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Rossgraben Bridge
Schwarzwasser, Rossgraben (road), Spilmannswald, canton of Berne, Switzerland
Rossgraben Bridge
associated engineer
Robert Maillart
Ingenieurbureau Maillart
date  August - October 1932, opened 19th November 1932
era  Modern  |  category  Bridge  |  reference  Tf165016
photo  Jane Joyce
The bridge carrying the Rossgraben road over the Schwarzwasser near Spilmannswald, south of Berne in western Switzerland, is an open box three-hinged arch of reinforced concrete. Its design is reminiscent of Maillart’s famous Salginatobel Bridge, completed two years earlier in Graubünden canton. The Rossgraben Bridge has been refurbished and remains in use.
Until the 1930s, there was no road over the Schwarzwasser (black water) in this area of the Bernese hinterland. Goods were transported by cart along the dangerous paths of the deep valley, and by fording the river when water levels were low or using footbridges.
By 1931, planning for a 6.4km road between Rüeggisberg and Wahlern in Schwarzenburg was underway, and its route included bridges over the Schwarzwasser and the Schwandbach (dwindling brook), one of its tributaries. The road was constructed in 1932-8, at a total cost of 473,000 Swiss Francs.
The canton's official design for the Rossgraben Bridge (Rossgrabenbrücke) was a conventional three-span beam scheme on two river piers, with heavy vertical abutments and a central clear span of 50m. Contractors were allowed to propose alternatives and in 1931, local builder Albert Binggeli (1898-1994) submitted a design for a 60m span three-hinge arch.
In August and September of that year, the highly-regarded Swiss reinforced concrete pioneer Robert Maillart (1872-1940) entered the competition. He offered two designs, of 60m and 80m span, based on the Salginatobel Bridge (Salginatobelbrücke, 1930), though the crossing height is very low here by comparison. Maillart and the cantonal authorities favoured the longer span on practical and aesthetic grounds, and he was awarded the design contract on 18th February 1932. Binggeli's firm shared the contractor work with Losinger & Cie, founded in 1917 by brothers Eugen (1891-1951) and Oskar.
The Rossgraben Bridge is 102m long. Its 82m single span is Maillart's second-longest open box three-hinged arch, after Salginatobel. The arch rises 9.7m and is mostly 3.6m wide, broadening to 4.6m at each end. The deck follows the arch in width, measuring 3.6m at midspan and 4.6m above the base hinges, but continues to widen at the ends to join the 5.6m wide road at the approaches. Metal railings bolted to the outside edges of the kerb, as opposed to solid parapets, help the deck appear very slender in comparison to the substructure.
The arch is 180mm deep over the central third and 200mm deep at the banks. It was constructed on curved timber centring supported by six fan-shaped timber trusses, each carried on two temporary concrete piers. The scaffolding was designed for the weight of the arch only as, once the concrete had set and gained strength, the arch in turn became the support for the construction of the cross walls and deck slab.
The 100mm thick vertical cross walls connect the deck and the arch below. Small circular openings in the walls provide access to near the crown of the arch. Two further cross walls, 140mm thick, are located on the anchorages adjacent to the base hinges.
The central section of the bridge below the deck is a closed box, with full-height side walls. About a quarter of the way out from the centre hinge, the side walls reduce in height, and follow the curve of the arch down to the base hinges at a constant height.
The concrete crown hinge isn't flush with the edge of the deck slab, which makes it a noticeable feature of the bridge. The hinge's two halves are connected by 17 pairs of crossed hooked steel bars, 36mm in diameter. A pad of hardwood with cork inserts formed the original hinge packing.
Rossgraben Bridge was constructed between August and October 1932, in world record time according to Maillart, though this hasn't been verified. It costs only 82,840 Swiss Francs. The opening ceremony was held on 19th November 1932, after a final inspection by cantonal official and politician Hans Stähli (1889-1963). A year later, the nearby Schwandbach Bridge (Schwandbachbrücke), also by Maillart, opened.
In 1978-9, the bridge's carriageway was refurbished. In 1984, the bridge was listed as a Denkmalgeschützt (protected monument, No. 485.4.2) of national importance.
In 1991 and 2001, investigation of the concrete showed some reinforcement corrosion and spalling, though the arch generally was in good condition except for a leaking crown hinge. The carriageway sealing was defective. However, assessment showed the bridge’s load capacity (maximum 10 tonnes) remained sufficient for predicted future traffic loading.
Application of chemical corrosion inhibitors in 2002 did not prove successful. Initial cost estimates of conventional repairs — removing surface concrete down to reinforcement level and applying a layer of new concrete — were in excess of 2m Swiss Francs, for the Rossgraben and the Schwandbach together.
However, in 2004, Professor Eugen Brühwiler (b.1958) of the Ecole Polytechnique Fédérale de Lausanne (Swiss Federal Institute of Technology in Lausanne) suggested that Rossgraben and other bridges with "high cultural value and aesthetic quality deserve respectful treatment". Henceforth, works to extend their service life should adhere to the principles of sustainable development and keep structural intervention to a minimum to preserve original fabric and limit costs. A programme of repair work was undertaken in March-September 2005, and was closely monitored.
Works included improving the bridge’s drainage system, installing a waterproof membrane to the deck slab, renewing the road surface, repairing areas of concrete spalling and reinforcement bar corrosion, and protecting exposed concrete surfaces with a water-repellent (permeating up to 20 mm).
Care was taken to reinstate the original formwork pattern on the concrete surfaces left by the planks used when the concrete was first cast. The heavily timber-patterned finish is a characteristic feature of Maillart's structures. The metal railings were replaced like-for-like. The 556,000 Swiss Franc refurbishment program has made little visible alteration to the bridge.
Rossgraben Bridge has been a part of Naturpark Gantrisch (Gantrisch Wildlife Park) since 2012.
Assistant engineer: Ernst Stettler
Contractors: Losinger & Cie AG and Ernst & Albert Binggeli
Research: ECPK
bibliography
"Extending the service life of Swiss bridges of cultural value" by Eugen Brühwiler, in Proceedings of the Institution of Civil Engineers: Engineering History & Heritage, Vol.165, Issue 4, pp.235-240, London, Nov 2012
"Erhaltung historischer Bausubstanz: Maillart-Brücken im Kanton Bern werden saniert" by Ueli Salvisberg, in Geomatik Schweiz, pp.637-640, Berne, December 2008
"Robert Maillart e l’emancipazione del Cemento Armato", Studio Giovannardi e Rontini, Italy, Oct 2007
"Robert Maillart: Builder, Designer, and Artist" by David P. Billington, Cambridge University Press, 1997
"Robert Maillart and the art of reinforced concrete" by David P. Billington, Architectural History Foundation, MIT Press, 1990
"Robert Maillart: Bridges and Constructions" by Max Bill, translated by W.P.M.K. Clay, Pall Mall Press, 3rd revised edition, November 1969
http://happypontist.blogspot.co.uk
www.gantrisch.ch
www.swiss-timber-bridges.ch
Location

Rossgraben Bridge