supported spans.

While this seems at first sight somewhat baffling, Torroja's writings make clear that he felt a continuous bridge of this type was simply too unproven. His thinking on bridge design was often to eliminate stiff connections so as to reduce the effects of secondary restraint stresses, whether caused by settlement, temperature, shrinkage or any other source. He acknowledged that with the benefit of further experience, he would certainly have adopted a continuous beam design for any similar opportunities.

Related links:

• Tordera Bridge at Structurae
• Tordera Bridge on Google Maps (I think this is right but would welcome confirmation)
• Composite Truss Bridge Decks (article discusses the Torroja bridges and others, PDF)
• Las estructuras mixtas y el puente de Tordera (Spanish, PDF, article by Torroja on his Tordera design, with construction photos, technical diagrams etc)
• Rehabilitation of the Tordera Bridge (PDF)

• Posadas Bridge on Google Maps
• Posadas Bridge tourist information (Spanish)

• La Muga Bridge at Structurae (can anyone locate it on Google Maps for me?)

Torroja Bridges: 4. Alloz Aqueduct

The bridges of Eduardo Torroja which I've discussed so far included some innovative (Tempul Aqueduct) and unusual (Quince Ojos) structures, but none to rival the engineer's greatest structural designs aesthetically. But continuing a few years onwards, to the construction of the Alloz Aqueduct in 1939, and that changes (photo courtesy of Bridge Ink).

The Tempul Aqueduct used a concrete box to contain the aqueduct pipeline. At Alloz, water is carried in an open-topped channel, and the structure itself forms the channel. As a result, the avoidance of cracking in the concrete became the dominant design concern, and Torroja used a combination of methods to ensure the concrete remained in compression in all situations.

First, every alternate span is split at midspan (each span being 19m long). This establishes a bending moment diagram such that no part of the viaduct is in sagging, and hence the bottom edge of the channel is always in longitudinal compression (the image on the right and those below are all taken from The Structures of Eduardo Torroja). The joints are sealed using sheets of corrugated lead embedded in the concrete and protected by bituminous mastic.

To ensure that the top edge is similarly compressed, Torroja incorporated prestressing cables into the channel, with a somewhat unusual method for loading the cables. Unlike a modern prestressed concrete bridge, where prestress is applied by jacks at the ends of the cables, Torroja introduced transverse jacks between pairs of cables, and by jacking the cables apart, lengthened and hence stressed the cables. If more prestress is required than can be provided by such a jack, it is simply applied at multiple locations. A similar approach to prestressing has been used by Eladio Dieste in his brick shell structures.

Transverse prestress is also applied, using bars spanning between the channel's top flanges, placed at roughly 5m intervals. By tightening turnbuckles on the bars and drawing the two flanges slightly together, the inner face of the channel becomes compressed.

These techniques all seem highly unorthodox to a modern engineer, given the way that other methods have come to dominate in the intervening decades. But they exhibit a remarkable practically-minded ingenuity, and a concern to dimension and arrange the structure to produce the desired behaviour, rather than simply to design a more-or-less arbitrary structure and make it strong enough to cope with the resulting distribution of forces (as is so often the norm in structural engineering).

However, the real reason that the Alloz Aqueduct has lasted so well is not the technology, but its appearance. It is fundamentally difficult to make a long, deep concrete viaduct look attractive, but Torroja undoubtedly succeeded. The use of the parabolic channel cross-section is important, with the curved underside working well against the structure's overriding linearity. But the compass-shape legs, each of them offering a saddle support to the aqueduct, are the standout feature, carefully proportioned, and strikingly memorable.

This is my favourite Torroja bridge.

Related links:

• Alloz Aqueduct at Structurae
• Alloz Aqueduct at Google Maps
• Celebration of 70th anniversary, with driving directions (Spanish)

Torroja Bridges: 3. Aire & Quince Ojos viaducts

I'm continuing my series of posts on the bridges of Eduardo Torroja, with his next two significant structures chronologically.

These two very different bridges were both completed in 1933 as part of infrastructure construction in the University City (Ciudad Universitaria) area of Madrid.

El Viaducto del Aire ("Viaduct of the air") was built to carry trams across the Cantarranas watercourse. It's a reinforced concrete arch (pictured left, images taken from The Structures of Eduardo Torroja), spanning 36m and with 17.4m rise, with column spandrels.

It's clearly not a classic of its type, certainly compared to the concrete arch bridges of Robert Maillart built in the same era (e.g. the Valtschielbach Bridge of 1929), which were more ambitious technically and had a more lasting sense of aesthetics.

High amongst its ungainly elements are the unfortunate double columns above the arch springings. The deck cantilevers also throw a shadow onto the crown of the arch, breaking its profile. Torroja consciously varied the spacing of the columns above the arch to make it look better, but overall it's a very clunky design, partly because of the constraints inherent in such a tall arch form.

