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Flienglibach Bridge (1923), site of
Seestrasse, Wagitalersee, Schwyz canton, Switzerland
Flienglibach Bridge (1923), site of
associated engineer
Robert Maillart
Ingenieurbureau Maillart
date  1923
era  Modern  |  category  Bridge  |  reference  Tg280475
photo  copyright ETH-Bibliothek Zürich, Bildarchiv
With the deck stiffened arch of his Flienglibach Bridge, reinforced concrete pioneer Robert Maillart further advanced the structural design of bridges. He had made radical changes to traditional methods with his earlier bridges, and now realised that the bridge deck and the arch could act together to strengthen the whole structure. His noteworthy road bridge was replaced by the present one in 1969.
In August 1923, Swiss engineer Robert Maillart (1872-1940) designed a single 38.7m span bridge to cross the Flienglibach watercourse, which flows into Lake Wägital (Wägitalersee). The lake is a pumped storage reservoir for a hydro-electric power plant, formed by the construction of the Schräh Dam at Innerthal in 1922-4. It is encircled by the Seestrasse road, which crosses the many brooks and streams that feed into the lake.
Maillart’s concrete bridge was 41m long overall and 4.6m wide. It had a very thin arch, only 250mm thick at the crown and 350mm at the abutments. Its horizontal deck was supported by vertical transverse walls on top of the arch, four either side of the crown. The 330mm-thick deck had structural parapets 1.3m deep that acted as integral stiff beams. The arrangement enabled the deck to brace the arch and carry some of its load.
The new design was almost a visual refinement of his 1912 hingeless arch bridge over the Aare River in Aarburg, Aargau canton, which had rows of columns supporting the deck from the arch. However, at Aarburg the deck and the arch were not working as a combined structure, as the arch was much thicker (800mm-1.02m) and carried all the load. Cracks appeared on the underside of the deck beams near the column heads, showing that the deck was moving downwards relative to the arch.
He realised that to avoid cracking, deck and arch would have to work together. It was an idea that his former university professor Wilhelm Ritter (1847-1906) had explored in an 1883 article. In addition, Maillart wanted to make his elegant arches extremely thin, which was only possible when the arch loads were reduced because some live loading was carried by the comparatively substantial parapets. Another advantage of slenderness was that the bridge would need less materials, and therefore cost less to construct.
Maillart went on to design two more bridges around the shores of the lake, both completed in 1924. The Schrähbach Bridge, west of the dam wall, is another deck stiffened arch (slated for demolition in2014) and the Ziggenbach Bridge at the south east of the reservoir (extant), is a hingeless arch.
Unfortunately, the Flienglibach Bridge began to suffer frost damage soon after completion and it was discovered that the inferior chemical composition of its concrete was not frost-resistant. Maillart’s design was not at fault.
Around 1933, the frost damage was repaired using sprayed concrete, and longitudinal reinforced concrete walls were added to both sides of the bridge, closing the openings between deck and arch.
A 1937 report by Professor Mirko Ros (1879-1962) evaluated test measurements taken by the Swiss Federal Material Research Station on the Flienglibach and Schrähbach bridges. He found the results of stress recording and load testing very satisfactory, confirming Maillart’s design as structurally robust.
In 1969, Maillart’s ground-breaking bridge over the Flienglibach was replaced by a wider bridge of much plainer design — a horizontal concrete deck supported on two piers.
Contractor: Prader & Cie, Zürich
Contractor: F. Favetto, Brunnen
Research: ECPK
"State-of-the-art of spatial arch bridges" by Marta Samiento-Comesías, Ana M. Ruiz-Teran and Ángel C. Aparicio, in Bridge Engineering, ICE Proceedings, London, May 2012
"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

Flienglibach Bridge (1923), site of