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Salginatobel Bridge
Salginabach, Crausch, above Schiers, Graubunden canton, Switzerland
Salginatobel Bridge
associated engineer
Robert Maillart
Ingenieurbureau Maillart
date  June 1929 - August 1930, 1973 - 1976, 1997 - 1998
era  Modern  |  category  Bridge  |  reference  Tg906427
photo  Jane Joyce
Robert Maillart’s exceptional bridge high above the Salgina ravine is an International Historic Engineering Landmark. His graphical, rather than computational, design method resulted in a slender span that has won worldwide acclaim. The bridge is the oldest surviving example of its type — a three-hinge arch formed from a trough of reinforced concrete. It has been repaired seamlessly several times.
The Salginatobelbrücke (Salginatobel Bridge) was built to connect the settlements of Schiers and Schuders in the Graubünden canton of eastern Switzerland. From 1900 until a referendum in June 1925 changed the law, private car traffic had been banned in this remote alpine location.
On 12th July 1928, the canton’s chief engineer J. Solca advertised a competition for the bridge in the publication Schweizerischen Bauzeitung. The article asked for designs of a "reinforced concrete bridge (possibly an iron bridge)" approximately 134m long. On 18th July, interested contractors and representatives of the Schiers community were offered a tour of the construction site. Tenders opened on 7th September, and among the 19 received, the proposal by Robert Maillart (1872-1940) was the least expensive at 135,000 Swiss Francs.
Maillart completed the detailed design in spring 1929, and by June construction drawings were ready. Salginatobelbrücke is a visual development of his shorter bridge at Tavanasa (1905, destroyed 1927) and is built at the narrowest point of the gorge, where the sandstone is hardest and least crumbly. Maillart spent much time on site overseeing preparation of the slopes for anchoring the ends of the arch.
The reinforced concrete bridge is 132.3m long between abutments and carries a single lane roadway, 3.5m wide, sloping at 3% gradient from Schiers (west) to Schuders (east), gaining 4m in height. The single span soars 90m over the Salginabach (Salgina Brook), its western end carried on five transverse walls above the slope of the gorge. The deck is flanked by solid parapets 1.33m high. Semi-circular drainage holes, 100mm radius and spaced at 3m intervals, are set into the parapet walls at deck level.
The arch rises 13m from ends to centre, to 93m above brook-level. It is thicker and wider at the supports (400mm x 6m) than at the crown (200mm x 3.8m). A three-hinge arch was a good choice for the main span, as small movements at the supports can be accommodated, minimising the likelihood of significant cracking.
Solid spandrel walls, 290mm thick and 2.58m apart, support the deck directly on the arch over a 53.6m central section of the span, forming a box section. Thereafter the spandrel walls curve downwards, making a trough. The deck is also supported from the arch by tapering I-section (web 120mm wide, flange 600mm) vertical transverse walls, 120mm thick set at 6m centres, with access openings in them. The flanges of these walls curve outwards at the base, joining with the edge of the arch.
Maillart derived the final shape graphically using a series of parabolas. As the hinge is central, the design could be determined by evaluating the equilibrium forces on one half of the bridge (the other half being the same). He adjusted the geometry so that bending of the arch was minimised and the concrete was predominantly in compression. In effect, the shape of the arch reflected the bending moment diagram produced by the load path along it.
From his surviving design drawings, it seems that he completed four iterations of graphical plotting to arrive at built solution. He used 6m as a repeating unit, dividing the span into 6m segments (apart from 8.8m sections either side of the crown hinge) and widening the ends of the arch to 6m at the support hinges.
Interestingly, he concentrated on using dead loads (2.5 tonnes per cu m) in the design process. Though he did simulate one case of live loading (350kg per sq m), which might well have produced a more economical result, he chose not to pursue it, possibly because the bridge would not be in continual use, and also because long-term concrete creep under live loads was not fully understood at the time.
Once Maillart had completed the arch design, he knew what the reactions at the support hinges would be and so could design their foundation blocks, which are 7.8m wide. Again using graphical methods, principally vectorial equilibrium, he arranged the geometry to keep the thrust close to the central axis and prevent the blocks from rotating.
The three hinges are highly-stressed narrow concrete sections, each with the extra reinforcement of two crossed hooked steel bars 36mm in diameter. The support hinges had to be strong enough to transmit the bridge loads to the foundation blocks, yet flexible enough to absorb the movements that would otherwise have caused cracks to form.
Maillart’s structurally economical design, like all his arch bridges, could not have been built without substantial temporary works. The timber scaffolding for the centring, which supported the arch while under construction, was devised by Richard Coray (1869-1946), and cost an additional 45,000 Swiss Francs.
Coray was highly regarded as an engineer, having worked previously on bridges in Fribourg (Pont de Pérolles 1921, Pont de Zaehringen 1923), Vallorbe (Viaduc du Day 1925) and Geneva (Pont Butin 1926). In fact, before vehicular traffic, in 1918, he had designed a suspension footbridge with a timber deck for almost the same spot as the Salginatobelbrücke, though it was never built. In July 1929, he survived a fall of 35m in the ravine while carrying out surveys for Maillart's scaffolding.
During the summer, just six workmen assembled and erected the Salginatobelbrücke’s centring. It consisted of two fan-shaped lattices springing from the gorge’s sides, connected by a 30m truss span. The timber used to build it was hewn from trees felled locally in the community-owned forest.
Maillart's experience in the casting of arch spans enabled the scaffolding to be lightweight. He knew that once the concrete arch had cured, it would be strong enough to support the construction of the deck and parapets above — effectively acting as another layer of falsework.