At some point in its history, the Cantarranas valley was infilled, and it appears the bridge may still be present, but buried, in what is now the grounds of the Moncloa Palace, home to the Spanish president.

The nearby Quince Ojos ("Fifteen Eyes") Viaduct (pictured left, first two photographs courtesy of Carlos Viñas) is in many ways even less satisfying, although at least it has not suffered the unfortunate fate of the Aire Viaduct. Quince Ojos has 25 arches, each spanning a mere 7.8m. The bridge is 35m wide, so in essence it's a causeway cut through with a series of dim tunnels.

It looks in some respects like a traditional masonry-inspired concrete arch viaduct, reminding me a little of the 21-arch Glenfinnan Viaduct in Scotland, an unreinforced concrete structure built three decades beforehand (if not in the photos here, then at least in older images). But Glenfinnan's arches are to a larger scale, up to 30m tall and spanning 15m each. The proportions of the arches at Quince Ojos are simply too small to be attractive.

The real puzzle is that it isn't actually an arch viaduct at all. Instead, there is a joint at the crown of every arch, and in fact the bridge is a procession of columns supporting curved cantilevers. In The Structures of Eduardo Torroja (from where I've taken the image on the left), Torroja states that this choice was because of his concern about concrete rigidity in a viaduct of this length, an issue which didn't seem to unduly disturb the Glenfinnan designers, even though it lacks the reinforcement used at Quince Ojos.

Torroja deals explicitly with the potential criticism that his design is structurally deceptive:

"The superficial impression that the viaduct is a series of arches instead of separate cantilevers might be interpreted as a weakness. But in fact, what is the actual structural function of these cantilevers ... the stress distribution is certainly no less functional than it would be in an arch".

It's notable throughout Torroja's bridge work that he would incorporate joints wherever they simplified the design. Nowhere in his writing does he indicate any awareness of the problems that they introduce. While their poor durability may not have been well recognised in the early 1930s, they also attract damage due to dynamic impact from rail or carriageway loads, are less able to transmit longitudinal loads, and reduce structural redundancy and hence robustness overall.

The photo on the left (taken by Nicolas Janberg of Structurae in 2003, as is the next image below) shows the bridge to be in very poor condition, with staining and exposed reinforcement. Some of this may be due to lack of cover, poor concrete etc, but all those joints can hardly have helped. To be honest, I'm amazed the bridge hasn't been demolished and replaced a long time ago, although the fact that it carries a major highway into Madrid may mean this is simply too disruptive to contemplate.

Others had designed far more appealing reinforced concrete bridges at this time. Torroja's older rival, Eugène Freyssinet, had designed attractive structures such as the Boutiron Bridge two decades previously, and completed his 188m span masterpiece at Plougastel in 1933. Robert Maillart's bridges had shown that a pragmatic rather than theoretical approach to engineering could produce largely successful joint-free design.

Torroja's best bridges remained in the future.

Related links:

• Quince Ojos Viaduct at Structurae
• Discussion on burial of Quince Ojos Viaduct (in Spanish), with archive photos
• University construction photos showing Quince Ojos Viaduct
• Aire Viaduct at Structurae
• Aire Viaduct at Wikipedia (Spanish), with archive photos
• Aire Viaduct on Google Maps (I'm not quite sure of the Quince Ojos location, can anyone help?)
• Roadworks for the development of University City (1935 article in Spanish, PDF)

Torroja Bridges: 2. Tempul Aqueduct

Completed in 1926, when Eduardo Torroja was only 27, the Tempul Aqueduct is a truly remarkable bridge design. Replacing an earlier structure which had been destroyed by flooding in 1917, it was one of the first examples of that very modern structural form, the cable-stayed bridge.

It wasn't quite the first. Albert Gisclard's Cassagne Bridge of 1907, and Gaston Leinekugel-Lecocq's Lézardrieux Bridge of 1925 both predate it, and before that there were a large number of hybrid suspension and cable-stay designs, such as Albert Bridge in London and Brooklyn Bridge in New York. Even further back, there were proposals such as C T Loescher's timber design of 1784.

In Torroja's design, the main 60m span over the River Guadalete is supported by single stays from each tower (pictured right), with the central part of the span resting on half-joints. Torroja introduced these hinges (which seem an odd choice for an aqueduct) because he was uncertain about the compatibility of deflections of the various structural elements, especially because of the way in which the stays were stressed. Reflecting on the design in later life, he considered he could have been less conservative and adopted a continuous beam.

The motivation to adopt a stayed bridge was to provide an alternative to a design using three equal spans of 20m, which required the construction of piers within the riverbed. Concerns were raised that even with piled foundations, these piers could be undermined. Torroja proposed to use cable stays to replace the the river supports entirely.

The twisted steel cable stays passed over saddles on top of each tower, which were jacked upwards to tension the cables and also to lift the bridge off its formwork. This worked very effectively in practice, as the river flooded heavily during construction, and the tower-heads were jacked up to release the formwork, which was allowed to wash away without damaging the aqueduct. The cable forces could also be readily adjusted to deal with the effects of concrete shrinkage and cable relaxation.

