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)