Spanish Bridges: 2. R-3 Footbridge, Madrid

I saw two more footbridges in Madrid. Both form part of a footway and cycleway connecting the districts of Vicálvaro and San Blas, running through the Parque de la Maceta, and crossing two major highways.

The first, and the larger of these two footbridges, spans the R-3 motorway. It is a suspension bridge with a main span of 110m across the dual 3-lane motorway and connecting slip roads, and two 40m side spans. The approach structures comprise a further 3 spans at each end.

The bridge is nicknamed by locals the "Chupa Chups" bridge as a result of its lollipop-shaped masts. It is a distinctive bridge, and unusual in many ways, not all of them good. It was completed in 2007 to a design by Leonardo Fernández Troyano of Carlos Fernández Casado S.L.

For any suspension bridge of this span, a key design issue is how to reduce bending moments and associated vertical displacement in the bridge deck. The worst case for design (as with an arch bridge) is with only half of the main span loaded, which results in S-shaped moments and deflection in the bridge deck.

The most common solution is to stiffen the bridge deck sufficiently to minimise deflections and distribute loads. The mirror-image approach is to have a slender bridge deck but to stiffen the suspension cable, but this is difficult to construct and consequently rare.

Another common approach is to use stay cables supported on the bridge towers to stiffen the deck close to the supports, with the most famous example being Brooklyn Bridge. This comes in two flavours: overlapping vertical hangers and diagonal stays, as in Brooklyn, or with the stayed section and vertically suspended section not overlapping. Saint Laurent Bridge in France is an example of the latter.

The R-3 footbridge takes a different approach, and one not commonly used (although the same design team has used it previously). Here, the stays are reversed, and rather than supporting the deck directly, they radiate upwards from the junction of the tower and the deck. These negative stays are connected to the main suspension cable, and they work by restricting movement of the main cable. This in turn reduces vertical movement of the bridge deck, and the associated bending moments.

The system can be highly effective, and without this system (or one of the alternatives) the R-3 footbridge could not have such a slender and economical bridge deck. You might ask why would a designer choose this system over the alternatives, and why is it so uncommon?

Fernández Troyano explains that it was done for reasons of construction economy. It allowed the bridge deck to be built out of repeated, identical, slender precast concrete panels. The more common positive-stayed alternative (Brooklyn et al) would have required custom deck panels to connect the diagonal stays onto.

I don't find this argument very persuasive, as I can't really believe that the complications in the deck would have been significant. Instead, the complication has been transferred to a series of customised cable clamps, each slightly different.

The real problem with this design solution is not the principle, but the detailing. At the masts, the negative stays pass through holes and are anchored in steel pipes projecting from the other side. It's an absolutely atrocious detail, a complete mess visually, and was almost certainly very awkward during construction, with the pipes so close together. It creates a length of hidden cable which can neither be inspected nor maintained and seems arranged deliberately to create water traps.

This is only one of the bridge's many flaws.

There are two basic options for a suspension footbridge when considering how to deal with the main cable as it passes over the supporting masts.

The first is to split the cable, anchoring it at the mast, as in the Nesciobrug. This allows a slender mast, but requires multiple suspension cable anchorages and potentially the need to adjust the cables during construction.

The second option, used on the R-3 footbridge, is a cable saddle, where the cable passes over the mast. The cable saddle must be large in radius, as otherwise bending stresses in the cable become unacceptable. For a footbridge, the size of saddle required is problematic, inevitably exceeding what is appropriate for a slender mast, and solutions include a fanned support (as on the Wingtip Bridge), or altering the mast to suit the saddle width (as on the Peramola Bridge, which is also negatively stayed).

For the R-3 footbridge, the designers opted for a possibly unique approach of "lollipop" mast heads, which feel like perhaps the worst possible option, visually.

The bridge's suspension cables are also anchored in a peculiar and somewhat thoughtless manner. The cables are connected to steel anchors at the top of inverted-V concrete piers. The anchors cannot be seen, as they are embedded into the pier heads, covered over in concrete and steel. There is, again, no facility for inspection or maintenance, and bituminous protection has oozed out of the anchorage and stained the facing concrete. It is a terrible detail.