Concrete casting took place in 1930, and lasted only three months. All concrete was mixed by hand and transported in wheelbarrows. Forming the arch was crucial — it had to be cast symmetrically from both sides without stopping, to prevent 'cold' joints between batches of concrete — and took 40 hours of continuous work on 21st and 22nd July.
On 18th August 1930, the scaffolding was struck and the bridge opened for traffic shortly afterwards.
The bridge was built without waterproofing measures, as reinforced concrete was a relatively new construction medium and its long-term durability had not been explored. The drainage was to prove inadequate. The concrete was of poor quality and cover to the reinforcement was much less than would be used nowadays.
A programme of renovation was undertaken in 1973-6. To rectify the problems of water ingress and drainage that had been made worse by the increasing use of road salt in winter, the semi-circular openings in the parapets were filled with concrete (coloured black to preserve the original appearance), which prevented runoff dripping onto the arch.
Repairs were made to the badly worn support hinge on the Schuders side, and areas of carbonation and spalling over the whole bridge. To refurbish the cracked deck, the wearing surface and some of the original concrete (total 60mm) were removed and replaced by 80mm of new reinforced concrete, increasing the structural depth to 180-220mm. The deck was surfaced with 30-40mm of new asphalt.
On 21st August 1991, the American Society of Civil Engineers (ASCE) designated the bridge an International Historic Civil Engineering Landmark. ASCE President James E. Sawyer remarked, “The Salginatobel Bridge looks as if it belongs in its magnificent setting. It is not an intrusion, but it is an elegant, serviceable, important structure”. At the time, it was one of only 13 such landmarks, seven of them bridges.
After landmark status was conferred, rules were defined for the bridge’s permanent conservation. These included no change of use, the maintenance the original fabric as far as possible, replacing only portions that are irreparably damaged, and no strengthening to the support structure because of the adverse effect of increasing its weight.
In 1995 and in 1997-8, the bridge was refurbished extensively. The unstable slope at the western abutment required structural measures. New bearings were constructed to prevent ground pressure being exerted on the structure, apparently replacing those installed in 1994.
Particular attention was paid to reversing the adverse effects of water damage and to protecting the reinforcement. The expansion joints were protected from rainwater ingress and the 1976 deck sealant replaced.
The damaged concrete parapets were replaced completely. The new ones are more heavily reinforced and a little larger at 1.35m tall and 180mm thick, though of similar appearance to the originals. A section of Maillart’s parapet can be seen outside the Prättigauerhof in Schiers.
The surfaces of the arch and the lateral and transverse walls, to a depth of 10-20 mm, were cut back using high pressure water jets and refinished with a 30mm layer of shotcrete (sprayed concrete). The original surface pattern provided by the timber formwork was then faithfully reinstated, with the help of photographs.
The project cost 2.1 million Swiss Francs, of which 300,000 Swiss Francs was for the scaffolding used during the shotcreting and parapet work.
Contractor: Florian Prader & Cie, Zurich/Chur
Arch formwork (timber): Gerüstbauunternehmung Richard Coray, Trin
Research: ECPK
bibliography
"Welt Monument Salginatobel Brücke" leaflet available on site published in Switzerland, possibly by the canton of Graubünden, undated
“Robert Maillart’s key methods from the Salginatobel Bridge design process (1928)” by Corentin Fivet and Denis Zastavni, in Journal of the International Association for Shell and Spatial Structures, Vol.53 No 1, pp.39-47, March 2012
"Arch Bridges, Design - Construction – Perception" by Stefania Palaoro, University of Trento, Doctoral School of Civil and Mechanical Structural Systems Engineering, Italy, April 2011
"What Was Truly Innovative about Maillart’s Designs Using Reinforced Concrete?" by Denis Zastavni, in Proceedings of the Third International Congress on Construction History, Cottbus, Germany, May 2009
“Robert Maillart e l’emancipazione del Cemento Armato”, Studio Giovannardi e Rontini, Italy, October 2007 [in Italian]
"The Salginatobel Bridge" by Andrew J. Tapping, Bridge Engineering 2 Conference 2007, University of Bath, 4th May 2007
"Rehabilitation of the Salginatobel Bridge" by Heinrich Figi, in Structural Engineering International, Vol.10, No.1, pp.21-23, February 2000
"Performance and Repair of the Structures of Robert Maillart" by E.M. Hines, University of California, and D.P. Billington, Princeton University, 1998
"Robert Maillart: Builder, Designer, and Artist" by David P. Billington, Cambridge University Press, 1997
"Robert Maillart, Beton-Virtuose" ed. Gesellschaft für Ingenieurbaukunst, VDF Hochschulverlag AG an der ETH Zürich, 1996, reprinted 2007 [in German]
"Concrete pioneer: Robert Maillart (1873-1940)" by David P. Billington, in Concrete Quarterly, pp.14-17, London, Winter 199
"Robert Maillart and the art of reinforced concrete" by David P. Billington, Architectural History Foundation, MIT Press, 1990
"Robert Maillart's bridges: the art of engineering" by David P. Billington, University Press, Princeton, New Jersey, 1979
"Robert Maillart: Bridges and Constructions" by Max Bill, translated by W.P.M.K. Clay, Pall Mall Press, 3rd revised edition, November 1969
http://dgcwww.epfl.ch
http://happypontist.blogspot.co.uk
www.bernd-nebel.de
www.worldmonument.ch
Location

Salginatobel Bridge