Once all adjustments were complete, the tower-heads were concreted in place, and the cables surrounded in concrete to provide long-term protection against corrosion. This approach never found great favour in cable-stay bridge design, perhaps because of the weight of concrete required and difficulty ensuring it would be self-supporting without cracking.

The horizontal component of the cable-stay force acts to prestress the deck, which is a reinforced concrete box containing a cast iron pipeline, with a very bulky appearance, especially on the spans away from the river.

In the modern cable-stay bridge, the cables are largely thought of as providing a series of vertical points of support to the deck to reduce its effective span, although it can be thought of as simply a variant on a post-tensioned bridge where the stressing cables are brought above the deck surface, hence providing a much greater angle to resist shear and lever arm to resist bending (this concept being most clear in the extradosed bridge). It's clear that Torroja thought of this bridge in both ways. Writing in connection with reinforced concrete cantilevers, he wrote in the Philosophy of Structures:

"For longer spans, the tension zone can be separated in the form of an independent cable, in a triangular assembly with the beam compressed over the pier."

Judged by modern standards, the bridge isn't visually successful. The deck is heavier than it needs to be, the stays are unattractive, and the series of short approach spans is obtrusive. This is a bridge which marks Torroja's technical achievements, it's not on a par with some of his later designs, which were far more sensitive aesthetically.

In 2008, the go-ahead was given to carry out 405k euros of rehabilitation work on the bridge, now know as the Puente de San Patricio (Bridge of St Patrick), although I don't know whether this has been completed.

All pictures have been taken from either Torroja's Philosophy of Structures or The Structures of Eduardo Torroja, there are very few photographs of the bridge online, and I haven't been able to locate any without copyright restrictions. Click on any image for a larger version.

Related links:

• Tempul Aqueduct at Structurae
• Tempul Aqueduct on Google Maps
• Acueducto-sifón sobre el río Guadalete (Spanish, PDF, 1927 article by Torroja on his design, with technical diagrams etc)

Torroja Bridges: 1. Introduction

I've recently been reading Eduardo Torroja's The Philosophy of Structures, and was wondering quite what to make of it here online. Most of the book relates to building structures, and although I have a great interest in structures and architecture beyond matters gephyrological, I would like to keep to my self-imposed restriction that this blog is for Pontism, and nothing else.

Torroja (1899-1961) was one of the most notable structural engineers of the twentieth century. His built work was less prolific than, say, Félix Candela or Heinz Isler, and certainly less spectacular than the likes of Pier Luigi Nervi or Eladio Dieste. In part, his renown is due to his wide-ranging commitment to structural engineering as a profession: as well as being a designer, he ran various companies and research laboratories, and he was the founding president of the International Association for Shell and Spatial Structures, who still award the Torroja Medal in his honour.

One reason he remains widely recognised was that he was one of very few engineers to articulate his design philosophy in writing. While The Philosophy of Structures is sometimes heavy going, The Structures of Eduardo Torroja is a simple pleasure, mainly comprising photographs and sketches, with the designer's explanations of his work. Both, incidentally, are out-of-print, but relatively easy to locate secondhand.

Torroja has some interesting things to say about design, many of which are relevant to bridges if aimed at other targets. His basic point of view is summarised in the introduction to Philosophy:

"Structural design is concerned with much more than science and techniques: it is also very much concerned with art, common sense, sentiment, aptitude, and enjoyment of the task of creating opportune outlines to which scientific calculations will add finishing touches, substantiating that the structure is sound and strong in accordance with the requirements".

Elsewhere in the same book, Torroja makes clear his view on how engineers should balance art and technology:

"Every art demands a technique ... In our case, the basic technique is essentially static balance and strength, against which so many objections are leveled on the count that technicians lack adequate culture and aesthetic feeling. Conversely, how much nonsense germinates in the mind of the artist who has not the requisite technical training and understanding!"

Over the next few posts, I'll cover several of Torroja's bridge designs, in chronological order. While doing so, it's worth noting that with one exception, they don't really rank amongst his finest designs, such as the Zarzuela Hippodrome (pictured right, courtesy of Paco Garate on flickr), Algeciras Market Hall, and Frontón Recoletos. I'd encourage interested readers to look at those if not familiar with Torroja's work.

I won't cover all his bridges: there were several built in Morocco late in his life for which I haven't been able to locate sufficient information, and there are a couple of unbuilt designs in The Structures which I'll also leave alone for now.

Related links:

• Torroja at Structurae
• Instituto de Ciencias de la Construcción Eduardo Torroja
• Eduardo Torroja - Ingeniero (by Navarro Vera & Fernández Ordóñez)
• Eduardo Torroja - from the philosophy of structures to the art and science of bulding (ed. Levi, Chiorino & Cestari)