Those inverted-V piers serve a dual purpose. They carry the cable anchorage forces into the ground, with the front leg in compression and the rear leg carrying the tension force.

They also act as a punctuation mark, separating the differing structural forms of the suspension bridge deck from its approach spans. The main bridge is supported on its edges, with the cable forces taken into the ground on both edges of the bridge. The approach spans, however, have a central spine beam, supported on single inclined columns. The inverted-Vs provide a visual break between the two different typologies, successfully, I think.

The approach spans are supported on further V-shaped piers, although here the designer presumably came up against difficulties in dealing with thermal movement of the bridge.

These piers have a cut-line near the base, indicating the presence of support bearings, presumably to allow the V-pier to move under thermal effects. It seems to me yet another poor detail, as the form of the pier is clearly unsuited to the loads and movements it experiences.

Seen from afar, the R-3 footbridge is an impressive and appropriate structure, with an impressively slender deck. It's unfortunate that the more closely you examine the details, the more you can see the very real flaws in the design,

Further information:

• Google maps
• Structurae
• The Art of Spanish Bridge Design
• Video interview with designer, Leonardo Fernández Troyano
• Two Suspended Footbridges in Madrid Stiffened with Negative Stays (Troyano, Triola, Fernández, Calle, and Pérez, Footbridge 2008)
• Pasarelas sobre la R-3 y la M-40 entre los distritos de Vicálvaro y San Blas en Madrid ( Troyano, Fernández, and Calle, IV Congreso de la Asociación Científico-Tecnica del Hormigón Estructural - Congreso Internacional de Estructuras, 2008)

Spanish Bridges: 1. Paloma Footbridge, Madrid

In October, I joined a trip to Madrid to see various interesting engineering structures. I've picked three of the footbridges to feature here.

The first is the Paloma Footbridge, a remarkable structure which carries pedestrians across a busy urban motorway. The 191m long bridge was designed by Cesma Ingenieros, with construction engineering by Ines Ingenieros. It was completed in October 2010 at a reported cost of only €2m.

The bridge is fascinating from a structural engineering viewpoint, but what is most immediate about it is its sense of drama. Its four spans sweep through a 90° curve to address a huge change in ground level, rising 8m in order to clear the motorway. The form is highly unusual, a group of two horizontal trusses and one inclined truss arranged in a 4m tall "C" cross-section.

It takes some time to understand quite how the structure works.

The bridge spans are supported on giant "Y" shaped columns, the upper arms of which also form part of the main load-bearing truss. This truss is then subject to a series of lateral forces which are resisted through the roof and floor trusses.

The lateral forces arise from three types of structural behaviour.

First, the vertical load from the bridge is eccentric to the piers, to the inside of the pier where the deck is straight, and to the outside of the pier due to the plan curvature where the deck is more highly curved. This eccentricity introduces a torsional warping stress which creates lateral forces in the upper and lower horizontal trusses.

Secondly, the loads are eccentric to the shear centre of the overall cross-section. For any channel-shaped section, the shear centre is outside the area enclosed by the section, on the outside of the vertical element. All loads therefore establish a further torsion acting towards the inside of the piers.

Finally, there are compression and tension forces in the horizontally-curved upper and lower chords of the main truss. These introduce lateral destabilising forces, the magnitude and direction varying according to the position along the main truss. The two horizontal trusses provide lateral stiffening to the main truss chords, and carry the out-of-balance forces back to the support piers.

The bridge is structurally ambitious but each element also fulfils an architectural purpose, with sun-shade elements on the (south-facing) main truss and also on the roof truss. These are very welcome on such an exposed site in central Spain.

The detailing of the bridge is, for the most part, excellent, and I especially like the way the Y-shaped columns are combined with, but stand out from, the main truss.

I also noted that some of the screening in the side of the structure has an enhanced density in one area, where the bridge overlooks some apartments, enhancing privacy. This has been cleverly done, so that very few bridge users will even notice it's there.

