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Schwandbach Bridge
Schwandbach, east of Spilmannswald, Berne canton, Switzerland
Schwandbach Bridge
associated engineer
Robert Maillart
Ingenieurbureau Maillart
date  August - November 1933, opened 29th November 1933
era  Modern  |  category  Bridge  |  reference  Tf168018
photo  Jane Joyce
Schwandbach Bridge, in the thickly wooded hinterland south of Berne, is a striking example of Maillart’s reinforced concrete deck-stiffened arch designs. Its roadway follows a sweeping curve in plan, supported by a series of transverse walls on a single thin straight arch.
The 6.4km road between Rüeggisberg and Wahlern in Schwarzenburg was constructed in 1932-8. The project included bridging the valleys of the Schwarzwasser (black water) and one of its many tributary streams, the Schwandbach (dwindling brook). Both bridges were designed by Swiss engineer Robert Maillart (1872–1940), who had become well known by expressing his then-revolutionary ideas for structures in reinforced concrete.
However, the first design for the Schwandbach Bridge (Schwandbachbrücke) was proposed by local contractor Albert Binggeli (1898-1994). In October 1932, he offered a straight bridge and approaches with sharp bends. Some sources declare it was in fact a dam, which would have been costly to construct. The cantonal authorities asked Maillart to find a solution, as they had done a year previously for the Schwarzwasser crossing (Rossgraben Bridge).
Maillart's Schwandbach Bridge is 55.6m long and 4.9m wide. The roadway bursts out of the forest right onto the bridge, already curving on an elliptical path some 50m long, then plunges into the forest once again. At the north end, the approach road meets the deck at a curve of radius 30m, and on the south side the deck continues at a radius of 22m until it joins the road. The southern approach bend is cut into the hillside at a 20m radius.
The supporting arch springs from abutments that consist of concrete retaining walls. These are a departure for Maillart, as his earlier bridges used masonry abutments — as well as the natural bedrock.
The cast concrete arch is set approximately perpendicular to the stream but varies in width. It's straight on the upstream (east) side but widens near the banks on the downstream side, following the sweep of the deck. This shape helps it resist the torsional twisting forces imposed by the horizontal deck curve, an effect similar to the combination of gravity and wind loads on high bridges.
The arch spans 37.4m, with a slab uniformly 200mm deep. It rises 6m from spring point to apex. The widths vary from 7.2m at the anchorage points to 4.2m at mid-span. The top of the curving arch meets the underside of the deck directly in the central 2.8m section, bounded by transverse walls 200mm thick, effectively forming a hollow box. The remainder of the arch effectively consists of a series of straight chords, four each side of the box, that change gradient at each transverse wall junction.
The transverse walls are trapezoidal in shape, as wide as the deck at their tops and as wide as the arch at their bases. They are spaced at 4.4m. Those at the ends of the arch are 200mm thick, and the others are 160mm.
Possibly because the contractors were the same for both bridges, some of the timbers from Rossgraben Bridge scaffolding were reused here for the arch centring. A lattice truss was used to form the arch, supported on a fan-shaped truss (founded on a temporary concrete pier) and timber struts angled into the sides of the gorge. As at Rossgraben, the centring was designed for the weight of the arch only, as the arch itself was then used to support the casting of structure above it.
The deck gradient is two percent, downhill in the western direction. The deck is 3.6m wide and 150mm deep with a wearing course on top. Longitudinal reinforced concrete beams, 900mm by 300mm in section, provide stiffening on both sides. A 1m walkway cantilevers from the top of the stiffening beam on the downstream side, overhanging by 700mm. Metal guard rails are bolted to the outer faces of the concave stiffening beam and convex walkway kerb.
Construction began on 15th August 1933. The concrete work was completed on 24th October, the scaffolding struck on 2nd November and the bridge opened to traffic on 29th November. Its maximum weight limit is 7 tonnes. The bridge cost 47,300 Swiss Francs to construct, of which 5,300 Francs was Maillart’s fee and 9,500 Francs the cost of the centring.
The sweeping form of the superstructure combined with the unusual arch made the Schwandbach Bridge unlike any other. In 1934, Professor Mirko Ros (1879-1962) of the Eidgenüssische Materialprüfungs- und Forschungsanstalt (EMPA, Swiss Federal Laboratories for Materials Science & Technology), and a friend of Maillart's, expressed an interest in load testing the bridge to compare its structural performance with Maillart’s concise design — captured succinctly in a couple of pages.
