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Thur Bridge, Felsegg
Thur, Henauerstrasse, west of Felsegg, St Gallen canton, Switzerland
Thur Bridge, Felsegg
associated engineer
Robert Maillart
Ingenieurbureau Maillart
date  1933
UK era  Modern  |  category  Bridge  |  reference  Tg370894
photo  Jane Joyce
Swiss engineer Robert Maillart used a pointed arch for the first time on the reinforced concrete Thur Bridge at Felsegg in St Gallen canton, north east Switzerland. The main span has twin ribs, each an identical three-hinge hollow box arch. In poor condition when visited in 2013 (pictured), the bridge is about to undergo renovation and strengthening.
The Thurbrücke (Thur Bridge) was planned as part of the construction of a trunk road between Zuzwil and Henau. In September 1932, St Gallen’s cantonal authorities produced a design, consisting of four 20m-span Romanesque masonry arches. In October, Maillart (1872-1940), a pioneer in the design of reinforced concrete structures, began negotiations with Kantonsingenieur Altweg about providing a more streamlined alternative with a longer span.
Maillart had been building three-hinge arch bridges since 1901. The most famous surviving examples of his open box schemes are the Salginatobelbrücke (Salginatobel Bridge, completed 1930) and Rossgrabenbrücke (Rossgraben Bridge, 1932).
For the Thurbrücke, Maillart proposed a single span of 72m, with an eastern approach viaduct, giving a total length of 132.4m. Altweg’s opposition was overruled, and Maillart was awarded the design contract in late December 1932. The detailed drawings and calculations were made by Marcel Fornerod (1903-2004), who worked in Maillart’s Zürich office.
Unlike his earlier bridges, which carried minor roads, this bridge had to be wide and robust enough to carry traffic loading for a two-lane major road. It is also sited in a level landscape and visual intrusion had to be considered. Maillart increased the loadbearing capacity without raising the height of the arch by flattening its curvature and deepening the spandrels walls at the quarter spans. This led to a break in the arch at midspan, making a point at the central hinge — his first broken arch.
The bridge loading was further reduced by treating the arch as two arches, side by side. Each is 3.2m wide and they are 1.6m apart, carrying a 10m wide deck. Thriftily, he centring constructed for one arch. Once that was cast, the centring was lowered and moved sideways for the casting of the second arch..
Each arch is an open hollow rectangular trough with vertical spandrel walls 320mm thick. These increase in height from the base hinge to join with the longitudinal deck beams approximately 13m from the hinge. The arch-rise between base and midspan hinges is 8.5m. The hinges were formed using cork and hardwood.
The arches are connected by four vertical transverse walls at 6.8m centres on each side of the central hinge, with cast-in access openings. The wall closest to the base hinges is 140mm thick and the others are 100mm thick. Below the base hinges, wedge-shaped anchorages sunk into the river bed support the arches.
The approach superstructures are carried on 340mm thick vertical portal frames founded on bedrock. One portal completes the west end of the bridge, while the east side viaduct has four portals set at 9.6m centres.
The superstructure is 7.4m wide and consists of four longitudinal beams 300mm thick, surmounted by a 230mm slab. The deck is cantilevered from the superstructure 900mm on each side. The 6.5m wide asphalt roadway is flanked by 1.5m footpaths and concrete parapets 1.12m high and 250mm thick. The heavy parapets draw the eye and detract from the bridge’s overall slenderness, but were required by the canton.
The bridge was constructed in nine months during 1933 and the cost, excluding roadway surfacing, was 169,100 Swiss Francs. In keeping with Swiss procedures, load testing was carried out to verify structural soundness. Four trucks weighing a total of 51.6 tonnes produced deflections of less than 3mm.
On 12th March 1936, Professor Mirko Ros (1879-1962) of the Eidgenössische Materialprüfungs- und Forschungsanstalt (Swiss Federal Laboratories for Materials Science and Technology) confirmed the stress results as satisfactory. He concluded that the "road bridge over the Thur between Gossau and Wil is structurally a very interesting, original and economical solution".
Until the late 1960s, when the A1 (E60) road opened, the bridge formed part of the main traffic artery between Wil and St Gallen. It now has protected heritage status.
Over the years, its structural condition has deteriorated. The roadway was reduced in width to prevent it being used by trucks. In February 2014, it was decided that the bridge would be strengthened and completely renovated, at an estimated cost of 6m Swiss Francs. Preparatory work is to begin in 2014.
The main road, Henauerstrasse, which passes over the bridge, also crosses a smaller bridge less than 100m to the south east, over a stream and a by-road. It has four unequal spans, and the roadway deck is supported on rows of vertical columns, four columns per row. Its style, though plainer, is similar to the Thurbrücke — including the heavy parapets — and it is likely to have been constructed at the same time (1933), possibly as part of Ingenieurbureau Maillart’s contract.
Contractor: Th. Bertschinger AG, Zürich
Contractor: K. Bendel, St Gallen
Research: ECPK
"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
"Einige neuere Eisenbetonbrücken" by Robert Maillart, in Schweizerische Bauzeitung, Vol.107, pp.157-163, April 1936

Thur Bridge, Felsegg