Design of the support piers cannot have been straightforward, as they need to be strong enough to resist the complex loads applied to them (I'm not sure whether they are also designed for highway impact loads), but also flexible enough not to experience undue stress when the bridge experiences thermal expansion and contraction.

There is one feature of the bridge design that I certainly can't commend. The sun screens are a welcome feature, but with an unintended side effect, which can be seen in the first video below.

There's a pronounced light flicker which can be clearly seen as you traverse the bridge. It's caused by the orientation and spacing of the screen slats, as can be seen in this second video.

If the slats were oriented the other way, or were angled or spaced differently, the flicker would be reduced or eliminated (although obviously it will vary anyway according to the direction and elevation of the sun).

I think most bridge users will hardly notice, but I can easily see it being a significant issue for people with photosensitive epilepsy, or visually dependent vertigo. As a bridge designer, it's yet another thing to add to my list of things to consider for accessible design. Many designers seem to think that just providing a shallow gradient is enough to accommodate disabled or less able bridge users, but of course disability takes many different forms.

Another aspect which detracts from the general high quality of the bridge is the parapets, which have over-sized handrails (again, not good from an accessibility point of view), and inadequate detailing at the expansion joints. As you can see in the photo, there appears to be some kind of painted duct tape over the sleeved top rail, and an intermediate element has been damaged due to poor alignment.

On the whole, however, the Paloma Footbridge is a bold, carefully considered design. Trussed footbridges are often the least-cost, most mundane solution to crossing a motorway, but nothing could be further from the truth in this case.

Further information:

• Google maps
• Structurae
• The Art of Spanish Bridge Design
• Video interview with designer, Peter Tanner
• Paloma Footbridge, Madrid, Spain (Tanner, Bellod and Sanz, Structural Engineering International, 2011)
• Footbridges: Small is Beautiful (ECCE, 2014)

Footbridge 2011: papers roundup pt 2

I've had time to jot down a few more comments on the papers presented at Footbridge 2011. Again, this is a highly selective bunch, there were many others which were interesting but which I simply won't have time to cover.

The Arganzuela helicoidal bridge over the Manzanares River, designed by MC2 Engineering Cosultants with Dominique Perrault Architecture, belongs to the increasingly popular genre of helical truss bridges, a genre I have surveyed previously. There are plenty of images online, but I've just chosen one from the conference paper.

It's actually two separate bridges, with a total length of 278m. Each is in the form of a cone, varying in diameter from 6m to 12m, and structurally they are helical trusses, with straight upper and lower chords intersected with spiral diagonals, taking the general form of a through Warren truss. This is pretty much as rational as this helical form ever gets, but as the authors noted, it still imposes significant secondary stresses on the steelwork, requiring substantially more material than would the more conventional solution.

I've included it here because while I have my doubts about the conic geometry (attractive in the widening direction, but visually constricting in the other direction), this is one of the best of its genre. This is largely, I think, because of the cladding treatment, which with its varying densities of mesh, distracts from the structural frame and uses its apparent solidity to create an object with a greater sculptural weight.

I enjoyed Andreas Keil's presentation, Passerelle sur Nanterre - spatial play of forces, on the La Defense footbridge designed jointly by Schlaich Bergermann und Partner and Deitmar Feichtinger Architectes. This is a remarkable 88m long bridge which passes around the perimeter of a curved building, without touching it, and with supports only towards the ends. This is a challenge in itself, but the choice was made to support it on only one edge using the "inverted Fink truss" typology (a system which has only limited resemblance to a genuine Fink truss).

The result is a truly superlative example of high-tech structural engineering. The main vertical support comes from the truss, which is essentially a series of successively cantilevering cable-stays. The number of cables in each bay of the truss are varied according to their order in the cantilever i.e. the number of cables used is proportional to the force to be carried. This system is assisted by a set of ring-cables offset below the deck level, which are cranked back up to deck level at the ends and hence carry a share of the vertical load in a manner similar to an external prestressing tendon. This also reduces the possibility of uplift forces on the bridge's end bearings.