The canton’s highway authority wouldn't pay for the testing, so Ros applied to the Eidgenössische Stiftung zur Förderung schweizerischer Volkswirtschaft durch wissenschaftliche Forschung (Swiss Foundation for the Promotion of Swiss economy through Scientific Research) for a grant of 6,000 Francs. Influenced by foundation board member and old adversary of Maillart's, Max Ritter (1884-1946), the application was rejected.
Ritter thought that mathematical theories could describe the bridge’s behaviour and asked his assistant engineer, Karl Hofacker (1897-1991), to study the matter. Hofacker made two models and produced some 30 sheets of calculations but still did not complete the task. Eventually, after a year-long hiatus, the foundation agreed to sponsor the test programme.
Four EMPA engineers carried out stress tests on 19th- 22nd June and 29th-31st October 1935. They used 82 instruments to measure the effects of a 12.5 tonne load placed in 28 different positions, generating approximately 2,500 individual measurements. Of particular interest were the results of eccentric loading on the bridge’s anticipated lateral twisting and the associated unequal edge stresses in arch and deck.
Analysis of the measurements verified Maillart’s design, as stated in the conclusions of Ros' report: "Das Verhalten des Tragsystems der Schwandbachbrücke hat sich als regelmässig und vollkommen elastisch erwiesen. Als wesentliches Ergebnis kann entschieden die Tatsache festgestellt werden, dass das Verhalten dieser im Grundriss verhältnismässig stark gekrümmten Brücke von demjenigen einer geraden praktisch so unwesentlich abweicht, dass einfache konstruktive Ueberlegungen genügen, um den Einfluss der Kr¨mmung gebührend zu berücksichtigen" .
In English this reads: "The Schwandbach Bridge’s structural behaviour has proved to be regular and perfectly elastic. Essentially it was determined that the behaviour of this comparatively sharp plan curve differs so negligibly from that of a straight bridge that simple structural considerations are sufficient to take due account of the influence of the curvature".
No bridge repairs have been noted until 1978-9, when the carriageway was refurbished. In 1984, the structure was listed on the national inventory of cultural heritage as a Denkmalgeschützt (protected monument, No. 485.4.1) of national importance.
In 1991 and 2001, inspections found some reinforcement corrosion and concrete spalling, rusting railings and defective carriageway sealing. However, the bridge’s load capacity of 7 tonnes satisfied predicted future traffic loading. In 2002, chemical corrosion inhibitors were applied to the concrete but did not prove successful.
In 2004, Professor Eugen Brühwiler (b.1958) of the Ecole Polytechnique Fédérale de Lausanne (Swiss Federal Institute of Technology in Lausanne) stated that the Schwandbach Bridge and others of "high cultural value and aesthetic quality deserve respectful treatment".
Initial cost estimates for repairs to the Schwandbach and Rossgraben bridges together exceeded 2m Swiss Francs, for removing and replacing surface concrete down to reinforcement level. So Brühwiler’s suggestion that construction interventions should adhere to the principles of sustainable development, keeping structural work to a minimum to preserve original fabric and limit costs, was well received.
From March to September 2005, a schedule of closely monitored repairs was carried out. The work was designed to maintain and, where necessary in some areas, reinstate the structure’s original appearance.
Bridge refurbishment included improving drainage, waterproofing the deck slab, renewing the roadway wearing course, replacing metal railings, repairing areas of concrete spalling and reinforcement bar corrosion, and impregnating exposed concrete surfaces with a water-repellent (to 20 mm depth). The finish on all concrete repairs replicates the pattern of planked formwork that typifies Maillart’s structures. The low-impact project cost 374,000 Swiss Francs.
The Schwandbach Bridge is now acknowledged as one of Maillart's masterpieces. It has been a part of Naturpark Gantrisch (Gantrisch Wildlife Park) since 2012.
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
"The Revolutionary Bridges of Robert Maillart" by David P. Billington, in Scientific American, pp.84-91, July 2000
"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'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
"Belastungsversuche an der gebogenen Eisenbetonbrücke über den Schwandbach, Kt. Bern" by Mirko Ros, in Schweizerische Bauzeitung, Vol.113, pp.53-58, February 1939
www.gantrisch.ch
www.swiss-timber-bridges.ch
Location

Schwandbach Bridge