20 February 2010

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.

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