The deck is a hollow steel box, providing some of the much needed torsional stiffness. However, this is far from sufficient on its own, and the lower ring-cables provide the remaining torsional restraint. The vector sum of their forces at each mast position imparts a net inwards force, which, because of its level, acts to counterbalance the torsion resulting from the deck dead and live loads only being supported on one edge. For some reason, the bridge as a whole reminds me of a set of gimbals, as in a gyroscope, possibly because of the way the whole ensemble appears to teeter on only a few pin-point supports.

It's a remarkable bridge.

Another bridge which impressed me was Link 27 - a new footbridge in the west of Vienna, Austria, presented by Rudolf Branstötter. This 38m span bridge was completed on site in September 2010, and was the end result of a student design contest which Branstötter had won. I thought this was an exceptional structure, where the structural form, although unusual, was a carefully thought-through response to the site's very specific constraints.

It spans over both a railway line and the River Wien. The different permissible clearances led to a structural form which has a low level springing on the river bank, and which is supported over the railway from above. One end resembles a concrete arch, while the other is a concrete cantilever, with fixity provided by steel tie-down bars. However, both aspects appear fully integrated. The method of construction was also unusual, with both edge girders precast on site, lying on their side, before being rotated and lifted into place.

Footbridge 2011: papers roundup pt 1

Okay, I thought I would attempt a quick round-up of some of the papers and presentations from Footbridge 2011 which particularly caught my eye. Apologies to anyone whose presentation I saw and found interesting, but have missed out, I only have so much time so will have to be quite selective!

In addition to the three keynotes previously mentioned, I enjoyed Henryk Zobel's Contemporary structural solutions of timber pedestrian bridges, which offered a nice survey of the range of timber designs currently in use. One that struck me as especially attractive was the treetop walkwway in Tharandt, Germany, pictured right (click any image for a larger version). This 118m long curved, stress-laminated timber structure was explained further in its own paper, although I missed the presentation.

There were relatively few other presentations to suggest any acceleration of the use of timber footbridges, although the Margaretengürtel design in Vienna was presented (see my previous post for details), and another presentation addressed some of the reasons why timber bridges remain unpopular in the UK. Client inertia appeared to be the deciding factor, although I know from my own experience as a designer that client concerns over vandalism, fire damage and general durability are often hard to refute. Having said that, at one time, the UK was claimed as home to the longest timber arch footbridge in Europe (the Middlewood Way Bridge near Macclesfield, a 50m span built in 1992), although that claim has certainly now been overtaken by bridges over the River Lora, near Florence (72m span) and in Rimini, Italy (92m span).

One aspect of the conference which I thought was quite unfortunate was the separation of most of the footbridge dynamics papers into a separate strand, which meant that those of us interested in more general design case studies or topics probably saw very little about dynamics at all. The ghettoisation of the dynamics specialists did mean that the entirely non-technical could avoid the subject, but runs the risk of re-opening a gulf between academia and practising engineers which had been temporarily narrowed in the post-Millennium Bridge years.

Nonetheless, some of the dynamics presentations which I did see were quite staggeringly esoteric, and it was tempting to wonder to what extent they were researching areas which were likely to have real practical relevance.

One dynamics paper which clearly was highly relevant, was Aleksandar Pavic's keynote Vertical crowd dynamic action on footbridges: Review of design guidelines and their application. Pavic's paper reviews five current published methods of dealing with this problem (see table, left), provides worked example calculations for a simple structure (itself very useful, given the ambiguities present in some of these documents), and observes that they can lead to very different results, with serious implications both for economy of design and the potential for error.

Two of the guidelines, HIVOSS and SETRA, are available online. It's notable that when Eurocodes 0 and 1 were published, a methodology for dynamic analysis of footbridges was conspicuous by its absence, including the load models which had been expected to appear (and which are given in the 2005 fib document Guidelines for the design of footbridges). Some of this is because the state-of-the-art in this area has been a constantly moving target, but I think it's a shame that Eurocratic deadlines for publication of the Eurocodes were allowed to take precedence over the presentation of standards which would assist rather than confuse designers. For anyone who doesn't follow the footbridge dynamics literature, it must be a real struggle working out how to proceed.

I saw two presentations where weathering steel was treated in very different ways. Martin Knight and Simon Fryer presented Combining engineering and aesthetics: The Town Centre Link, London, which must be one of the largest scale footbridges shown, at some 135m long and 12m wide. It's enormous weathering steel Vierendeel trusses (pictured, right) are detailed to avoid water traps, and are protected against the risk of graffiti by their height above the rail station platforms below, and by the use of full-height glazing on their inner face.

In contrast, Xavier Font discussed the lovely Can Gili Footbridge (pictured left), which I have covered on this blog before. Here, the pedestrians are kept away from direct contact with the rusty weathering steel by an internal guardrail. However, the potential for graffiti clearly exists, and I understand graffiti is now present. The absorbent surface patina on weathering steel makes graffiti hard to clean off, and only a fresh blast cleaning can reliably remove it (in the UK, this is covered by Highways Agency standard BD 7/01). In addition to the cost, I guess that will affect the uniformity of colour of the weathering steel patina, which may be undesirable. I greatly admire the appearance of weathering steel bridges, but as with the timber designs already mentioned, clients may have to adopt a more flexible attitude to the issues of durability and vandalism.

I was also impressed by a couple of weathering steel footbridges presented by Mario Guisasola, but would like to prepare a full post on them some day, as they were easily amongst the best designs presented at the conference.

Okay, that's all I have time for now, I will say more on the conference presentations later.

Can Gili Footbridge

I've been waiting for an opportunity to feature one of the bridges of Xavier Font Solà (of the design firm Alfa Polaris), and with his Can Gili Footbridge completed on site in January at Granollers, near Barcelona in Spain, that opportunity has arrived.

Alfa Polaris is perhaps best known for their multiple award-winning restoration of the Pont Trencat, a historic masonry arch bridge which had been partly destroyed in 1811. A new span was added in weathering steel, retaining the arch form, and achieving a unique blend of historically sympathetic restoration with brutalist modern intervention. I also very much admire their Joan Camps footbridge, also in Granollers, which again uses weathering steel and gives a very distinctive take on the traditional Warren truss design.

If rust orange-brown steel seems something of an Alfa Polaris signature (much as white paint is for Calatrava), then the €574k Can Gili footbridge will do nothing to dispel that impression. The bridge design commenced in 2001, but funding for construction was only secured in 2009. The structure was built by ACSA Obras e Infraestructuras, with steelwork by Talleres Plain.

The design's heavily asymmetric appearance is a direct response to the profile of the highway cutting that is bridged. This is also asymmetric, with an embankment much taller on one side than the other, and there is poorer ground to one side of the highway. This has led to the single offset V-shaped pier, although you could also see the bridge as arch-like in its form.

The bridge was erected with only four one-day road closures, which the designer notes was a key constraint leading to the adoption of a steel rather than concrete design. However, my own experience is that in the UK at least, it is exceptionally rare for a road to be closed for so long, and most footbridges over highways have to be installed within a single night-time closure. Four days is positively luxurious.

The bridge's main superstructure consists of two 40m long hollow weathering steel box girders, supporting a plastic lumber deck 2m wide (plastic lumber is a cheaper alternative to the fibre-reinforced plastic which is becoming more and more common as a bridge deck material). The main span is 33m.

There's absolutely nothing about the site which makes the design choices inevitable. Plenty of other structural forms could have bridged this divide, most without any need for an intermediate support within the highway cutting. The design seems positively willful, but the result is sufficiently striking to justify that.

I don't know how weathering steel will develop its characteristic protective patina over time in Spain, but the current orange colour looks spectacular against a blue sky. The dark purple colour it can take on later will have less character, but most of the bridge's visual impact is from its shape anyway.

The contrasting use of stainless steel handrails, infill bars, and approach rails looks good. The bridge uses low-level LED lighting for illumination, an increasingly common choice.

Do I like it? Very definitely. It's strong and bold, and shows that a form with almost brutal solidity can still make an attractive structure in the right context. That's in marked contrast to the trend for modern footbridges to be lightweight, almost to dematerialise in favour of ultra-slim decks and cable support networks.

Further information

• Google maps

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)

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?)